Clean up examples

This reverts commit 5e685d6641.

.gitignore

@@ -1 +1,2 @@
 target/
+node_modules/

Cargo.toml

@@ -1,46 +1,50 @@
 [package]
 name = "dust-lang"
-version = "0.1.1"
-description = "Data-oriented programming language and interactive shell."
-authors = ["jeff <dev@jeffa.io.com>"]
+description = "General purpose programming language"
+version = "0.4.2"
 repository = "https://git.jeffa.io/jeff/dust.git"
-homepage = "https://git.jeffa.io/jeff/dust"
-readme = "README.md"
+edition = "2021"
 license = "MIT"
-edition = "2018"
+authors = ["Jeff Anderson"]
 default-run = "dust"
 
 [[bin]]
 name = "dust"
+path = "src/main.rs"
 
-[[bin]]
-name = "gui"
-
-[lib]
-name = "dust_lib"
-path = "src/lib.rs"
+[profile.dev]
+opt-level = 1
+[profile.dev.package."*"]
+opt-level = 3
 
 [dependencies]
-rand = "0.8.5"
-chrono = "0.4.26"
-trash = "3.0.3"
-rayon = "1.7.0"
-serde = { version = "1.0.171", features = ["derive"] }
-sysinfo = "0.29.6"
-toml = "0.7.6"
-toml_edit = "0.19.14"
-comfy-table = "7.0.1"
-clap = { version = "4.3.19", features = ["derive"] }
-git2 = "0.17.2"
+clap = { version = "4.4.4", features = ["derive"] }
 csv = "1.2.2"
-json = "0.12.4"
-reqwest = { version = "0.11.18", features = ["blocking", "json"] }
-serde_json = "1.0.104"
-egui_extras = "0.22.0"
-rustyline = { version = "12.0.0", features = ["with-file-history", "derive"] }
-ansi_term = "0.12.1"
-iced = "0.10.0"
-egui = "0.22.0"
-eframe = "0.22.0"
-env_logger = "0.10.0"
-once_cell = "1.18.0"
+libc = "0.2.148"
+log = "0.4.20"
+rand = "0.8.5"
+rayon = "1.8.0"
+reqwest = { version = "0.11.20", features = ["blocking", "json"] }
+serde = { version = "1.0.188", features = ["derive"] }
+serde_json = "1.0.107"
+toml = "0.8.1"
+tree-sitter = "0.20.10"
+enum-iterator = "1.4.1"
+env_logger = "0.10"
+reedline = { version = "0.28.0", features = ["clipboard", "sqlite"] }
+crossterm = "0.27.0"
+nu-ansi-term = "0.49.0"
+humantime = "2.1.0"
+stanza = "0.5.1"
+colored = "2.1.0"
+lyneate = "0.2.1"
+
+[target.'cfg(not(target_arch = "wasm32"))'.dependencies]
+env_logger = "0.10"
+
+[target.'cfg(target_arch = "wasm32")'.dependencies]
+getrandom = { version = "0.2", features = ["js"] }
+wasm-bindgen-futures = "0.4"
+
+[build-dependencies]
+cc = "1.0"

README.md

@@ -1,259 +1,100 @@
 # Dust
 
-Dust is a data-oriented programming language and interactive shell. Dust can be used as a replacement for a traditional command line shell, as a scripting language and as a tool create or manage data. Dust is expression-based, has first-class functions, lexical scope and lightweight syntax.
+High-level programming language with effortless concurrency, automatic memory management, type safety and strict error handling.
 
-A basic dust program:
-
-```dust
-output "Hello world!"
-```
-
-Dust can do two (or more) things at the same time with effortless concurrency:
-
-```dust
-run(
-    'output "will this one finish first?"',
-    'output "or will this one?"'
-)
-```
-
-Dust can do amazing things with data. To load CSV data, isolate a column and render it as a line plot in a GUI window:
-
-```dust
-read_file("examples/assets/faithful.csv")
-    -> from_csv(input)
-    -> rows(input)
-    -> transform(input, 'input.1')
-    -> plot(input)
-```
+![Dust version of an example from The Rust Programming Language.](https://git.jeffa.io/jeff/dust/raw/branch/main/docs/assets/example_0.png)
 
 <!--toc:start-->
 - [Dust](#dust)
   - [Features](#features)
-  - [Usage](#usage)
-  - [Installation](#installation)
-  - [Contributing](#contributing)
-  - [The Dust Programming Language](#the-dust-programming-language)
-    - [Variables and Data Types](#variables-and-data-types)
-    - [Tools](#tools)
-    - [Lists](#lists)
-    - [Maps](#maps)
-    - [Tables](#tables)
-    - [The Yield Operator](#the-yield-operator)
-    - [Functions](#functions)
-    - [Time](#time)
+    - [Easy to Read and Write](#easy-to-read-and-write)
+    - [Effortless Concurrency](#effortless-concurrency)
+    - [Helpful Errors](#helpful-errors)
+    - [Static analysis](#static-analysis)
+    - [Debugging](#debugging)
+    - [Automatic Memory Management](#automatic-memory-management)
+    - [Error Handling](#error-handling)
+  - [Installation and Usage](#installation-and-usage)
 <!--toc:end-->
 
 ## Features
 
-- Data visualization: GUI (not TUI) plots, graphs and charts are available from directly within dust. No external tools are needed.
-- Powerful tooling: Built-in commands reduce complex tasks to plain, simple code. You can even partition disks or install software.
-- Pipelines: Like a pipe in bash, dust features the yield `->` operator.
-- Format conversion: Effortlessly convert between dust and formats like JSON, CSV and TOML.
-- Structured data: Dust can represent data with more than just strings. Lists, maps and tables are easy to make and manage.
-- Developer tools: Dust has a complete tree sitter grammar, allowing syntax highlighting and completion in most code editors.
+### Easy to Read and Write
 
-## Usage
+Dust has simple, easy-to-learn syntax.
 
-Dust is an experimental project under active development. At this stage, features come and go and the API is always changing. It should not be considered for serious use yet.
-
-To get help with the shell you can use the "help" tool.
-
-```dust
-help()         # Returns a table will all tool info.
-help("random") # Returns a table with info on tools in the specified group.
-# The above is simply a shorthand for this:
-help() -> where(input, 'tool == "random"')    
+```js
+output('Hello world!')
 ```
 
-## Installation
+### Effortless Concurrency
 
-You must have the default rust toolchain installed and up-to-date. Install [rustup] if it is not already installed. Run `cargo install dust-lang` then run `dust` to start the interactive shell. Use `dust --help` to see the full command line options.
+Write multi-threaded code as easily as you would write code for a single thread.
 
-To build from source, clone the repository and run `cargo run` to start the shell. To see other command line options, use `cargo run -- --help`.
-
-## Contributing
-
-Please submit any thoughts or suggestions for this project. To contribute a new command, see the library documentation. Implementation tests are written in dust and are run by a corresponding rust test so dust tests will be run when `cargo test` is called.
-
-## The Dust Programming Language
-
-Dust is a hard fork of [evalexpr]; a simple expression language. Dust's core language features maintain this simplicity. But it can manage large, complex sets of data and perform complicated tasks through commands. It should not take long for a new user to learn the language, especially with the assistance of the shell.
-
-If your editor supports tree sitter, you can use [tree-sitter-dust] for syntax highlighting and completion support. Aside from this guide, the best way to learn dust is to read the examples and tests to get a better idea of what dust can do.
-
-### Variables and Data Types
-
-Variables have two parts: a key and a value. The key is always a text string. The value can be any of the following data types:
-
-- string
-- integer
-- floating point value
-- boolean
-- list
-- map
-- table
-- function
-- time
-- empty
-
-Here are some examples of variables in dust.
-
-```dust
-string = "The answer is 42.";
-integer = 42;
-float = 42.42;
-list = (1, 2, string, integer, float);
-map.key = "value";
-empty = ();
+```js
+async {
+    output('Will this one print first?')
+    output('Or will this one?')
+    output('Who knows! Each "output" will run in its own thread!')
+}
 ```
 
-### Tools
+### Helpful Errors
 
-**Tools** are dust's built-in functions. Some of them can reconfigure your whole system while others do very little. They may accept different inputs, or none at all. For example, commands in the `random` group can be run without input, but the `random_integer` command can optionally take two numbers as in inclusive range.
+Dust shows you exactly where your code went wrong and suggests changes.
+
+![Example of syntax error output.](https://git.jeffa.io/jeff/dust/raw/branch/main/docs/assets/syntax_error.png)
+
+### Static analysis
+
+Your code is always validated for safety before it is run.
+
+![Example of type error output.](https://git.jeffa.io/jeff/dust/raw/branch/main/docs/assets/type_error.png)
+
+Dust
+
+### Debugging
+
+Just set the environment variable `DUST_LOG=info` and Dust will tell you exactly what your code is doing while it's doing it. If you set `DUST_LOG=trace`, it will output detailed logs about parsing, abstraction, validation, memory management and runtime. Here are some of the logs from the end of a simple [fizzbuzz example](https://git.jeffa.io/jeff/dust/src/branch/main/examples/fizzbuzz.ds).
+
+![Example of debug output.](https://git.jeffa.io/jeff/dust/raw/branch/main/docs/assets/debugging.png)
+
+### Automatic Memory Management
+
+Thanks to static analysis, Dust knows exactly how many times each variable is used. This allows Dust to free memory as soon as the variable will no longer be used, without any help from the user.
+
+### Error Handling
+
+Runtime errors are no problem with Dust. The `Result` type represents the output of an operation that might fail. The user must decide what to do in the case of an error.
 
 ```dust
-die_roll = random_integer(1, 6);
-d20_roll = random_integer(1, 20);
-coin_flip = random_boolean();
+match io:stdin() {
+    Result::Ok(input) -> output("We read this input: " + input)
+    Result::Error(message) -> output("We got this error: " + message)
+}
 ```
 
-```dust
-message = "I hate dust.";
-replace(message, "hate", "love")
+## Installation and Usage
+
+There are two ways to compile Dust. **It is best to clone the repository and compile the latest code**, otherwise the program may be a different version than the one shown on GitHub. Either way, you must have `rustup`, `cmake` and a C compiler installed.
+
+To install from the git repository:
+
+```fish
+git clone https://git.jeffa.io/jeff/dust
+cd dust
+cargo run --release
 ```
 
-### Lists
+To install with cargo:
 
-Lists are sequential collections. They can be built by grouping values with parentheses and separating them with commas. Values can be indexed by their position to access their contents. Lists are used to represent rows in tables and most commands take a list as an argument. Their contents can be indexed using dot notation with an integer.
-
-```dust
-list = (true, 41, "Ok");
-
-assert_equal(list.0, true);
-
-the_answer = list.1 + 1;
-
-assert_equal(the_answer, 42);
+```fish
+cargo install dust-lang
+dust
 ```
 
-### Maps
+## Benchmarks
 
-Maps are flexible collections with arbitrary key-value pairs, similar to JSON objects. Under the hood, all of dust's runtime variables are stored in a map, so, as with variables, the key is always a string.
+## Development Status
 
-```dust
-reminder.message = "Buy milk";
-reminder.tags = ("groceries", "home");
-
-json = to_json(reminder);
-append(json, "info.txt");
-```
-
-### Tables
-
-Tables are strict collections, each row must have a value for each column. Empty cells must be explicitly set to an empty value.
-
-```dust
-animals = create_table (
-    ("name", "species", "age"),
-    (
-        ("rover", "cat", 14),
-        ("spot", "snake", 9),
-        ("bob", "giraffe", 2)
-    )
-);
-```
-
-Querying a table is similar to SQL.
-  
-```dust
-names = select(animals, "name");
-youngins = where(animals, 'age < 5');
-```
-
-The commands `create_table` and `insert` make sure that all of the memory used to hold the rows is allocated at once, so it is good practice to group your rows together instead of using a call for each row.
-
-```dust
-insert(
-    animals,
-    (
-        ("eliza", "ostrich", 4),
-        ("pat", "white rhino", 7),
-        ("jim", "walrus", 9)
-    )
-);
-
-assert_equal(count(animals.all), 6);
-
-sorted = sort(animals);
-```
-
-### The Yield Operator
-
-Like a pipe in bash, zsh or fish, the yield operator evaluates the expression on the left and passes it as input to the expression on the right. That input is always assigned to the **`input` variable** for that context. These expressions may simply contain a value or they can call a command or function that returns a value.
-
-```dust
-"Hello dust!" -> output(input)
-```
-
-This can be useful when working on the command line but to make a script easier to read or to avoid fetching the same resource multiple times, we can also declare variables. You should use `->` and variables together to write efficient, elegant scripts.
-
-```dust
-json = download("https://api.sampleapis.com/futurama/characters");
-from_json(json)
-    -> select(input, "name");
-    -> input.4
-```
-
-### Functions
-
-Functions are first-class values in dust, so they are assigned to variables like any other value. The function body is wrapped in single parentheses. To call a function, it's just like calling a command: simply pass it an argument or use an empty set of parentheses to pass an empty value.
-
-In the function bod, the **`input` variable** represents whatever value is passed to the function when called.
-
-```dust
-say_hi = 'output "hi"';
-add_one = 'input + 1';
-
-say_hi();
-assert_equal(add_one(3), 4);
-```
-
-This function simply passes the input to the shell's standard output.
-
-```dust
-print = 'output(input)';
-```
-
-Because functions are stored in variables, we can use collections like maps to
-organize them.
-
-```dust
-math.add = 'input.0 + input.1';
-math.subtract = 'input.0 - input.1';
-
-assert_equal(math.add(2, 2), 4);
-assert_equal(math.subtract(100, 1), 99);
-```
-
-### Time
-
-Dust can record, parse and convert time values. Dust can parse TOML datetime
-values or can create time values using commands.
-
-```dust
-dob = from_toml("1979-05-27T07:32:00-08:00")
-
-output "Date of birth = " + local(dob);
-```
-
-```dust
-time = now();
-
-output "Universal time is " + utc(time);
-output "Local time is " + local(time);
-```
-
-[dnf]: https://dnf.readthedocs.io/en/latest/index.html
-[evalexpr]: https://github.com/ISibboI/evalexpr
-[rustup]: https://rustup.rs
+Currently, Dust is being prepared for version 1.0. Until then, there may be breaking changes to the language and CLI.

build.rs

@@ -0,0 +1,17 @@
+fn main() {
+    let src_dir = std::path::Path::new("tree-sitter-dust/src");
+    let mut c_config = cc::Build::new();
+
+    c_config.include(src_dir);
+    c_config
+        .flag_if_supported("-Wno-unused-parameter")
+        .flag_if_supported("-Wno-unused-but-set-variable")
+        .flag_if_supported("-Wno-trigraphs");
+
+    let parser_path = src_dir.join("parser.c");
+
+    c_config.file(&parser_path);
+    c_config.compile("parser");
+
+    println!("cargo:rerun-if-changed={}", parser_path.to_str().unwrap());
+}

docs/language.md

@@ -0,0 +1,515 @@
+# Dust Language Reference
+
+!!! This is a **work in progress** and has incomplete information. !!!
+
+This is an in-depth description of the syntax and abstractions used by the Dust language. It is not
+necessary to read or understand all of it before you start using Dust. Instead, refer to it when
+you need help with the syntax or understanding how the code is run.
+
+Each section of this document corresponds to a node in the concrete syntax tree. Creating this tree
+is the first step in interpreting Dust code. Second, the syntax tree is traversed and an abstract
+tree is generated. Each node in the syntax tree corresponds to a node in the abstract tree. Third,
+the abstract tree is verified to ensure that it will not generate any values that violate the type
+restrictions. Finally, the abstract tree is run, beginning at the [root](#root).
+
+You may reference the [grammar file](tree-sitter-dust/grammar.js) and the [Tree Sitter docs]
+(https://tree-sitter.github.io/) while reading this guide to understand how the language is parsed.
+
+<!--toc:start-->
+- [Dust Language Reference](#dust-language-reference)
+  - [Root](#root)
+  - [Values](#values)
+    - [Boolean](#boolean)
+    - [Integer](#integer)
+    - [Float](#float)
+    - [Range](#range)
+    - [String](#string)
+    - [List](#list)
+    - [Map](#map)
+    - [Function](#function)
+    - [Option](#option)
+    - [Structure](#structure)
+  - [Types](#types)
+    - [Basic Types](#basic-types)
+    - [Number](#number)
+    - [Any](#any)
+    - [None](#none)
+    - [List Type](#list-type)
+    - [Map Type](#map-type)
+    - [Iter](#iter)
+    - [Function Type](#function-type)
+    - [Option Type](#option-type)
+    - [Custom Types](#custom-types)
+  - [Statements](#statements)
+    - [Assignment](#assignment)
+    - [Blocks](#blocks)
+      - [Synchronous Blocks](#synchronous-blocks)
+      - [Asynchronous Blocks](#asynchronous-blocks)
+    - [Break](#break)
+    - [For Loop](#for-loop)
+    - [While Loop](#while-loop)
+    - [If/Else](#ifelse)
+    - [Match](#match)
+    - [Pipe](#pipe)
+    - [Expression](#expression)
+  - [Expressions](#expressions)
+      - [Identifier](#identifier)
+      - [Index](#index)
+      - [Logic](#logic)
+      - [Math](#math)
+      - [Value](#value)
+      - [New](#new)
+      - [Command](#command)
+  - [Built-In Values](#built-in-values)
+  - [Comments](#comments)
+<!--toc:end-->
+
+## Root
+
+The root node represents all of the source code. It is a sequence of [statements](#statements) that
+are executed synchronously, in order. The output of the program is always the result of the final
+statement or the first error encountered.
+
+## Values
+
+There are ten kinds of value in Dust. Some are very simple and are parsed directly from the source
+code, some are collections and others are used in special ways, like functions and structures. All
+values can be assinged to an [identifier](#identifiers).
+
+Dust does not have a null type. Absent values are represented with the `none` value, which is a
+kind of [option](#option). You may not create a variable without a value and no variable can ever
+be in an 'undefined' state during execution.
+
+### Boolean
+
+Booleans are true or false. They are represented by the literal tokens `true` and `false`.
+
+### Integer
+
+Integers are whole numbers that may be positive, negative or zero. Internally, an integer is a
+signed 64-bit value.
+
+```dust
+42
+```
+
+Integers always **overflow** when their maximum or minimum value is reached. Overflowing means that
+if the value is too high or low for the 64-bit integer, it will wrap around. You can use the built-
+in values `int:max` and `int:min` to get the highest and lowest possible values.
+
+```dust
+assert_equal(int:max + 1, int:min)
+assert_equal(int:min - 1, int:max)
+```
+
+### Float
+
+A float is a numeric value with a decimal. Floats are 64-bit and, like integers, will **overflow**
+at their bounds.
+
+```dust
+42.0
+```
+
+### Range
+
+A range represents a contiguous sequence of integers. Dust ranges are **inclusive** so both the high
+and low bounds will be represented.
+
+```dust
+0..100
+```
+
+### String
+
+A string is a **utf-8** sequence used to represent text. Strings can be wrapped in single or double quotes as well as backticks.
+
+```dust
+'42'
+"42"
+`42`
+'forty-two'
+```
+
+### List
+
+A list is **collection** of values stored as a sequence and accessible by [indexing](#index) their position with an integer. Lists indexes begin at zero for the first item.
+
+```dust
+[ 42 'forty-two' ]
+[ 123, 'one', 'two', 'three' ]
+```
+
+Note that the commas are optional, including trailing commas.
+
+```dust
+[1 2 3 4 5]:2
+# Output: 3
+```
+
+### Map
+
+Maps are flexible collections with arbitrary **key-value pairs**, similar to JSON objects. A map is
+created with a pair of curly braces and its entries are variables declared inside those braces. Map
+contents can be accessed using a colon `:`. Commas may optionally be included after the key-value
+pairs.
+
+```dust
+reminder = {
+    message = "Buy milk"
+    tags = ["groceries", "home"]
+}
+
+reminder:message
+# Output: Buy milk
+```
+
+Internally a map is represented by a B-tree. The implicit advantage of using a B-tree instead of a
+hash map is that a B-tree is sorted and therefore can be easily compared to another. Maps are also
+used by the interpreter as the data structure for holding variables. You can even inspect the active
+**execution context** by calling the built-in `context()` function.
+
+The map stores each [identifier](#identifiers)'s key with a value and the value's type. For internal
+use by the interpreter, a type can be set to a key without a value. This makes it possible to check
+the types of values before they are computed.
+
+### Function
+
+A function encapsulates a section of the abstract tree so that it can be run seperately and with
+different arguments. The function body is a [block](#block), so adding `async` will cause the body
+to run like any other `async` block. Unlike some languages, there are no concepts like futures or
+async functions in Dust.
+
+Functions are **first-class values** in Dust, so they can be assigned to variables like any other
+value.
+
+```dust
+# This simple function has no arguments and no return value.
+say_hi = () <none> {
+    output("hi") # The "output" function is a built-in that prints to stdout.
+}
+
+# This function has one argument and will return a value.
+add_one = (number <num>) <num> {
+    number + 1
+}
+
+say_hi()
+assert_equal(add_one(3), 4)
+```
+
+Functions can also be **anonymous**. This is useful for using **callbacks** (i.e. functions that are
+called by another function).
+
+```dust
+# Use a callback to retain only the numeric characters in a string.
+str:retain(
+	'a1b2c3'
+	(char <str>) <bool> {
+		is_some(int:parse(char))
+	}
+)
+```
+
+### Option
+
+An option represents a value that may not be present. It has two variants: **some** and **none**. 
+
+```dust
+say_something = (message <option(str)>) <str> {
+    either_or(message, "hiya")
+}
+
+say_something(some("goodbye"))
+# goodbye
+
+say_something(none)
+# hiya
+```
+
+Dust includes built-in functions to work with option values: `is_none`, `is_some` and `either_or`.
+
+### Structure
+
+A structure is a **concrete type value**. It is a value, like any other, and can be [assigned]
+(#assignment) to an [identifier](#identifier). It can then be instantiated as a [map](#map) that
+will only allow the variables present in the structure. Default values may be provided for each
+variable in the structure, which will be propagated to the map it creates. Values without defaults
+must be given a value during instantiation.
+
+```dust
+struct User {
+    name <str>
+    email <str>
+    id <int> = generate_id()
+}
+
+bob = new User {
+    name = "Bob"
+    email = "bob@example.com"
+}
+
+# The variable "bob" is a structured map.
+```
+
+A map created by using [new](#new) is called a **structured map**. In other languages it may be
+called a "homomorphic mapped type". Dust will generate errors if you try to set any values on the
+structured map that are not allowed by the structure.
+
+## Types
+
+Dust enforces strict type checking. To make the language easier to write, **type inference** is used
+to allow variables to be declared without specifying the type. Instead, the interpreter will figure
+it out and set the strictest type possible.
+
+To make the type-setting syntax easier to distinguish from the rest of your code, a **type
+specification** is wrapped in pointed brackets. So variable assignment using types looks like this:
+
+```dust
+my_float <float> = 666.0
+```
+
+### Basic Types
+
+The simple types, and their notation are:
+
+- boolean `bool`
+- integer `int`
+- float `float`
+- string `str`
+
+### Number
+
+The `num` type may represent a value of type `int` or `float`.
+
+### Any
+
+The `any` type does not enforce type bounds.
+
+### None
+
+The `none` type indicates that no value should be found after executing the statement or block, with
+one expection: the `none` variant of the `option` type.
+
+### List Type
+
+A list's contents can be specified to create type-safe lists. The `list(str)` type would only allow
+string values. Writing `list` without the parentheses and content type is equivalent to writing
+`list(any)`.
+
+### Map Type
+
+The `map` type is unstructured and can hold any key-value pair.
+
+### Iter
+
+The `iter` type refers to types that can be used with a [for loop](#for-loop). These include `list`,
+`range`, `string` and `map`.
+
+### Function Type
+
+A function's type specification is more complex than other types. A function value must always have
+its arguments and return type specified when the **function value** is created.
+
+```dust
+my_function = (number <int>, text <str>) <none> {
+    output(number)
+    output(text)
+}
+```
+
+But what if we need to specify a **function type** without creating the function value? This is
+necessary when using callbacks or defining structures that have functions set at instantiation.
+
+```dust
+use_adder = (adder <(int) -> int>, number <int>) -> <int> {
+    adder(number)
+}
+
+use_adder(
+    (i <int>) <int> { i + 2 }
+    40
+)
+
+# Output: 42
+```
+
+```dust
+struct Message {
+    send_n_times <(str, int) -> none>
+}
+
+stdout_message = new Message {
+    send_n_times = (content <str>, n <int>) <none> {
+        for _ in 0..n {
+            output(content)
+        }
+    }
+}
+```
+
+### Option Type
+
+The `option(type)` type is expected to be either `some(value)` or `none`. The type of the value
+inside the `some` is always specified.
+
+```dust
+result <option(str)> = none
+
+for file in fs:read_dir("./") {
+    if file:size > 100 {
+        result = some(file:path)
+        break
+    }
+}
+
+output(result)
+```
+
+```dust
+get_line_break_index(text <str>) <some(int)> {
+    str:find(text, '\n')
+}
+```
+
+### Custom Types
+
+Custom types such as **structures** are referenced by their variable identifier.
+
+```dust
+File = struct {
+    path <str>
+    size <int>
+    type <str>
+}
+
+print_file_info(file <File>) <none> {
+  	info = file:path
+    		+ '\n'
+    		+ file:size
+    		+ '\n' 
+    		+ file:type
+		
+  	output(info)
+}
+```
+
+## Statements
+
+TODO
+
+### Assignment
+
+TODO
+
+### Blocks
+
+TODO
+
+#### Synchronous Blocks
+
+TODO
+
+#### Asynchronous Blocks
+
+```dust
+# An async block will run each statement in its own thread.
+async {
+    output(random_integer())
+    output(random_float())
+    output(random_boolean())
+}
+```
+
+```dust
+data = async {
+    output("Reading a file...")
+    read("examples/assets/faithful.csv")
+}
+```
+
+### Break
+
+TODO
+
+### For Loop
+
+TODO
+
+```dust
+list = [ 1, 2, 3 ]
+
+for number in list {
+    output(number + 1)
+}
+```
+
+### While Loop
+
+TODO
+
+A **while** loop continues until a predicate is false.
+
+```dust
+i = 0
+while i < 10 {
+    output(i)
+    i += 1
+}
+```
+
+### If/Else
+
+TODO
+
+### Match
+
+TODO
+
+### Pipe
+
+TODO
+
+### Expression
+
+TODO
+
+## Expressions
+
+TODO
+
+#### Identifier
+
+TODO
+
+#### Index
+
+TODO
+
+#### Logic
+
+TODO
+
+#### Math
+
+TODO
+
+#### Value
+
+TODO
+
+#### New
+
+TODO
+
+#### Command
+
+TODO
+
+## Built-In Values
+
+TODO
+
+## Comments
+
+TODO

examples/assets/faithful.csv

@@ -1,273 +0,0 @@
-"Index", "Eruption length (mins)","Eruption wait (mins)"
-  1, 3.600, 79
-  2, 1.800, 54
-  3, 3.333, 74
-  4, 2.283, 62
-  5, 4.533, 85
-  6, 2.883, 55
-  7, 4.700, 88
-  8, 3.600, 85
-  9, 1.950, 51
- 10, 4.350, 85
- 11, 1.833, 54
- 12, 3.917, 84
- 13, 4.200, 78
- 14, 1.750, 47
- 15, 4.700, 83
- 16, 2.167, 52
- 17, 1.750, 62
- 18, 4.800, 84
- 19, 1.600, 52
- 20, 4.250, 79
- 21, 1.800, 51
- 22, 1.750, 47
- 23, 3.450, 78
- 24, 3.067, 69
- 25, 4.533, 74
- 26, 3.600, 83
- 27, 1.967, 55
- 28, 4.083, 76
- 29, 3.850, 78
- 30, 4.433, 79
- 31, 4.300, 73
- 32, 4.467, 77
- 33, 3.367, 66
- 34, 4.033, 80
- 35, 3.833, 74
- 36, 2.017, 52
- 37, 1.867, 48
- 38, 4.833, 80
- 39, 1.833, 59
- 40, 4.783, 90
- 41, 4.350, 80
- 42, 1.883, 58
- 43, 4.567, 84
- 44, 1.750, 58
- 45, 4.533, 73
- 46, 3.317, 83
- 47, 3.833, 64
- 48, 2.100, 53
- 49, 4.633, 82
- 50, 2.000, 59
- 51, 4.800, 75
- 52, 4.716, 90
- 53, 1.833, 54
- 54, 4.833, 80
- 55, 1.733, 54
- 56, 4.883, 83
- 57, 3.717, 71
- 58, 1.667, 64
- 59, 4.567, 77
- 60, 4.317, 81
- 61, 2.233, 59
- 62, 4.500, 84
- 63, 1.750, 48
- 64, 4.800, 82
- 65, 1.817, 60
- 66, 4.400, 92
- 67, 4.167, 78
- 68, 4.700, 78
- 69, 2.067, 65
- 70, 4.700, 73
- 71, 4.033, 82
- 72, 1.967, 56
- 73, 4.500, 79
- 74, 4.000, 71
- 75, 1.983, 62
- 76, 5.067, 76
- 77, 2.017, 60
- 78, 4.567, 78
- 79, 3.883, 76
- 80, 3.600, 83
- 81, 4.133, 75
- 82, 4.333, 82
- 83, 4.100, 70
- 84, 2.633, 65
- 85, 4.067, 73
- 86, 4.933, 88
- 87, 3.950, 76
- 88, 4.517, 80
- 89, 2.167, 48
- 90, 4.000, 86
- 91, 2.200, 60
- 92, 4.333, 90
- 93, 1.867, 50
- 94, 4.817, 78
- 95, 1.833, 63
- 96, 4.300, 72
- 97, 4.667, 84
- 98, 3.750, 75
- 99, 1.867, 51
-100, 4.900, 82
-101, 2.483, 62
-102, 4.367, 88
-103, 2.100, 49
-104, 4.500, 83
-105, 4.050, 81
-106, 1.867, 47
-107, 4.700, 84
-108, 1.783, 52
-109, 4.850, 86
-110, 3.683, 81
-111, 4.733, 75
-112, 2.300, 59
-113, 4.900, 89
-114, 4.417, 79
-115, 1.700, 59
-116, 4.633, 81
-117, 2.317, 50
-118, 4.600, 85
-119, 1.817, 59
-120, 4.417, 87
-121, 2.617, 53
-122, 4.067, 69
-123, 4.250, 77
-124, 1.967, 56
-125, 4.600, 88
-126, 3.767, 81
-127, 1.917, 45
-128, 4.500, 82
-129, 2.267, 55
-130, 4.650, 90
-131, 1.867, 45
-132, 4.167, 83
-133, 2.800, 56
-134, 4.333, 89
-135, 1.833, 46
-136, 4.383, 82
-137, 1.883, 51
-138, 4.933, 86
-139, 2.033, 53
-140, 3.733, 79
-141, 4.233, 81
-142, 2.233, 60
-143, 4.533, 82
-144, 4.817, 77
-145, 4.333, 76
-146, 1.983, 59
-147, 4.633, 80
-148, 2.017, 49
-149, 5.100, 96
-150, 1.800, 53
-151, 5.033, 77
-152, 4.000, 77
-153, 2.400, 65
-154, 4.600, 81
-155, 3.567, 71
-156, 4.000, 70
-157, 4.500, 81
-158, 4.083, 93
-159, 1.800, 53
-160, 3.967, 89
-161, 2.200, 45
-162, 4.150, 86
-163, 2.000, 58
-164, 3.833, 78
-165, 3.500, 66
-166, 4.583, 76
-167, 2.367, 63
-168, 5.000, 88
-169, 1.933, 52
-170, 4.617, 93
-171, 1.917, 49
-172, 2.083, 57
-173, 4.583, 77
-174, 3.333, 68
-175, 4.167, 81
-176, 4.333, 81
-177, 4.500, 73
-178, 2.417, 50
-179, 4.000, 85
-180, 4.167, 74
-181, 1.883, 55
-182, 4.583, 77
-183, 4.250, 83
-184, 3.767, 83
-185, 2.033, 51
-186, 4.433, 78
-187, 4.083, 84
-188, 1.833, 46
-189, 4.417, 83
-190, 2.183, 55
-191, 4.800, 81
-192, 1.833, 57
-193, 4.800, 76
-194, 4.100, 84
-195, 3.966, 77
-196, 4.233, 81
-197, 3.500, 87
-198, 4.366, 77
-199, 2.250, 51
-200, 4.667, 78
-201, 2.100, 60
-202, 4.350, 82
-203, 4.133, 91
-204, 1.867, 53
-205, 4.600, 78
-206, 1.783, 46
-207, 4.367, 77
-208, 3.850, 84
-209, 1.933, 49
-210, 4.500, 83
-211, 2.383, 71
-212, 4.700, 80
-213, 1.867, 49
-214, 3.833, 75
-215, 3.417, 64
-216, 4.233, 76
-217, 2.400, 53
-218, 4.800, 94
-219, 2.000, 55
-220, 4.150, 76
-221, 1.867, 50
-222, 4.267, 82
-223, 1.750, 54
-224, 4.483, 75
-225, 4.000, 78
-226, 4.117, 79
-227, 4.083, 78
-228, 4.267, 78
-229, 3.917, 70
-230, 4.550, 79
-231, 4.083, 70
-232, 2.417, 54
-233, 4.183, 86
-234, 2.217, 50
-235, 4.450, 90
-236, 1.883, 54
-237, 1.850, 54
-238, 4.283, 77
-239, 3.950, 79
-240, 2.333, 64
-241, 4.150, 75
-242, 2.350, 47
-243, 4.933, 86
-244, 2.900, 63
-245, 4.583, 85
-246, 3.833, 82
-247, 2.083, 57
-248, 4.367, 82
-249, 2.133, 67
-250, 4.350, 74
-251, 2.200, 54
-252, 4.450, 83
-253, 3.567, 73
-254, 4.500, 73
-255, 4.150, 88
-256, 3.817, 80
-257, 3.917, 71
-258, 4.450, 83
-259, 2.000, 56
-260, 4.283, 79
-261, 4.767, 78
-262, 4.533, 84
-263, 1.850, 58
-264, 4.250, 83
-265, 1.983, 43
-266, 2.250, 60
-267, 4.750, 75
-268, 4.117, 81
-269, 2.150, 46
-270, 4.417, 90
-271, 1.817, 46
-272, 4.467, 74

examples/assets/seaCreatures.json

@@ -0,0 +1,6 @@
+[
+    { "name": "Sammy", "type": "shark", "clams": 5 },
+    { "name": "Bubbles", "type": "orca", "clams": 3 },
+    { "name": "Splish", "type": "dolphin", "clams": 2 },
+    { "name": "Splash", "type": "dolphin", "clams": 2 }
+]

examples/async.ds

@@ -0,0 +1,19 @@
+create_random_numbers = (count <int>) <none> {
+	numbers = []
+
+	while length(numbers) < count {
+		numbers += random:integer()
+	}
+
+	output("Made " + length(numbers) + " numbers.")
+}
+
+output("This will print first.")
+
+async {
+	create_random_numbers(1000)
+ 	create_random_numbers(100)
+	create_random_numbers(10)
+}
+
+output("This will print last.")

examples/async_commands.ds

@@ -0,0 +1,17 @@
+async {
+	{
+		^echo 'Starting 1...'
+		^sleep 1
+		^echo 'Finished 1.'
+	}
+	{
+		^echo 'Starting 2...'
+		^sleep 2
+		^echo 'Finished 2.'
+	}
+	{
+		^echo 'Starting 3...'
+		^sleep 3
+		^echo 'Finished 3.'
+	}
+}

examples/async_download.ds

@@ -0,0 +1,20 @@
+cast_len = 0
+characters_len = 0
+episodes_len = 0
+
+async {	
+	{
+		cast = download("https://api.sampleapis.com/futurama/cast")
+		cast_len = length(from_json(cast))		
+	}
+	{
+		characters = download("https://api.sampleapis.com/futurama/characters")	
+		characters_len = length(from_json(characters))
+	}
+	{
+		episodes = download("https://api.sampleapis.com/futurama/episodes")
+		episodes_len = length(from_json(episodes))
+	}
+}
+
+output ([cast_len, characters_len, episodes_len])

examples/clue_solver.ds

@@ -0,0 +1,53 @@
+cards = {
+	rooms = ['Library' 'Kitchen' 'Conservatory']
+	suspects = ['White' 'Green' 'Scarlett']
+	weapons = ['Rope' 'Lead_Pipe' 'Knife']
+}
+
+is_ready_to_solve = (cards <map>) <bool> {
+	(length(cards:suspects) == 1)
+	&& (length(cards:rooms) == 1)
+	&& (length(cards:weapons) == 1)
+}
+
+remove_card = (cards <map>, opponent_card <str>) <none> {
+	cards:rooms -= opponent_card
+	cards:suspects -= opponent_card
+	cards:weapons -= opponent_card
+}
+
+make_guess = (cards <map>, current_room <str>) <none> {
+	if is_ready_to_solve(cards) {
+		output(
+			'I accuse '
+			+ cards:suspects:0
+			+ ' in the ' 
+			+ cards:rooms:0  
+			+ ' with the ' 
+			+ cards:weapons:0 
+			+ '!'
+		)
+	} else {
+		output(
+			'I question '
+			+ random:from(cards:suspects)
+			+ ' in the '
+			+ current_room
+			+ ' with the '
+			+ random:from(cards:weapons)
+			+ '.'
+		)
+	}
+}
+
+take_turn = (cards <map>, opponent_card <str>, current_room <str>) <none> {
+	remove_card(cards opponent_card)
+	make_guess(cards current_room)
+}
+
+take_turn(cards 'Rope' 'Kitchen')
+take_turn(cards 'Library' 'Kitchen')
+take_turn(cards 'Conservatory' 'Kitchen')
+take_turn(cards 'White' 'Kitchen')
+take_turn(cards 'Green' 'Kitchen')
+take_turn(cards 'Knife' 'Kitchen')

examples/collections.ds

@@ -1,114 +0,0 @@
-# transform
-
-list = (1, 2, 3);
-test = transform(list, 'input + 1');
-assert_equal((2, 3, 4), test);
-
-# string
-
-test = string(42);
-assert_equal("42", test);
-
-test = string(42.42);
-assert_equal("42.42", test);
-
-test = string(false);
-assert_equal("false", test);
-
-# create_table
-
-table = create_table(
-	("text", "num"),
-	(
-		("foo", 1),
-		("bar", 2)
-	)
-);
-
-# rows
-
-test = rows(table);
-assert_equal(
-	(
-		("foo", 1),
-		("bar", 2)
-	),
-	test
-);
-
-# insert
-
-test = insert(
-	table,
-	(
-		("foo", 1),
-		("bar", 2)
-	)
-);
-assert_equal(
-	create_table(
-		("text", "num"),
-		(
-			("foo", 1),
-			("bar", 2),
-			("foo", 1),
-			("bar", 2)
-		)
-	),
-	test
-);
-
-# select
-
-table = create_table(
-	("text", "number", "bool"),
-	(
-		("a", 1, true),
-		("b", 2, true),
-		("a", 3, true)
-	)
-);
-
-test_table = create_table(
-	("text", "bool"),
-	(
-		("a", true),
-		("b", true),
-		("a", true)
-	)
-);
-
-assert_equal(select(table, ("text", "bool")), test_table);
-
-test_table = create_table(
-	("text", "number", "bool"),
-	(
-		("a", 1, true),
-		("a", 3, true)
-	)
-);
-
-assert_equal(where(table, 'text == "a"'), test_table);
-
-# count
-
-table = create_table(
-	("text", "number", "bool"),
-	(
-		("a", 1, true),
-		("b", 2, true),
-		("a", 3, true)
-	)
-);
-test = count(table);
-assert_equal(3, test);
-
-test = count("123");
-assert_equal(3, test);
-
-test = count(1, 2, 3);
-assert_equal(3, test);
-
-map.x.z.y = 1;
-test = count(map);
-assert_equal(1, test);

examples/data_formats.ds

@@ -1,4 +0,0 @@
-dob = from_toml("1979-05-27T07:32:00-08:00");
-toml = to_toml(dob);
-
-assert_equal(toml, "1979-05-27T07:32:00-08:00");

examples/download.ds

@@ -0,0 +1,10 @@
+raw_data = download("https://api.sampleapis.com/futurama/cast")
+cast_data = from_json(raw_data)
+
+names = []
+
+for cast_member in cast_data {
+	names += cast_member:name
+}
+
+assert_equal("Billy West", names:0)

examples/fetch.ds

@@ -1,4 +0,0 @@
-raw_data = download("https://api.sampleapis.com/futurama/cast");
-data = from_json(raw_data);
-
-assert_equal("Billy West", data.0.name);

examples/fibonacci.ds

@@ -0,0 +1,9 @@
+fib = (i <int>) <int> {
+	if i <= 1 {
+		1
+	} else {
+		fib(i - 1) + fib(i - 2)
+	}
+}
+
+fib(8)

examples/fizzbuzz.ds

@@ -0,0 +1,18 @@
+count = 1
+
+while count <= 15 {
+	divides_by_3 = count % 3 == 0
+	divides_by_5 = count % 5 == 0
+
+	if divides_by_3 && divides_by_5 { 
+		output('fizzbuzz')
+	} else if divides_by_3 { 
+		output('fizz')
+	} else if divides_by_5 {
+		output('buzz')
+	} else { 
+		output(count)
+	}
+	
+	count += 1
+}

examples/guessing_game.ds

@@ -0,0 +1,26 @@
+# This is a Dust version of an example from the Rust Book.
+#
+# https://doc.rust-lang.org/book/ch02-00-guessing-game-tutorial.html
+
+output("Guess the number.")
+
+secret_number = int:random_range(0..=100)
+
+loop {
+	output("Please input your guess.")
+
+	input = io:stdin():expect("Failed to read line.")
+	guess = int:parse(input)
+
+	output("You guessed: " + guess)
+
+	match cmp(guess, secret_number) {
+		Ordering::Less -> output("Too small!")
+		Ordering::Greater -> output("Too big!")
+		Ordering::Equal -> {
+			output("You win!")
+			break
+		}
+	}
+}
+

examples/gui_rendering.ds

@@ -1,6 +0,0 @@
-# This will read a CSV file and display it as a line plot in a GUI window.
-read_file("examples/assets/faithful.csv")
-    -> from_csv(input)
-    -> rows(input)
-    -> transform(input, 'input.1')
-    -> plot(input);

examples/hello_world.ds

@@ -0,0 +1 @@
+output('Hello, world!')

examples/jq_data.ds

@@ -0,0 +1,12 @@
+data = json:parse(fs:read_file('examples/assets/jq_data.json'))
+
+new_data = []
+
+for commit_data in data as collection {	
+	new_data += {
+		message = commit_data:commit:message
+		name = commit_data:commit:committer:name
+	}
+}
+
+new_data

examples/list.ds

@@ -1,12 +0,0 @@
-# Lists are created by grouping items in partheses and separating them with
-# commas.
-
-numbers = (1, 2, 3);
-
-# To access the values in a list, use an integer as an index.
-
-x = numbers.0;
-y = numbers.1;
-z = numbers.2;
-
-assert_equal(x + y, z);

examples/random.ds

@@ -0,0 +1,13 @@
+stuff = [
+	random:integer()
+	random:integer()
+	random:integer()
+	random:float()
+	random:float()
+	random:float()
+	random:boolean()
+	random:boolean()
+	random:boolean()
+]
+
+random:from(stuff)

examples/scope.ds

@@ -1,9 +0,0 @@
-foo = "bar";
-func = 'foo';
-
-assert_equal("bar", func());
-
-foo = "xyz";
-
-assert_equal("xyz", func());
-

examples/sea_creatures.ds

@@ -0,0 +1,19 @@
+raw_data = fs:read_file('examples/assets/seaCreatures.json')
+sea_creatures = json:parse(raw_data)
+
+data = {
+	creatures = []
+	total_clams = 0
+	dolphin_clams = 0
+}
+
+for creature in sea_creatures {
+	data:creatures += creature:name
+	data:total_clams += creature:clams
+
+	if creature:type == 'dolphin' {
+		data:dolphin_clams += creature:clams
+	}
+}
+
+data

examples/table.ds

@@ -1,29 +0,0 @@
-table = create_table(
-	("text", "number", "bool"),
-	(
-		("a", 1, true),
-		("b", 2, true),
-		("a", 3, true)
-	)
-);
-
-test_table = create_table(
-	("text", "bool"),
-	(
-		("a", true),
-		("b", true),
-		("a", true)
-	)
-);
-
-assert_equal(select(table, ("text", "bool")), test_table);
-
-test_table = create_table(
-	("text", "number", "bool"),
-	(
-		("a", 1, true),
-		("a", 3, true)
-	)
-);
-
-assert_equal(where(table, 'text == "a"'), test_table);

examples/variables.ds

@@ -1,13 +0,0 @@
-# Dust is data-oriented, so variables are declared with minimal syntax. A
-# single character, the assignment operator (`=`), sets a variable.
-
-x = 1;
-y = "hello dust!";
-z = 42.0;
-list = (3, 2, x);
-big_list = (x, y, z, list);
-map.x = "foobar";
-function = '
-	message = "I am a function!";
-	output message;
-';

scripts/bench.fish

@@ -0,0 +1,35 @@
+#!/usr/bin/fish
+# This script is has the following prerequisites (aside from fish):
+# - hyperfine
+# - dust (can be installed with "cargo install dust-lang")
+# - jq
+# - nodejs
+# - nushell
+# - dielectron.json (can be downloaded from https://opendata.cern.ch/record/304)
+
+hyperfine \
+      --shell none \
+      --parameter-list data_path examples/assets/seaCreatures.json \
+      --warmup 3 \
+      "target/release/dust -c 'length(json:parse(fs:read_file(\"{data_path}\")))'" \
+      "jq 'length' {data_path}" \
+      "node --eval \"require('node:fs').readFile('{data_path}', (err, data)=>{console.log(JSON.parse(data).length)})\"" \
+      "nu -c 'open {data_path} | length'"
+
+hyperfine \
+      --shell none \
+      --parameter-list data_path examples/assets/jq_data.json \
+      --warmup 3 \
+      "target/release/dust -c 'length(json:parse(fs:read_file(\"{data_path}\")))'" \
+      "jq 'length' {data_path}" \
+      "node --eval \"require('node:fs').readFile('{data_path}', (err, data)=>{console.log(JSON.parse(data).length)})\"" \
+      "nu -c 'open {data_path} | length'"
+
+hyperfine \
+      --shell none \
+      --parameter-list data_path dielectron.json \
+      --warmup 3 \
+      "target/release/dust -c 'length(json:parse(fs:read_file(\"{data_path}\")))'" \
+      "jq 'length' {data_path}" \
+      "node --eval \"require('node:fs').readFile('{data_path}', (err, data)=>{console.log(JSON.parse(data).length)})\"" \
+      "nu -c 'open {data_path} | length'"

scripts/build_debug.fish

@@ -0,0 +1,8 @@
+#!/usr/bin/fish
+# Build the project in debug mode.
+
+cd tree-sitter-dust/
+tree-sitter generate --debug-build --no-bindings
+
+cd ..
+cargo build

scripts/build_release.fish

@@ -0,0 +1,8 @@
+#!/bin/fish
+# Build the project in release mode.
+
+cd tree-sitter-dust/
+tree-sitter generate --no-bindings
+
+cd ..
+cargo build --release

scripts/test.fish

@@ -0,0 +1,9 @@
+#!/usr/bin/fish
+# Build the project in debug mode.
+
+cd tree-sitter-dust/
+tree-sitter generate --debug-build --no-bindings
+tree-sitter test
+
+cd ..
+cargo test

src/abstract_tree/as.rs

@@ -0,0 +1,131 @@
+use serde::{Deserialize, Serialize};
+use tree_sitter::Node;
+
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Context, Expression, Format, List, SourcePosition, Type, Value,
+};
+
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub struct As {
+    expression: Expression,
+    r#type: Type,
+    position: SourcePosition,
+}
+
+impl AbstractTree for As {
+    fn from_syntax(node: Node, source: &str, context: &Context) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("as", node)?;
+
+        let expression_node = node.child(0).unwrap();
+        let expression = Expression::from_syntax(expression_node, source, context)?;
+
+        let type_node = node.child(2).unwrap();
+        let r#type = Type::from_syntax(type_node, source, context)?;
+
+        Ok(As {
+            expression,
+            r#type,
+            position: SourcePosition::from(node.range()),
+        })
+    }
+
+    fn expected_type(&self, _context: &Context) -> Result<Type, ValidationError> {
+        Ok(self.r#type.clone())
+    }
+
+    fn validate(&self, _source: &str, context: &Context) -> Result<(), ValidationError> {
+        let initial_type = self.expression.expected_type(context)?;
+
+        if self.r#type.accepts(&initial_type) {
+            return Ok(());
+        }
+
+        if let Type::ListOf(item_type) = &self.r#type {
+            match &initial_type {
+                Type::ListOf(expected_item_type) => {
+                    println!("{item_type} {expected_item_type}");
+
+                    if !item_type.accepts(&expected_item_type) {
+                        return Err(ValidationError::TypeCheck {
+                            expected: self.r#type.clone(),
+                            actual: initial_type.clone(),
+                            position: self.position,
+                        });
+                    }
+                }
+                Type::String => {
+                    if let Type::String = item_type.as_ref() {
+                    } else {
+                        return Err(ValidationError::ConversionImpossible {
+                            initial_type,
+                            target_type: self.r#type.clone(),
+                        });
+                    }
+                }
+
+                Type::Any => {
+                    // Do no validation when converting from "any" to a list.
+                    // This effectively defers to runtime behavior, potentially
+                    // causing a runtime error.
+                }
+                _ => {
+                    return Err(ValidationError::ConversionImpossible {
+                        initial_type,
+                        target_type: self.r#type.clone(),
+                    })
+                }
+            }
+        }
+
+        Ok(())
+    }
+
+    fn run(&self, source: &str, context: &Context) -> Result<Value, RuntimeError> {
+        let value = self.expression.run(source, context)?;
+        let converted_value = if self.r#type.accepts(&value.r#type()?) {
+            return Ok(value);
+        } else if let Type::ListOf(_) = self.r#type {
+            match value {
+                Value::List(list) => Value::List(list),
+                Value::String(string) => {
+                    let chars = string
+                        .chars()
+                        .map(|char| Value::String(char.to_string()))
+                        .collect();
+
+                    Value::List(List::with_items(chars))
+                }
+                _ => {
+                    return Err(RuntimeError::ConversionImpossible {
+                        from: value.r#type()?,
+                        to: self.r#type.clone(),
+                        position: self.position.clone(),
+                    });
+                }
+            }
+        } else if let Type::Integer = self.r#type {
+            match value {
+                Value::Integer(integer) => Value::Integer(integer),
+                Value::Float(float) => Value::Integer(float as i64),
+                _ => {
+                    return Err(RuntimeError::ConversionImpossible {
+                        from: value.r#type()?,
+                        to: self.r#type.clone(),
+                        position: self.position.clone(),
+                    })
+                }
+            }
+        } else {
+            todo!()
+        };
+
+        Ok(converted_value)
+    }
+}
+
+impl Format for As {
+    fn format(&self, _output: &mut String, _indent_level: u8) {
+        todo!()
+    }
+}

src/abstract_tree/assignment.rs

@@ -0,0 +1,186 @@
+use serde::{Deserialize, Serialize};
+
+use crate::{
+    context::Context,
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, AssignmentOperator, Format, Function, Identifier, SourcePosition, Statement,
+    SyntaxNode, Type, TypeSpecification, Value,
+};
+
+/// Variable assignment, including add-assign and subtract-assign operations.
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub struct Assignment {
+    identifier: Identifier,
+    type_specification: Option<TypeSpecification>,
+    operator: AssignmentOperator,
+    statement: Statement,
+    syntax_position: SourcePosition,
+}
+
+impl AbstractTree for Assignment {
+    fn from_syntax(
+        syntax_node: SyntaxNode,
+        source: &str,
+        context: &Context,
+    ) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("assignment", syntax_node)?;
+
+        let child_count = syntax_node.child_count();
+
+        let identifier_node = syntax_node.child(0).unwrap();
+        let identifier = Identifier::from_syntax(identifier_node, source, context)?;
+
+        let type_node = syntax_node.child(1).unwrap();
+        let type_specification = if type_node.kind() == "type_specification" {
+            Some(TypeSpecification::from_syntax(type_node, source, context)?)
+        } else {
+            None
+        };
+
+        let operator_node = syntax_node.child(child_count - 2).unwrap();
+        let operator = AssignmentOperator::from_syntax(operator_node, source, context)?;
+
+        let statement_node = syntax_node.child(child_count - 1).unwrap();
+        let statement = Statement::from_syntax(statement_node, source, context)?;
+
+        Ok(Assignment {
+            identifier,
+            type_specification,
+            operator,
+            statement,
+            syntax_position: syntax_node.range().into(),
+        })
+    }
+
+    fn validate(&self, source: &str, context: &Context) -> Result<(), ValidationError> {
+        if let AssignmentOperator::Equal = self.operator {
+            let r#type = if let Some(definition) = &self.type_specification {
+                definition.inner().clone()
+            } else {
+                self.statement.expected_type(context)?
+            };
+
+            log::info!("Setting type: {} <{}>", self.identifier, r#type);
+
+            context.set_type(self.identifier.clone(), r#type)?;
+        }
+
+        if let Some(type_specification) = &self.type_specification {
+            match self.operator {
+                AssignmentOperator::Equal => {
+                    let expected = type_specification.inner();
+                    let actual = self.statement.expected_type(context)?;
+
+                    if !expected.accepts(&actual) {
+                        return Err(ValidationError::TypeCheck {
+                            expected: expected.clone(),
+                            actual,
+                            position: self.syntax_position,
+                        });
+                    }
+                }
+                AssignmentOperator::PlusEqual => {
+                    if let Type::ListOf(expected) = type_specification.inner() {
+                        let actual = self.identifier.expected_type(context)?;
+
+                        if !expected.accepts(&actual) {
+                            return Err(ValidationError::TypeCheck {
+                                expected: expected.as_ref().clone(),
+                                actual,
+                                position: self.syntax_position,
+                            });
+                        }
+                    } else {
+                        let expected = type_specification.inner();
+                        let actual = self.identifier.expected_type(context)?;
+
+                        if !expected.accepts(&actual) {
+                            return Err(ValidationError::TypeCheck {
+                                expected: expected.clone(),
+                                actual,
+                                position: self.syntax_position,
+                            });
+                        }
+                    }
+                }
+                AssignmentOperator::MinusEqual => todo!(),
+            }
+        } else {
+            match self.operator {
+                AssignmentOperator::Equal => {}
+                AssignmentOperator::PlusEqual => {
+                    if let Type::ListOf(expected) = self.identifier.expected_type(context)? {
+                        let actual = self.statement.expected_type(context)?;
+
+                        if !expected.accepts(&actual) {
+                            return Err(ValidationError::TypeCheck {
+                                expected: expected.as_ref().clone(),
+                                actual,
+                                position: self.syntax_position,
+                            });
+                        }
+                    }
+                }
+                AssignmentOperator::MinusEqual => todo!(),
+            }
+        }
+
+        self.statement.validate(source, context)?;
+
+        Ok(())
+    }
+
+    fn run(&self, source: &str, context: &Context) -> Result<Value, RuntimeError> {
+        let right = self.statement.run(source, context)?;
+
+        let new_value = match self.operator {
+            AssignmentOperator::PlusEqual => {
+                let left = self.identifier.run(source, context)?;
+
+                left.add(right, self.syntax_position)?
+            }
+            AssignmentOperator::MinusEqual => {
+                if let Some(left) = context.get_value(&self.identifier)? {
+                    left.subtract(right, self.syntax_position)?
+                } else {
+                    return Err(RuntimeError::ValidationFailure(
+                        ValidationError::VariableIdentifierNotFound(self.identifier.clone()),
+                    ));
+                }
+            }
+            AssignmentOperator::Equal => right,
+        };
+
+        if let Value::Function(Function::ContextDefined(function_node)) = &new_value {
+            function_node
+                .context()
+                .set_value(self.identifier.clone(), new_value.clone())?;
+        }
+
+        log::info!("RUN assignment: {} = {}", self.identifier, new_value);
+
+        context.set_value(self.identifier.clone(), new_value)?;
+
+        Ok(Value::none())
+    }
+
+    fn expected_type(&self, _context: &Context) -> Result<Type, ValidationError> {
+        Ok(Type::None)
+    }
+}
+
+impl Format for Assignment {
+    fn format(&self, output: &mut String, indent_level: u8) {
+        self.identifier.format(output, indent_level);
+
+        if let Some(type_specification) = &self.type_specification {
+            type_specification.format(output, indent_level);
+        }
+
+        output.push(' ');
+        self.operator.format(output, indent_level);
+        output.push(' ');
+
+        self.statement.format(output, 0);
+    }
+}

src/abstract_tree/assignment_operator.rs

@@ -0,0 +1,62 @@
+use serde::{Deserialize, Serialize};
+
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Context, Format, SyntaxNode, Type, Value,
+};
+
+/// Operators that be used in an assignment statement.
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub enum AssignmentOperator {
+    Equal,
+    PlusEqual,
+    MinusEqual,
+}
+
+impl AbstractTree for AssignmentOperator {
+    fn from_syntax(
+        node: SyntaxNode,
+        _source: &str,
+        _context: &Context,
+    ) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("assignment_operator", node)?;
+
+        let operator_node = node.child(0).unwrap();
+        let operator = match operator_node.kind() {
+            "=" => AssignmentOperator::Equal,
+            "+=" => AssignmentOperator::PlusEqual,
+            "-=" => AssignmentOperator::MinusEqual,
+            _ => {
+                return Err(SyntaxError::UnexpectedSyntaxNode {
+                    expected: "=, += or -=".to_string(),
+                    actual: operator_node.kind().to_string(),
+                    position: node.range().into(),
+                })
+            }
+        };
+
+        Ok(operator)
+    }
+
+    fn run(&self, _source: &str, _context: &Context) -> Result<Value, RuntimeError> {
+        Ok(Value::none())
+    }
+
+    fn expected_type(&self, _context: &Context) -> Result<Type, ValidationError> {
+        Ok(Type::None)
+    }
+
+    fn validate(&self, _source: &str, _context: &Context) -> Result<(), ValidationError> {
+        Ok(())
+    }
+}
+
+impl Format for AssignmentOperator {
+    fn format(&self, output: &mut String, _indent_level: u8) {
+        match self {
+            AssignmentOperator::Equal => output.push('='),
+            AssignmentOperator::PlusEqual => output.push_str("+="),
+            AssignmentOperator::MinusEqual => output.push_str("-="),
+        }
+    }
+}

src/abstract_tree/block.rs

@@ -0,0 +1,170 @@
+use std::{
+    fmt::{self, Formatter},
+    sync::RwLock,
+};
+
+use rayon::prelude::*;
+use serde::{Deserialize, Serialize};
+
+use crate::{
+    error::{rw_lock_error::RwLockError, RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Context, Format, Statement, SyntaxNode, Type, Value,
+};
+
+/// Abstract representation of a block.
+///
+/// A block is almost identical to the root except that it must have curly
+/// braces and can optionally be asynchronous. A block evaluates to the value of
+/// its final statement but an async block will short-circuit if a statement
+/// results in an error. Note that this will be the first statement to encounter
+/// an error at runtime, not necessarilly the first statement as they are
+/// written.
+#[derive(Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub struct Block {
+    is_async: bool,
+    contains_return: bool,
+    statements: Vec<Statement>,
+}
+
+impl Block {
+    pub fn contains_return(&self) -> bool {
+        self.contains_return
+    }
+}
+
+impl AbstractTree for Block {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("block", node)?;
+
+        let first_child = node.child(0).unwrap();
+        let is_async = first_child.kind() == "async";
+        let mut contains_return = false;
+
+        let statement_count = if is_async {
+            node.child_count() - 3
+        } else {
+            node.child_count() - 2
+        };
+        let mut statements = Vec::with_capacity(statement_count);
+        let block_context = Context::with_variables_from(context)?;
+
+        for index in 1..node.child_count() - 1 {
+            let child_node = node.child(index).unwrap();
+
+            if child_node.kind() == "statement" {
+                let statement = Statement::from_syntax(child_node, source, &block_context)?;
+
+                if statement.is_return() {
+                    contains_return = true;
+                }
+
+                statements.push(statement);
+            }
+        }
+
+        Ok(Block {
+            is_async,
+            contains_return,
+            statements,
+        })
+    }
+
+    fn validate(&self, _source: &str, _context: &Context) -> Result<(), ValidationError> {
+        for statement in &self.statements {
+            statement.validate(_source, _context)?;
+        }
+
+        Ok(())
+    }
+
+    fn run(&self, _source: &str, _context: &Context) -> Result<Value, RuntimeError> {
+        if self.is_async {
+            let statements = &self.statements;
+            let final_result = RwLock::new(Ok(Value::none()));
+
+            statements
+                .into_par_iter()
+                .enumerate()
+                .find_map_first(|(index, statement)| {
+                    let result = statement.run(_source, _context);
+                    let should_return = if self.contains_return {
+                        statement.is_return()
+                    } else {
+                        index == statements.len() - 1
+                    };
+
+                    if should_return {
+                        let get_write_lock = final_result.write();
+
+                        match get_write_lock {
+                            Ok(mut final_result) => {
+                                *final_result = result;
+                                None
+                            }
+                            Err(_error) => Some(Err(RuntimeError::RwLock(RwLockError))),
+                        }
+                    } else {
+                        None
+                    }
+                })
+                .unwrap_or(final_result.into_inner().map_err(|_| RwLockError)?)
+        } else {
+            for (index, statement) in self.statements.iter().enumerate() {
+                if statement.is_return() {
+                    return statement.run(_source, _context);
+                }
+
+                if index == self.statements.len() - 1 {
+                    return statement.run(_source, _context);
+                }
+            }
+
+            Ok(Value::none())
+        }
+    }
+
+    fn expected_type(&self, _context: &Context) -> Result<Type, ValidationError> {
+        for (index, statement) in self.statements.iter().enumerate() {
+            if statement.is_return() {
+                return statement.expected_type(_context);
+            }
+
+            if index == self.statements.len() - 1 {
+                return statement.expected_type(_context);
+            }
+        }
+
+        Ok(Type::None)
+    }
+}
+
+impl Format for Block {
+    fn format(&self, output: &mut String, indent_level: u8) {
+        if self.is_async {
+            output.push_str("async {\n");
+        } else {
+            output.push_str("{\n");
+        }
+
+        for (index, statement) in self.statements.iter().enumerate() {
+            if index > 0 {
+                output.push('\n');
+            }
+
+            statement.format(output, indent_level + 1);
+        }
+
+        output.push('\n');
+        Block::indent(output, indent_level);
+        output.push('}');
+    }
+}
+
+impl fmt::Debug for Block {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        f.debug_struct("Block")
+            .field("is_async", &self.is_async)
+            .field("statements", &self.statements)
+            .finish()
+    }
+}

src/abstract_tree/command.rs

@@ -0,0 +1,76 @@
+use std::process::{self, Stdio};
+
+use serde::{Deserialize, Serialize};
+use tree_sitter::Node as SyntaxNode;
+
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Context, Format, Type, Value,
+};
+
+/// An external program invokation.
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub struct Command {
+    command_text: String,
+    command_arguments: Vec<String>,
+}
+
+impl AbstractTree for Command {
+    fn from_syntax(
+        node: SyntaxNode,
+        source: &str,
+        _context: &Context,
+    ) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("command", node)?;
+
+        let command_text_node = node.child(1).unwrap();
+        let command_text = source[command_text_node.byte_range()].to_string();
+
+        let mut command_arguments = Vec::new();
+
+        for index in 2..node.child_count() {
+            let text_node = node.child(index).unwrap();
+            let mut text = source[text_node.byte_range()].to_string();
+
+            if (text.starts_with('\'') && text.ends_with('\''))
+                || (text.starts_with('"') && text.ends_with('"'))
+                || (text.starts_with('`') && text.ends_with('`'))
+            {
+                text = text[1..text.len() - 1].to_string();
+            }
+
+            command_arguments.push(text);
+        }
+
+        Ok(Command {
+            command_text,
+            command_arguments,
+        })
+    }
+
+    fn expected_type(&self, _context: &Context) -> Result<Type, ValidationError> {
+        Ok(Type::String)
+    }
+
+    fn validate(&self, _source: &str, _context: &Context) -> Result<(), ValidationError> {
+        Ok(())
+    }
+
+    fn run(&self, _source: &str, _context: &Context) -> Result<Value, RuntimeError> {
+        let output = process::Command::new(&self.command_text)
+            .args(&self.command_arguments)
+            .stdout(Stdio::piped())
+            .stderr(Stdio::inherit())
+            .spawn()?
+            .wait_with_output()?
+            .stdout;
+
+        Ok(Value::String(String::from_utf8(output)?))
+    }
+}
+
+impl Format for Command {
+    fn format(&self, _output: &mut String, _indent_level: u8) {
+        todo!()
+    }
+}

src/abstract_tree/enum_defintion.rs

@@ -0,0 +1,92 @@
+use serde::{Deserialize, Serialize};
+use tree_sitter::Node as SyntaxNode;
+
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Context, EnumInstance, Format, Identifier, Type, TypeDefinition, Value,
+};
+
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub struct EnumDefinition {
+    identifier: Identifier,
+    variants: Vec<(Identifier, Vec<Type>)>,
+}
+
+impl EnumDefinition {
+    pub fn new(identifier: Identifier, variants: Vec<(Identifier, Vec<Type>)>) -> Self {
+        Self {
+            identifier,
+            variants,
+        }
+    }
+
+    pub fn instantiate(&self, variant: Identifier, content: Option<Value>) -> EnumInstance {
+        EnumInstance::new(self.identifier.clone(), variant, content)
+    }
+
+    pub fn identifier(&self) -> &Identifier {
+        &self.identifier
+    }
+
+    pub fn variants(&self) -> &Vec<(Identifier, Vec<Type>)> {
+        &self.variants
+    }
+}
+
+impl AbstractTree for EnumDefinition {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("enum_definition", node)?;
+
+        let identifier_node = node.child(1).unwrap();
+        let identifier = Identifier::from_syntax(identifier_node, source, context)?;
+
+        let mut variants = Vec::new();
+        let mut current_identifier: Option<Identifier> = None;
+        let mut types = Vec::new();
+
+        for index in 3..node.child_count() - 1 {
+            let child = node.child(index).unwrap();
+
+            if child.kind() == "identifier" {
+                if let Some(identifier) = &current_identifier {
+                    variants.push((identifier.clone(), types));
+                }
+
+                current_identifier = Some(Identifier::from_syntax(child, source, context)?);
+                types = Vec::new();
+            }
+
+            if child.kind() == "type" {
+                let r#type = Type::from_syntax(child, source, context)?;
+
+                types.push(r#type);
+            }
+        }
+
+        Ok(EnumDefinition {
+            identifier,
+            variants,
+        })
+    }
+
+    fn expected_type(&self, _context: &Context) -> Result<Type, ValidationError> {
+        Ok(Type::None)
+    }
+
+    fn validate(&self, _source: &str, context: &Context) -> Result<(), ValidationError> {
+        context.set_definition(self.identifier.clone(), TypeDefinition::Enum(self.clone()))?;
+        self.identifier.validate(_source, context)?;
+
+        Ok(())
+    }
+
+    fn run(&self, _source: &str, _context: &Context) -> Result<Value, RuntimeError> {
+        Ok(Value::none())
+    }
+}
+
+impl Format for EnumDefinition {
+    fn format(&self, _output: &mut String, _indent_level: u8) {
+        todo!()
+    }
+}

src/abstract_tree/enum_pattern.rs

@@ -0,0 +1,70 @@
+use serde::{Deserialize, Serialize};
+use tree_sitter::Node as SyntaxNode;
+
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Context, Format, Identifier, Type, Value,
+};
+
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub struct EnumPattern {
+    name: Identifier,
+    variant: Identifier,
+    inner_identifier: Option<Identifier>,
+}
+
+impl EnumPattern {
+    pub fn name(&self) -> &Identifier {
+        &self.name
+    }
+
+    pub fn variant(&self) -> &Identifier {
+        &self.variant
+    }
+
+    pub fn inner_identifier(&self) -> &Option<Identifier> {
+        &self.inner_identifier
+    }
+}
+
+impl AbstractTree for EnumPattern {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("enum_pattern", node)?;
+
+        let enum_name_node = node.child(0).unwrap();
+        let name = Identifier::from_syntax(enum_name_node, source, context)?;
+
+        let enum_variant_node = node.child(2).unwrap();
+        let variant = Identifier::from_syntax(enum_variant_node, source, context)?;
+
+        let inner_identifier = if let Some(child) = node.child(4) {
+            Some(Identifier::from_syntax(child, source, context)?)
+        } else {
+            None
+        };
+
+        Ok(EnumPattern {
+            name,
+            variant,
+            inner_identifier,
+        })
+    }
+
+    fn expected_type(&self, _context: &Context) -> Result<Type, ValidationError> {
+        Ok(Type::None)
+    }
+
+    fn validate(&self, _source: &str, _context: &Context) -> Result<(), ValidationError> {
+        Ok(())
+    }
+
+    fn run(&self, _source: &str, _context: &Context) -> Result<Value, RuntimeError> {
+        Ok(Value::none())
+    }
+}
+
+impl Format for EnumPattern {
+    fn format(&self, _output: &mut String, _indent_level: u8) {
+        todo!()
+    }
+}

src/abstract_tree/expression.rs

@@ -0,0 +1,120 @@
+use serde::{Deserialize, Serialize};
+
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    value_node::ValueNode,
+    AbstractTree, As, Command, Context, Format, FunctionCall, Identifier, Index, Logic, Math,
+    SyntaxNode, Type, Value,
+};
+
+/// Abstract representation of an expression statement.
+///
+/// Unlike statements, which can involve complex logic, an expression is
+/// expected to evaluate to a value. However, an expression can still contain
+/// nested statements and may evaluate to an empty value.
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub enum Expression {
+    Value(ValueNode),
+    Identifier(Identifier),
+    Index(Box<Index>),
+    Math(Box<Math>),
+    Logic(Box<Logic>),
+    FunctionCall(Box<FunctionCall>),
+    Command(Command),
+    As(Box<As>),
+}
+
+impl AbstractTree for Expression {
+    fn from_syntax(
+        node: SyntaxNode,
+        source: &str,
+        _context: &Context,
+    ) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("expression", node)?;
+
+        let child = if node.child(0).unwrap().is_named() {
+            node.child(0).unwrap()
+        } else {
+            node.child(1).unwrap()
+        };
+
+        let expression = match child.kind() {
+            "as" => Expression::As(Box::new(As::from_syntax(child, source, _context)?)),
+            "value" => Expression::Value(ValueNode::from_syntax(child, source, _context)?),
+            "identifier" => {
+                Expression::Identifier(Identifier::from_syntax(child, source, _context)?)
+            }
+            "index" => Expression::Index(Box::new(Index::from_syntax(child, source, _context)?)),
+            "math" => Expression::Math(Box::new(Math::from_syntax(child, source, _context)?)),
+            "logic" => Expression::Logic(Box::new(Logic::from_syntax(child, source, _context)?)),
+            "function_call" => Expression::FunctionCall(Box::new(FunctionCall::from_syntax(
+                child, source, _context,
+            )?)),
+            "command" => Expression::Command(Command::from_syntax(child, source, _context)?),
+            _ => {
+                return Err(SyntaxError::UnexpectedSyntaxNode {
+                    expected: "value, identifier, index, math, logic, function call, as or command"
+                        .to_string(),
+                    actual: child.kind().to_string(),
+                    position: node.range().into(),
+                })
+            }
+        };
+
+        Ok(expression)
+    }
+
+    fn expected_type(&self, _context: &Context) -> Result<Type, ValidationError> {
+        match self {
+            Expression::Value(value_node) => value_node.expected_type(_context),
+            Expression::Identifier(identifier) => identifier.expected_type(_context),
+            Expression::Math(math) => math.expected_type(_context),
+            Expression::Logic(logic) => logic.expected_type(_context),
+            Expression::FunctionCall(function_call) => function_call.expected_type(_context),
+            Expression::Index(index) => index.expected_type(_context),
+            Expression::Command(command) => command.expected_type(_context),
+            Expression::As(r#as) => r#as.expected_type(_context),
+        }
+    }
+
+    fn validate(&self, _source: &str, _context: &Context) -> Result<(), ValidationError> {
+        match self {
+            Expression::Value(value_node) => value_node.validate(_source, _context),
+            Expression::Identifier(identifier) => identifier.validate(_source, _context),
+            Expression::Math(math) => math.validate(_source, _context),
+            Expression::Logic(logic) => logic.validate(_source, _context),
+            Expression::FunctionCall(function_call) => function_call.validate(_source, _context),
+            Expression::Index(index) => index.validate(_source, _context),
+            Expression::Command(command) => command.validate(_source, _context),
+            Expression::As(r#as) => r#as.validate(_source, _context),
+        }
+    }
+
+    fn run(&self, _source: &str, _context: &Context) -> Result<Value, RuntimeError> {
+        match self {
+            Expression::Value(value_node) => value_node.run(_source, _context),
+            Expression::Identifier(identifier) => identifier.run(_source, _context),
+            Expression::Math(math) => math.run(_source, _context),
+            Expression::Logic(logic) => logic.run(_source, _context),
+            Expression::FunctionCall(function_call) => function_call.run(_source, _context),
+            Expression::Index(index) => index.run(_source, _context),
+            Expression::Command(command) => command.run(_source, _context),
+            Expression::As(r#as) => r#as.run(_source, _context),
+        }
+    }
+}
+
+impl Format for Expression {
+    fn format(&self, _output: &mut String, _indent_level: u8) {
+        match self {
+            Expression::Value(value_node) => value_node.format(_output, _indent_level),
+            Expression::Identifier(identifier) => identifier.format(_output, _indent_level),
+            Expression::Math(math) => math.format(_output, _indent_level),
+            Expression::Logic(logic) => logic.format(_output, _indent_level),
+            Expression::FunctionCall(function_call) => function_call.format(_output, _indent_level),
+            Expression::Index(index) => index.format(_output, _indent_level),
+            Expression::Command(command) => command.format(_output, _indent_level),
+            Expression::As(r#as) => r#as.format(_output, _indent_level),
+        }
+    }
+}

src/abstract_tree/for.rs

@@ -0,0 +1,153 @@
+use rayon::prelude::*;
+use serde::{Deserialize, Serialize};
+
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Block, Context, Expression, Format, Identifier, SourcePosition, SyntaxNode, Type,
+    Value,
+};
+
+/// Abstract representation of a for loop statement.
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub struct For {
+    is_async: bool,
+    item_id: Identifier,
+    collection: Expression,
+    block: Block,
+    source_position: SourcePosition,
+
+    #[serde(skip)]
+    context: Context,
+}
+
+impl AbstractTree for For {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("for", node)?;
+
+        let for_node = node.child(0).unwrap();
+        let is_async = match for_node.kind() {
+            "for" => false,
+            "async for" => true,
+            _ => {
+                return Err(SyntaxError::UnexpectedSyntaxNode {
+                    expected: "for or async for".to_string(),
+                    actual: for_node.kind().to_string(),
+                    position: node.range().into(),
+                })
+            }
+        };
+
+        let identifier_node = node.child(1).unwrap();
+        let identifier = Identifier::from_syntax(identifier_node, source, context)?;
+
+        let expression_node = node.child(3).unwrap();
+        let expression = Expression::from_syntax(expression_node, source, context)?;
+
+        let loop_context = Context::with_variables_from(context)?;
+
+        let item_node = node.child(4).unwrap();
+        let item = Block::from_syntax(item_node, source, &loop_context)?;
+
+        Ok(For {
+            is_async,
+            item_id: identifier,
+            collection: expression,
+            block: item,
+            source_position: SourcePosition::from(node.range()),
+            context: loop_context,
+        })
+    }
+
+    fn expected_type(&self, _context: &Context) -> Result<Type, ValidationError> {
+        Ok(Type::None)
+    }
+
+    fn validate(&self, _source: &str, context: &Context) -> Result<(), ValidationError> {
+        self.collection.validate(_source, context)?;
+
+        let collection_type = self.collection.expected_type(context)?;
+        let item_type = match collection_type {
+            Type::Any => Type::Any,
+            Type::Collection => Type::Any,
+            Type::List => Type::Any,
+            Type::ListOf(_) => todo!(),
+            Type::ListExact(_) => todo!(),
+            Type::Map(_) => todo!(),
+            Type::String => todo!(),
+            Type::Range => todo!(),
+            _ => {
+                return Err(ValidationError::TypeCheck {
+                    expected: Type::Collection,
+                    actual: collection_type,
+                    position: self.source_position,
+                });
+            }
+        };
+        let key = self.item_id.clone();
+
+        self.context.inherit_all_from(context)?;
+        self.context.set_type(key, item_type)?;
+        self.item_id.validate(_source, &self.context)?;
+        self.block.validate(_source, &self.context)
+    }
+
+    fn run(&self, source: &str, context: &Context) -> Result<Value, RuntimeError> {
+        self.context.inherit_all_from(context)?;
+
+        let expression_run = self.collection.run(source, context)?;
+        let key = &self.item_id;
+
+        if let Value::Range(range) = expression_run {
+            if self.is_async {
+                range.into_par_iter().try_for_each(|integer| {
+                    self.context.add_allowance(key)?;
+                    self.context
+                        .set_value(key.clone(), Value::Integer(integer))?;
+                    self.block.run(source, &self.context).map(|_value| ())
+                })?;
+            } else {
+                for i in range {
+                    self.context.add_allowance(key)?;
+                    self.context.set_value(key.clone(), Value::Integer(i))?;
+                    self.block.run(source, &self.context)?;
+                }
+            }
+
+            return Ok(Value::none());
+        }
+
+        if let Value::List(list) = &expression_run {
+            if self.is_async {
+                list.items()?.par_iter().try_for_each(|value| {
+                    self.context.add_allowance(key)?;
+                    self.context.set_value(key.clone(), value.clone())?;
+                    self.block.run(source, &self.context).map(|_value| ())
+                })?;
+            } else {
+                for value in list.items()?.iter() {
+                    self.context.add_allowance(key)?;
+                    self.context.set_value(key.clone(), value.clone())?;
+                    self.block.run(source, &self.context)?;
+                }
+            }
+        }
+
+        Ok(Value::none())
+    }
+}
+
+impl Format for For {
+    fn format(&self, output: &mut String, indent_level: u8) {
+        if self.is_async {
+            output.push_str("async for ");
+        } else {
+            output.push_str("for ");
+        }
+
+        self.item_id.format(output, indent_level);
+        output.push_str(" in ");
+        self.collection.format(output, indent_level);
+        output.push(' ');
+        self.block.format(output, indent_level);
+    }
+}

src/abstract_tree/function_call.rs

@@ -0,0 +1,202 @@
+use serde::{Deserialize, Serialize};
+
+use crate::{
+    built_in_functions::Callable,
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Context, Expression, Format, Function, FunctionExpression, SourcePosition,
+    SyntaxNode, Type, Value,
+};
+
+/// A function being invoked and the arguments it is being passed.
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub struct FunctionCall {
+    function_expression: FunctionExpression,
+    arguments: Vec<Expression>,
+    syntax_position: SourcePosition,
+}
+
+impl FunctionCall {
+    /// Returns a new FunctionCall.
+    pub fn new(
+        function_expression: FunctionExpression,
+        arguments: Vec<Expression>,
+        syntax_position: SourcePosition,
+    ) -> Self {
+        Self {
+            function_expression,
+            arguments,
+            syntax_position,
+        }
+    }
+}
+
+impl AbstractTree for FunctionCall {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("function_call", node)?;
+
+        let function_node = node.child(0).unwrap();
+        let function_expression = FunctionExpression::from_syntax(function_node, source, context)?;
+
+        let mut arguments = Vec::new();
+
+        for index in 2..node.child_count() - 1 {
+            let child = node.child(index).unwrap();
+
+            if child.is_named() {
+                let expression = Expression::from_syntax(child, source, context)?;
+
+                arguments.push(expression);
+            }
+        }
+
+        Ok(FunctionCall {
+            function_expression,
+            arguments,
+            syntax_position: node.range().into(),
+        })
+    }
+
+    fn expected_type(&self, context: &Context) -> Result<Type, ValidationError> {
+        match &self.function_expression {
+            FunctionExpression::Identifier(identifier) => {
+                let identifier_type = identifier.expected_type(context)?;
+
+                if let Type::Function {
+                    parameter_types: _,
+                    return_type,
+                } = &identifier_type
+                {
+                    Ok(*return_type.clone())
+                } else {
+                    Ok(identifier_type)
+                }
+            }
+            FunctionExpression::FunctionCall(function_call) => function_call.expected_type(context),
+            FunctionExpression::Value(value_node) => {
+                let value_type = value_node.expected_type(context)?;
+
+                if let Type::Function { return_type, .. } = value_type {
+                    Ok(*return_type)
+                } else {
+                    Ok(value_type)
+                }
+            }
+            FunctionExpression::Index(index) => {
+                let index_type = index.expected_type(context)?;
+
+                if let Type::Function { return_type, .. } = index_type {
+                    Ok(*return_type)
+                } else {
+                    Ok(index_type)
+                }
+            }
+        }
+    }
+
+    fn validate(&self, _source: &str, context: &Context) -> Result<(), ValidationError> {
+        self.function_expression.validate(_source, context)?;
+
+        let function_expression_type = self.function_expression.expected_type(context)?;
+
+        let parameter_types = if let Type::Function {
+            parameter_types, ..
+        } = function_expression_type
+        {
+            parameter_types
+        } else {
+            return Err(ValidationError::TypeCheckExpectedFunction {
+                actual: function_expression_type,
+                position: self.syntax_position,
+            });
+        };
+
+        if self.arguments.len() != parameter_types.len() {
+            return Err(ValidationError::ExpectedFunctionArgumentAmount {
+                expected: parameter_types.len(),
+                actual: self.arguments.len(),
+                position: self.syntax_position,
+            });
+        }
+
+        for (index, expression) in self.arguments.iter().enumerate() {
+            expression.validate(_source, context)?;
+
+            if let Some(expected) = parameter_types.get(index) {
+                let actual = expression.expected_type(context)?;
+
+                if !expected.accepts(&actual) {
+                    return Err(ValidationError::TypeCheck {
+                        expected: expected.clone(),
+                        actual,
+                        position: self.syntax_position,
+                    });
+                }
+            }
+        }
+
+        Ok(())
+    }
+
+    fn run(&self, source: &str, context: &Context) -> Result<Value, RuntimeError> {
+        let value = match &self.function_expression {
+            FunctionExpression::Identifier(identifier) => {
+                if let Some(value) = context.get_value(identifier)? {
+                    value.clone()
+                } else {
+                    return Err(RuntimeError::ValidationFailure(
+                        ValidationError::VariableIdentifierNotFound(identifier.clone()),
+                    ));
+                }
+            }
+            FunctionExpression::FunctionCall(function_call) => {
+                function_call.run(source, context)?
+            }
+            FunctionExpression::Value(value_node) => value_node.run(source, context)?,
+            FunctionExpression::Index(index) => index.run(source, context)?,
+        };
+        let function = value.as_function()?;
+
+        match function {
+            Function::BuiltIn(built_in_function) => {
+                let mut arguments = Vec::with_capacity(self.arguments.len());
+
+                for expression in &self.arguments {
+                    let value = expression.run(source, context)?;
+
+                    arguments.push(value);
+                }
+
+                built_in_function.call(&arguments, source, context)
+            }
+            Function::ContextDefined(function_node) => {
+                let call_context = Context::with_variables_from(function_node.context())?;
+
+                call_context.inherit_from(context)?;
+
+                let parameter_expression_pairs =
+                    function_node.parameters().iter().zip(self.arguments.iter());
+
+                for (identifier, expression) in parameter_expression_pairs {
+                    let value = expression.run(source, context)?;
+
+                    call_context.set_value(identifier.clone(), value)?;
+                }
+
+                function_node.body().run(source, &call_context)
+            }
+        }
+    }
+}
+
+impl Format for FunctionCall {
+    fn format(&self, output: &mut String, indent_level: u8) {
+        self.function_expression.format(output, indent_level);
+        output.push('(');
+
+        for expression in &self.arguments {
+            expression.format(output, indent_level);
+        }
+
+        output.push(')');
+    }
+}

src/abstract_tree/function_expression.rs

@@ -0,0 +1,91 @@
+use serde::{Deserialize, Serialize};
+
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Context, Format, FunctionCall, Identifier, Index, SyntaxNode, Type, Value,
+    ValueNode,
+};
+
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub enum FunctionExpression {
+    Identifier(Identifier),
+    FunctionCall(Box<FunctionCall>),
+    Value(ValueNode),
+    Index(Index),
+}
+
+impl AbstractTree for FunctionExpression {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("function_expression", node)?;
+
+        let first_child = node.child(0).unwrap();
+        let child = if first_child.is_named() {
+            first_child
+        } else {
+            node.child(1).unwrap()
+        };
+
+        let function_expression = match child.kind() {
+            "identifier" => {
+                FunctionExpression::Identifier(Identifier::from_syntax(child, source, context)?)
+            }
+
+            "function_call" => FunctionExpression::FunctionCall(Box::new(
+                FunctionCall::from_syntax(child, source, context)?,
+            )),
+            "value" => FunctionExpression::Value(ValueNode::from_syntax(child, source, context)?),
+            "index" => FunctionExpression::Index(Index::from_syntax(child, source, context)?),
+            _ => {
+                return Err(SyntaxError::UnexpectedSyntaxNode {
+                    expected: "identifier, function call, value or index".to_string(),
+                    actual: child.kind().to_string(),
+                    position: node.range().into(),
+                })
+            }
+        };
+
+        Ok(function_expression)
+    }
+
+    fn expected_type(&self, context: &Context) -> Result<Type, ValidationError> {
+        match self {
+            FunctionExpression::Identifier(identifier) => identifier.expected_type(context),
+            FunctionExpression::FunctionCall(function_call) => function_call.expected_type(context),
+            FunctionExpression::Value(value_node) => value_node.expected_type(context),
+            FunctionExpression::Index(index) => index.expected_type(context),
+        }
+    }
+
+    fn validate(&self, _source: &str, _context: &Context) -> Result<(), ValidationError> {
+        match self {
+            FunctionExpression::Identifier(identifier) => identifier.validate(_source, _context),
+            FunctionExpression::FunctionCall(function_call) => {
+                function_call.validate(_source, _context)
+            }
+            FunctionExpression::Value(value_node) => value_node.validate(_source, _context),
+            FunctionExpression::Index(index) => index.validate(_source, _context),
+        }
+    }
+
+    fn run(&self, source: &str, context: &Context) -> Result<Value, RuntimeError> {
+        match self {
+            FunctionExpression::Identifier(identifier) => identifier.run(source, context),
+            FunctionExpression::FunctionCall(function_call) => function_call.run(source, context),
+            FunctionExpression::Value(value_node) => value_node.run(source, context),
+            FunctionExpression::Index(index) => index.run(source, context),
+        }
+    }
+}
+
+impl Format for FunctionExpression {
+    fn format(&self, output: &mut String, indent_level: u8) {
+        match self {
+            FunctionExpression::Value(value_node) => value_node.format(output, indent_level),
+            FunctionExpression::Identifier(identifier) => identifier.format(output, indent_level),
+            FunctionExpression::FunctionCall(function_call) => {
+                function_call.format(output, indent_level)
+            }
+            FunctionExpression::Index(index) => index.format(output, indent_level),
+        }
+    }
+}

src/abstract_tree/function_node.rs

@@ -0,0 +1,179 @@
+use std::fmt::{self, Display, Formatter};
+
+use serde::{Deserialize, Serialize};
+
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Block, Context, Format, Function, Identifier, SourcePosition, SyntaxNode, Type,
+    TypeSpecification, Value,
+};
+
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq, PartialOrd, Ord)]
+pub struct FunctionNode {
+    parameters: Vec<Identifier>,
+    body: Block,
+    r#type: Type,
+    syntax_position: SourcePosition,
+
+    #[serde(skip)]
+    context: Context,
+}
+
+impl FunctionNode {
+    pub fn parameters(&self) -> &Vec<Identifier> {
+        &self.parameters
+    }
+
+    pub fn body(&self) -> &Block {
+        &self.body
+    }
+
+    pub fn r#type(&self) -> &Type {
+        &self.r#type
+    }
+
+    pub fn syntax_position(&self) -> &SourcePosition {
+        &self.syntax_position
+    }
+
+    pub fn context(&self) -> &Context {
+        &self.context
+    }
+
+    pub fn return_type(&self) -> &Type {
+        match &self.r#type {
+            Type::Function {
+                parameter_types: _,
+                return_type,
+            } => return_type.as_ref(),
+            _ => &Type::None,
+        }
+    }
+}
+
+impl AbstractTree for FunctionNode {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("function", node)?;
+
+        let child_count = node.child_count();
+        let mut parameters = Vec::new();
+        let mut parameter_types = Vec::new();
+
+        for index in 1..child_count - 3 {
+            let child = node.child(index).unwrap();
+
+            if child.kind() == "identifier" {
+                let identifier = Identifier::from_syntax(child, source, context)?;
+
+                parameters.push(identifier);
+            }
+
+            if child.kind() == "type_specification" {
+                let type_specification = TypeSpecification::from_syntax(child, source, context)?;
+
+                parameter_types.push(type_specification.take_inner());
+            }
+        }
+
+        let return_type_node = node.child(child_count - 2).unwrap();
+        let return_type = TypeSpecification::from_syntax(return_type_node, source, context)?;
+
+        let function_context = Context::with_variables_from(context)?;
+
+        let body_node = node.child(child_count - 1).unwrap();
+        let body = Block::from_syntax(body_node, source, &function_context)?;
+
+        let r#type = Type::function(parameter_types, return_type.take_inner());
+        let syntax_position = node.range().into();
+
+        Ok(FunctionNode {
+            parameters,
+            body,
+            r#type,
+            syntax_position,
+            context: function_context,
+        })
+    }
+
+    fn expected_type(&self, _context: &Context) -> Result<Type, ValidationError> {
+        Ok(self.r#type().clone())
+    }
+
+    fn validate(&self, source: &str, context: &Context) -> Result<(), ValidationError> {
+        if let Type::Function {
+            parameter_types,
+            return_type,
+        } = &self.r#type
+        {
+            self.context.inherit_from(context)?;
+
+            for (parameter, r#type) in self.parameters.iter().zip(parameter_types.iter()) {
+                self.context.set_type(parameter.clone(), r#type.clone())?;
+            }
+
+            let actual = self.body.expected_type(&self.context)?;
+
+            if !return_type.accepts(&actual) {
+                return Err(ValidationError::TypeCheck {
+                    expected: return_type.as_ref().clone(),
+                    actual,
+                    position: self.syntax_position,
+                });
+            }
+
+            self.body.validate(source, &self.context)?;
+
+            Ok(())
+        } else {
+            Err(ValidationError::TypeCheckExpectedFunction {
+                actual: self.r#type.clone(),
+                position: self.syntax_position,
+            })
+        }
+    }
+
+    fn run(&self, _source: &str, context: &Context) -> Result<Value, RuntimeError> {
+        self.context.inherit_from(context)?;
+
+        let self_as_value = Value::Function(Function::ContextDefined(self.clone()));
+
+        Ok(self_as_value)
+    }
+}
+
+impl Format for FunctionNode {
+    fn format(&self, output: &mut String, indent_level: u8) {
+        let (parameter_types, return_type) = if let Type::Function {
+            parameter_types,
+            return_type,
+        } = &self.r#type
+        {
+            (parameter_types, return_type)
+        } else {
+            return;
+        };
+
+        output.push('(');
+
+        for (identifier, r#type) in self.parameters.iter().zip(parameter_types.iter()) {
+            identifier.format(output, indent_level);
+            output.push_str(" <");
+            r#type.format(output, indent_level);
+            output.push('>');
+        }
+
+        output.push_str(") <");
+        return_type.format(output, indent_level);
+        output.push_str("> ");
+        self.body.format(output, indent_level);
+    }
+}
+
+impl Display for FunctionNode {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        let mut string = String::new();
+
+        self.format(&mut string, 0);
+        f.write_str(&string)
+    }
+}

src/abstract_tree/identifier.rs

@@ -0,0 +1,167 @@
+use std::{
+    fmt::{self, Display, Formatter},
+    sync::Arc,
+};
+
+use serde::{de::Visitor, Deserialize, Serialize};
+
+use crate::{
+    built_in_identifiers::all_built_in_identifiers,
+    built_in_values::all_built_in_values,
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Context, Format, SyntaxNode, Type, Value,
+};
+
+/// A string by which a variable is known to a context.
+///
+/// Every variable is a key-value pair. An identifier holds the key part of that
+/// pair. Its inner value can be used to retrieve a Value instance from a Map.
+#[derive(Debug, Clone, Eq, PartialEq, PartialOrd, Ord, Hash)]
+pub struct Identifier(Arc<String>);
+
+impl Identifier {
+    pub fn new(key: &str) -> Self {
+        for built_in_identifier in all_built_in_identifiers() {
+            let identifier = built_in_identifier.get();
+
+            if &key == identifier.inner().as_ref() {
+                return identifier.clone();
+            }
+        }
+
+        Identifier(Arc::new(key.to_string()))
+    }
+
+    pub fn from_raw_parts(arc: Arc<String>) -> Self {
+        Identifier(arc)
+    }
+
+    pub fn inner(&self) -> &Arc<String> {
+        &self.0
+    }
+
+    pub fn contains(&self, string: &str) -> bool {
+        self.0.as_ref() == string
+    }
+}
+
+impl AbstractTree for Identifier {
+    fn from_syntax(
+        node: SyntaxNode,
+        source: &str,
+        _context: &Context,
+    ) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("identifier", node)?;
+
+        let text = &source[node.byte_range()];
+
+        debug_assert!(!text.is_empty());
+
+        Ok(Identifier::new(text))
+    }
+
+    fn validate(&self, _source: &str, context: &Context) -> Result<(), ValidationError> {
+        let variable_exists = context.add_allowance(self)?;
+
+        if variable_exists {
+            Ok(())
+        } else {
+            for built_in_value in all_built_in_values() {
+                if built_in_value.name() == self.inner().as_ref() {
+                    return Ok(());
+                }
+            }
+
+            Err(ValidationError::VariableIdentifierNotFound(self.clone()))
+        }
+    }
+
+    fn expected_type(&self, context: &Context) -> Result<Type, ValidationError> {
+        if let Some(r#type) = context.get_type(self)? {
+            Ok(r#type)
+        } else {
+            for built_in_value in all_built_in_values() {
+                if built_in_value.name() == self.inner().as_ref() {
+                    return Ok(built_in_value.get().r#type()?);
+                }
+            }
+
+            Err(ValidationError::VariableIdentifierNotFound(self.clone()))
+        }
+    }
+
+    fn run(&self, _source: &str, context: &Context) -> Result<Value, RuntimeError> {
+        if let Some(value) = context.get_value(self)? {
+            return Ok(value);
+        } else {
+            for built_in_value in all_built_in_values() {
+                if built_in_value.name() == self.inner().as_ref() {
+                    return Ok(built_in_value.get().clone());
+                }
+            }
+        }
+
+        Err(RuntimeError::ValidationFailure(
+            ValidationError::VariableIdentifierNotFound(self.clone()),
+        ))
+    }
+}
+
+impl Format for Identifier {
+    fn format(&self, output: &mut String, _indent_level: u8) {
+        output.push_str(&self.0);
+    }
+}
+
+impl From<String> for Identifier {
+    fn from(value: String) -> Self {
+        Identifier::from_raw_parts(Arc::new(value))
+    }
+}
+
+impl From<&str> for Identifier {
+    fn from(value: &str) -> Self {
+        Identifier::new(value)
+    }
+}
+
+impl Display for Identifier {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
+
+impl Serialize for Identifier {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        serializer.serialize_str(self.0.as_ref())
+    }
+}
+
+impl<'de> Deserialize<'de> for Identifier {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        deserializer.deserialize_string(IdentifierVisitor)
+    }
+}
+
+struct IdentifierVisitor;
+
+impl<'de> Visitor<'de> for IdentifierVisitor {
+    type Value = Identifier;
+
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        formatter.write_str("valid UFT-8 sequence")
+    }
+
+    fn visit_string<E>(self, v: String) -> Result<Self::Value, E>
+    where
+        E: serde::de::Error,
+    {
+        Ok(Identifier(Arc::new(v)))
+    }
+}

src/abstract_tree/if_else.rs

@@ -0,0 +1,160 @@
+use serde::{Deserialize, Serialize};
+
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Block, Context, Expression, Format, SourcePosition, SyntaxNode, Type, Value,
+};
+
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub struct IfElse {
+    if_expression: Expression,
+    if_block: Block,
+    else_if_expressions: Vec<Expression>,
+    else_if_blocks: Vec<Block>,
+    else_block: Option<Block>,
+    source_position: SourcePosition,
+}
+
+impl AbstractTree for IfElse {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
+        let if_expression_node = node.child(0).unwrap().child(1).unwrap();
+        let if_expression = Expression::from_syntax(if_expression_node, source, context)?;
+
+        let if_block_node = node.child(0).unwrap().child(2).unwrap();
+        let if_block = Block::from_syntax(if_block_node, source, context)?;
+
+        let child_count = node.child_count();
+        let mut else_if_expressions = Vec::new();
+        let mut else_if_blocks = Vec::new();
+        let mut else_block = None;
+
+        for index in 1..child_count {
+            let child = node.child(index).unwrap();
+
+            if child.kind() == "else_if" {
+                let expression_node = child.child(1).unwrap();
+                let expression = Expression::from_syntax(expression_node, source, context)?;
+
+                else_if_expressions.push(expression);
+
+                let block_node = child.child(2).unwrap();
+                let block = Block::from_syntax(block_node, source, context)?;
+
+                else_if_blocks.push(block);
+            }
+
+            if child.kind() == "else" {
+                let else_node = child.child(1).unwrap();
+                else_block = Some(Block::from_syntax(else_node, source, context)?);
+            }
+        }
+
+        Ok(IfElse {
+            if_expression,
+            if_block,
+            else_if_expressions,
+            else_if_blocks,
+            else_block,
+            source_position: SourcePosition::from(node.range()),
+        })
+    }
+
+    fn expected_type(&self, context: &Context) -> Result<Type, ValidationError> {
+        self.if_block.expected_type(context)
+    }
+
+    fn validate(&self, _source: &str, context: &Context) -> Result<(), ValidationError> {
+        self.if_expression.validate(_source, context)?;
+        self.if_block.validate(_source, context)?;
+
+        let expected = self.if_block.expected_type(context)?;
+        let else_ifs = self
+            .else_if_expressions
+            .iter()
+            .zip(self.else_if_blocks.iter());
+
+        for (expression, block) in else_ifs {
+            expression.validate(_source, context)?;
+            block.validate(_source, context)?;
+
+            let actual = block.expected_type(context)?;
+
+            if !expected.accepts(&actual) {
+                return Err(ValidationError::TypeCheck {
+                    expected,
+                    actual,
+                    position: self.source_position,
+                });
+            }
+        }
+
+        if let Some(block) = &self.else_block {
+            block.validate(_source, context)?;
+
+            let actual = block.expected_type(context)?;
+
+            if !expected.accepts(&actual) {
+                return Err(ValidationError::TypeCheck {
+                    expected,
+                    actual,
+                    position: self.source_position,
+                });
+            }
+        }
+
+        Ok(())
+    }
+
+    fn run(&self, source: &str, context: &Context) -> Result<Value, RuntimeError> {
+        let if_boolean = self.if_expression.run(source, context)?.as_boolean()?;
+
+        if if_boolean {
+            self.if_block.run(source, context)
+        } else {
+            let else_ifs = self
+                .else_if_expressions
+                .iter()
+                .zip(self.else_if_blocks.iter());
+
+            for (expression, block) in else_ifs {
+                let if_boolean = expression.run(source, context)?.as_boolean()?;
+
+                if if_boolean {
+                    return block.run(source, context);
+                }
+            }
+
+            if let Some(block) = &self.else_block {
+                block.run(source, context)
+            } else {
+                Ok(Value::none())
+            }
+        }
+    }
+}
+
+impl Format for IfElse {
+    fn format(&self, output: &mut String, indent_level: u8) {
+        output.push_str("if ");
+        self.if_expression.format(output, indent_level);
+        output.push(' ');
+        self.if_block.format(output, indent_level);
+
+        let else_ifs = self
+            .else_if_expressions
+            .iter()
+            .zip(self.else_if_blocks.iter());
+
+        for (expression, block) in else_ifs {
+            output.push_str("else if ");
+            expression.format(output, indent_level);
+            output.push(' ');
+            block.format(output, indent_level);
+        }
+
+        if let Some(block) = &self.else_block {
+            output.push_str("else ");
+            block.format(output, indent_level);
+        }
+    }
+}

src/abstract_tree/index.rs

@@ -0,0 +1,134 @@
+use serde::{Deserialize, Serialize};
+
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Context, Format, Identifier, IndexExpression, SourcePosition, SyntaxNode, Type,
+    Value,
+};
+
+/// Abstract representation of an index expression.
+///
+/// An index is a means of accessing values stored in list, maps and strings.
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub struct Index {
+    pub collection: IndexExpression,
+    pub index: IndexExpression,
+    source_position: SourcePosition,
+}
+
+impl AbstractTree for Index {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("index", node)?;
+
+        let collection_node = node.child(0).unwrap();
+        let collection = IndexExpression::from_syntax(collection_node, source, context)?;
+
+        let index_node = node.child(2).unwrap();
+        let index = IndexExpression::from_syntax(index_node, source, context)?;
+
+        Ok(Index {
+            collection,
+            index,
+            source_position: SourcePosition::from(node.range()),
+        })
+    }
+
+    fn expected_type(&self, context: &Context) -> Result<Type, ValidationError> {
+        match self.collection.expected_type(context)? {
+            Type::ListOf(item_type) => Ok(*item_type.clone()),
+            Type::Map(map_types_option) => {
+                if let (Some(map_type), IndexExpression::Identifier(identifier)) =
+                    (map_types_option, &self.index)
+                {
+                    if let Some(r#type) = map_type.get(&identifier) {
+                        Ok(r#type.clone())
+                    } else {
+                        Ok(Type::Any)
+                    }
+                } else {
+                    Ok(Type::Any)
+                }
+            }
+            Type::None => Ok(Type::None),
+            r#type => Ok(r#type),
+        }
+    }
+
+    fn validate(&self, _source: &str, _context: &Context) -> Result<(), ValidationError> {
+        self.collection.validate(_source, _context)?;
+
+        let collection_type = self.collection.expected_type(_context)?;
+
+        if let (Type::Map(type_map_option), IndexExpression::Identifier(identifier)) =
+            (collection_type, &self.index)
+        {
+            if let Some(type_map) = type_map_option {
+                if !type_map.contains_key(identifier) {
+                    return Err(ValidationError::VariableIdentifierNotFound(
+                        identifier.clone(),
+                    ));
+                }
+            }
+        } else {
+            self.index.validate(_source, _context)?;
+        }
+
+        Ok(())
+    }
+
+    fn run(&self, source: &str, context: &Context) -> Result<Value, RuntimeError> {
+        let value = self.collection.run(source, context)?;
+
+        match value {
+            Value::List(list) => {
+                let index = self.index.run(source, context)?.as_integer()? as usize;
+                let item = list.items()?.get(index).cloned().unwrap_or_default();
+
+                Ok(item)
+            }
+            Value::Map(map) => {
+                let map = map.inner();
+
+                let value = if let IndexExpression::Identifier(identifier) = &self.index {
+                    if let Some(value) = map.get(identifier) {
+                        value
+                    } else {
+                        return Err(RuntimeError::ValidationFailure(
+                            ValidationError::VariableIdentifierNotFound(identifier.clone()),
+                        ));
+                    }
+                } else {
+                    let index_value = self.index.run(source, context)?;
+                    let identifier = Identifier::new(index_value.as_string()?);
+
+                    if let Some(value) = map.get(&identifier) {
+                        value
+                    } else {
+                        return Err(RuntimeError::ValidationFailure(
+                            ValidationError::VariableIdentifierNotFound(identifier.clone()),
+                        ));
+                    }
+                };
+
+                Ok(value.clone())
+            }
+            Value::String(string) => {
+                let index = self.index.run(source, context)?.as_integer()? as usize;
+                let item = string.chars().nth(index).unwrap_or_default();
+
+                Ok(Value::string(item.to_string()))
+            }
+            _ => Err(RuntimeError::ValidationFailure(
+                ValidationError::ExpectedCollection { actual: value },
+            )),
+        }
+    }
+}
+
+impl Format for Index {
+    fn format(&self, output: &mut String, indent_level: u8) {
+        self.collection.format(output, indent_level);
+        output.push(':');
+        self.index.format(output, indent_level);
+    }
+}

src/abstract_tree/index_assignment.rs

@@ -0,0 +1,96 @@
+use serde::{Deserialize, Serialize};
+
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, AssignmentOperator, Context, Format, Identifier, Index, IndexExpression,
+    SourcePosition, Statement, SyntaxNode, Type, Value,
+};
+
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub struct IndexAssignment {
+    index: Index,
+    operator: AssignmentOperator,
+    statement: Statement,
+    position: SourcePosition,
+}
+
+impl AbstractTree for IndexAssignment {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("index_assignment", node)?;
+
+        let index_node = node.child(0).unwrap();
+        let index = Index::from_syntax(index_node, source, context)?;
+
+        let operator_node = node.child(1).unwrap();
+        let operator = AssignmentOperator::from_syntax(operator_node, source, context)?;
+
+        let statement_node = node.child(2).unwrap();
+        let statement = Statement::from_syntax(statement_node, source, context)?;
+
+        Ok(IndexAssignment {
+            index,
+            operator,
+            statement,
+            position: node.range().into(),
+        })
+    }
+
+    fn expected_type(&self, _context: &Context) -> Result<Type, ValidationError> {
+        Ok(Type::None)
+    }
+
+    fn validate(&self, _source: &str, _context: &Context) -> Result<(), ValidationError> {
+        self.index.validate(_source, _context)?;
+        self.statement.validate(_source, _context)
+    }
+
+    fn run(&self, source: &str, context: &Context) -> Result<Value, RuntimeError> {
+        let _index_collection = self.index.collection.run(source, context)?;
+        let index_identifier = if let IndexExpression::Identifier(identifier) = &self.index.index {
+            identifier
+        } else {
+            let index_run = self.index.index.run(source, context)?;
+            let expected_identifier = Identifier::new(index_run.as_string()?);
+
+            return Err(RuntimeError::ValidationFailure(
+                ValidationError::VariableIdentifierNotFound(expected_identifier),
+            ));
+        };
+
+        let value = self.statement.run(source, context)?;
+
+        let new_value = match self.operator {
+            AssignmentOperator::PlusEqual => {
+                if let Some(previous_value) = context.get_value(index_identifier)? {
+                    previous_value.add(value, self.position)?
+                } else {
+                    return Err(RuntimeError::ValidationFailure(
+                        ValidationError::VariableIdentifierNotFound(index_identifier.clone()),
+                    ));
+                }
+            }
+            AssignmentOperator::MinusEqual => {
+                if let Some(previous_value) = context.get_value(index_identifier)? {
+                    previous_value.subtract(value, self.position)?
+                } else {
+                    Value::none()
+                }
+            }
+            AssignmentOperator::Equal => value,
+        };
+
+        context.set_value(index_identifier.clone(), new_value)?;
+
+        Ok(Value::none())
+    }
+}
+
+impl Format for IndexAssignment {
+    fn format(&self, output: &mut String, indent_level: u8) {
+        self.index.format(output, indent_level);
+        output.push(' ');
+        self.operator.format(output, indent_level);
+        output.push(' ');
+        self.statement.format(output, indent_level);
+    }
+}

src/abstract_tree/index_expression.rs

@@ -0,0 +1,98 @@
+use serde::{Deserialize, Serialize};
+
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    value_node::ValueNode,
+    AbstractTree, Context, Format, FunctionCall, Identifier, Index, SyntaxNode, Type, Value,
+};
+
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub enum IndexExpression {
+    Value(ValueNode),
+    Identifier(Identifier),
+    Index(Box<Index>),
+    FunctionCall(Box<FunctionCall>),
+}
+
+impl AbstractTree for IndexExpression {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("index_expression", node)?;
+
+        let first_child = node.child(0).unwrap();
+        let child = if first_child.is_named() {
+            first_child
+        } else {
+            node.child(1).unwrap()
+        };
+
+        let abstract_node = match child.kind() {
+            "value" => IndexExpression::Value(ValueNode::from_syntax(child, source, context)?),
+            "identifier" => {
+                IndexExpression::Identifier(Identifier::from_syntax(child, source, context)?)
+            }
+            "index" => {
+                IndexExpression::Index(Box::new(Index::from_syntax(child, source, context)?))
+            }
+            "function_call" => IndexExpression::FunctionCall(Box::new(FunctionCall::from_syntax(
+                child, source, context,
+            )?)),
+            _ => {
+                return Err(SyntaxError::UnexpectedSyntaxNode {
+                    expected: "value, identifier, index or function call".to_string(),
+                    actual: child.kind().to_string(),
+                    position: node.range().into(),
+                })
+            }
+        };
+
+        Ok(abstract_node)
+    }
+
+    fn expected_type(&self, context: &Context) -> Result<Type, ValidationError> {
+        match self {
+            IndexExpression::Value(value_node) => value_node.expected_type(context),
+            IndexExpression::Identifier(identifier) => identifier.expected_type(context),
+            IndexExpression::Index(index) => index.expected_type(context),
+            IndexExpression::FunctionCall(function_call) => function_call.expected_type(context),
+        }
+    }
+
+    fn validate(&self, _source: &str, context: &Context) -> Result<(), ValidationError> {
+        match self {
+            IndexExpression::Value(value_node) => value_node.validate(_source, context),
+            IndexExpression::Identifier(identifier) => {
+                context.add_allowance(identifier)?;
+
+                Ok(())
+            }
+            IndexExpression::Index(index) => index.validate(_source, context),
+            IndexExpression::FunctionCall(function_call) => {
+                function_call.validate(_source, context)
+            }
+        }
+    }
+
+    fn run(&self, source: &str, context: &Context) -> Result<Value, RuntimeError> {
+        match self {
+            IndexExpression::Value(value_node) => value_node.run(source, context),
+            IndexExpression::Identifier(identifier) => identifier.run(source, context),
+            IndexExpression::Index(index) => index.run(source, context),
+            IndexExpression::FunctionCall(function_call) => function_call.run(source, context),
+        }
+    }
+}
+
+impl Format for IndexExpression {
+    fn format(&self, output: &mut String, indent_level: u8) {
+        match self {
+            IndexExpression::Value(value_node) => {
+                value_node.format(output, indent_level);
+            }
+            IndexExpression::Identifier(identifier) => identifier.format(output, indent_level),
+            IndexExpression::FunctionCall(function_call) => {
+                function_call.format(output, indent_level)
+            }
+            IndexExpression::Index(index) => index.format(output, indent_level),
+        }
+    }
+}

src/abstract_tree/logic.rs

@@ -0,0 +1,95 @@
+use serde::{Deserialize, Serialize};
+
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Context, Expression, Format, LogicOperator, SyntaxNode, Type, Value,
+};
+
+/// Abstract representation of a logic expression.
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub struct Logic {
+    left: Expression,
+    operator: LogicOperator,
+    right: Expression,
+}
+
+impl AbstractTree for Logic {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("logic", node)?;
+
+        let first_node = node.child(0).unwrap();
+        let (left_node, operator_node, right_node) = {
+            if first_node.is_named() {
+                (first_node, node.child(1).unwrap(), node.child(2).unwrap())
+            } else {
+                (
+                    node.child(1).unwrap(),
+                    node.child(2).unwrap(),
+                    node.child(3).unwrap(),
+                )
+            }
+        };
+        let left = Expression::from_syntax(left_node, source, context)?;
+        let operator = LogicOperator::from_syntax(operator_node, source, context)?;
+        let right = Expression::from_syntax(right_node, source, context)?;
+
+        Ok(Logic {
+            left,
+            operator,
+            right,
+        })
+    }
+
+    fn expected_type(&self, _context: &Context) -> Result<Type, ValidationError> {
+        Ok(Type::Boolean)
+    }
+
+    fn validate(&self, _source: &str, _context: &Context) -> Result<(), ValidationError> {
+        log::info!("VALIDATE logic expression");
+
+        self.left.validate(_source, _context)?;
+        self.right.validate(_source, _context)
+    }
+
+    fn run(&self, source: &str, context: &Context) -> Result<Value, RuntimeError> {
+        let left = self.left.run(source, context)?;
+        let right = self.right.run(source, context)?;
+
+        log::info!("RUN logic expression: {left} {} {right}", self.operator);
+
+        let result = match self.operator {
+            LogicOperator::Equal => {
+                if let (Ok(left_num), Ok(right_num)) = (left.as_number(), right.as_number()) {
+                    left_num == right_num
+                } else {
+                    left == right
+                }
+            }
+            LogicOperator::NotEqual => {
+                if let (Ok(left_num), Ok(right_num)) = (left.as_number(), right.as_number()) {
+                    left_num != right_num
+                } else {
+                    left != right
+                }
+            }
+            LogicOperator::And => left.as_boolean()? && right.as_boolean()?,
+            LogicOperator::Or => left.as_boolean()? || right.as_boolean()?,
+            LogicOperator::Greater => left > right,
+            LogicOperator::Less => left < right,
+            LogicOperator::GreaterOrEqual => left >= right,
+            LogicOperator::LessOrEqual => left <= right,
+        };
+
+        Ok(Value::Boolean(result))
+    }
+}
+
+impl Format for Logic {
+    fn format(&self, output: &mut String, indent_level: u8) {
+        self.left.format(output, indent_level);
+        output.push(' ');
+        self.operator.format(output, indent_level);
+        output.push(' ');
+        self.right.format(output, indent_level);
+    }
+}

src/abstract_tree/logic_operator.rs

@@ -0,0 +1,93 @@
+use std::fmt::{self, Display, Formatter};
+
+use serde::{Deserialize, Serialize};
+
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Context, Format, SyntaxNode, Type, Value,
+};
+
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub enum LogicOperator {
+    Equal,
+    NotEqual,
+    And,
+    Or,
+    Greater,
+    Less,
+    GreaterOrEqual,
+    LessOrEqual,
+}
+
+impl AbstractTree for LogicOperator {
+    fn from_syntax(
+        node: SyntaxNode,
+        _source: &str,
+        _context: &Context,
+    ) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("logic_operator", node)?;
+
+        let operator_node = node.child(0).unwrap();
+        let operator = match operator_node.kind() {
+            "==" => LogicOperator::Equal,
+            "!=" => LogicOperator::NotEqual,
+            "&&" => LogicOperator::And,
+            "||" => LogicOperator::Or,
+            ">" => LogicOperator::Greater,
+            "<" => LogicOperator::Less,
+            ">=" => LogicOperator::GreaterOrEqual,
+            "<=" => LogicOperator::LessOrEqual,
+            _ => {
+                return Err(SyntaxError::UnexpectedSyntaxNode {
+                    expected: "==, !=, &&, ||, >, <, >= or <=".to_string(),
+                    actual: operator_node.kind().to_string(),
+                    position: node.range().into(),
+                })
+            }
+        };
+
+        Ok(operator)
+    }
+
+    fn expected_type(&self, _context: &Context) -> Result<Type, ValidationError> {
+        Ok(Type::None)
+    }
+
+    fn validate(&self, _source: &str, _context: &Context) -> Result<(), ValidationError> {
+        Ok(())
+    }
+
+    fn run(&self, _source: &str, _context: &Context) -> Result<Value, RuntimeError> {
+        Ok(Value::none())
+    }
+}
+
+impl Format for LogicOperator {
+    fn format(&self, output: &mut String, _indent_level: u8) {
+        match self {
+            LogicOperator::Equal => output.push('='),
+            LogicOperator::NotEqual => output.push_str("!="),
+            LogicOperator::And => output.push_str("&&"),
+            LogicOperator::Or => output.push_str("||"),
+            LogicOperator::Greater => output.push('>'),
+            LogicOperator::Less => output.push('<'),
+            LogicOperator::GreaterOrEqual => output.push_str(">="),
+            LogicOperator::LessOrEqual => output.push_str("<="),
+        }
+    }
+}
+
+impl Display for LogicOperator {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        match self {
+            LogicOperator::Equal => write!(f, "="),
+            LogicOperator::NotEqual => write!(f, "!="),
+            LogicOperator::And => write!(f, "&&"),
+            LogicOperator::Or => write!(f, "||"),
+            LogicOperator::Greater => write!(f, ">"),
+            LogicOperator::Less => write!(f, "<"),
+            LogicOperator::GreaterOrEqual => write!(f, ">="),
+            LogicOperator::LessOrEqual => write!(f, "<="),
+        }
+    }
+}

src/abstract_tree/map_node.rs

@@ -0,0 +1,117 @@
+use std::collections::BTreeMap;
+
+use serde::{Deserialize, Serialize};
+use tree_sitter::Node as SyntaxNode;
+
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Context, Format, Identifier, Map, SourcePosition, Statement, Type,
+    TypeSpecification, Value,
+};
+
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub struct MapNode {
+    properties: BTreeMap<Identifier, (Statement, Option<Type>)>,
+    position: SourcePosition,
+}
+
+impl MapNode {
+    pub fn properties(&self) -> &BTreeMap<Identifier, (Statement, Option<Type>)> {
+        &self.properties
+    }
+}
+
+impl AbstractTree for MapNode {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("map", node)?;
+
+        let mut properties = BTreeMap::new();
+        let mut current_identifier = None;
+        let mut current_type = None;
+
+        for index in 0..node.child_count() - 1 {
+            let child = node.child(index).unwrap();
+
+            if child.kind() == "identifier" {
+                current_identifier = Some(Identifier::from_syntax(child, source, context)?);
+                current_type = None;
+            }
+
+            if child.kind() == "type_specification" {
+                current_type =
+                    Some(TypeSpecification::from_syntax(child, source, context)?.take_inner());
+            }
+
+            if child.kind() == "statement" {
+                let statement = Statement::from_syntax(child, source, context)?;
+
+                if let Some(identifier) = &current_identifier {
+                    properties.insert(identifier.clone(), (statement, current_type.clone()));
+                }
+            }
+        }
+
+        Ok(MapNode {
+            properties,
+            position: SourcePosition::from(node.range()),
+        })
+    }
+
+    fn expected_type(&self, _context: &Context) -> Result<Type, ValidationError> {
+        if self.properties.is_empty() {
+            return Ok(Type::Map(None));
+        }
+
+        let mut type_map = BTreeMap::new();
+
+        for (identifier, (statement, r#type_option)) in &self.properties {
+            let r#type = if let Some(r#type) = type_option {
+                r#type.clone()
+            } else {
+                statement.expected_type(_context)?
+            };
+
+            type_map.insert(identifier.clone(), r#type);
+        }
+
+        Ok(Type::Map(Some(type_map)))
+    }
+
+    fn validate(&self, _source: &str, context: &Context) -> Result<(), ValidationError> {
+        for (_key, (statement, r#type)) in &self.properties {
+            statement.validate(_source, context)?;
+
+            if let Some(expected) = r#type {
+                let actual = statement.expected_type(context)?;
+
+                if !expected.accepts(&actual) {
+                    return Err(ValidationError::TypeCheck {
+                        expected: expected.clone(),
+                        actual,
+                        position: self.position.clone(),
+                    });
+                }
+            }
+        }
+
+        Ok(())
+    }
+
+    fn run(&self, _source: &str, _context: &Context) -> Result<Value, RuntimeError> {
+        let mut map = Map::new();
+
+        for (key, (statement, _)) in &self.properties {
+            let value = statement.run(_source, _context)?;
+
+            map.set(key.clone(), value);
+        }
+
+        Ok(Value::Map(map))
+    }
+}
+
+impl Format for MapNode {
+    fn format(&self, _output: &mut String, _indent_level: u8) {
+        todo!()
+    }
+}

src/abstract_tree/match.rs

@@ -0,0 +1,145 @@
+//! Pattern matching.
+//!
+//! Note that this module is called "match" but is escaped as "r#match" because
+//! "match" is a keyword in Rust.
+use serde::{Deserialize, Serialize};
+use tree_sitter::Node as SyntaxNode;
+
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Context, Expression, Format, MatchPattern, Statement, Type, Value,
+};
+
+/// Abstract representation of a match statement.
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub struct Match {
+    matcher: Expression,
+    options: Vec<(MatchPattern, Statement)>,
+    fallback: Option<Box<Statement>>,
+
+    #[serde(skip)]
+    context: Context,
+}
+
+impl AbstractTree for Match {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("match", node)?;
+
+        let matcher_node = node.child(1).unwrap();
+        let matcher = Expression::from_syntax(matcher_node, source, context)?;
+
+        let mut options = Vec::new();
+        let mut previous_pattern = None;
+        let mut next_statement_is_fallback = false;
+        let mut fallback = None;
+
+        for index in 2..node.child_count() {
+            let child = node.child(index).unwrap();
+
+            if child.kind() == "match_pattern" {
+                previous_pattern = Some(MatchPattern::from_syntax(child, source, context)?);
+            }
+
+            if child.kind() == "statement" {
+                let statement = Statement::from_syntax(child, source, context)?;
+
+                if next_statement_is_fallback {
+                    fallback = Some(Box::new(statement));
+                    next_statement_is_fallback = false;
+                } else if let Some(expression) = &previous_pattern {
+                    options.push((expression.clone(), statement));
+                }
+            }
+        }
+
+        Ok(Match {
+            matcher,
+            options,
+            fallback,
+            context: Context::default(),
+        })
+    }
+
+    fn expected_type(&self, context: &Context) -> Result<Type, ValidationError> {
+        let (_, first_statement) = self.options.first().unwrap();
+
+        first_statement.expected_type(context)
+    }
+
+    fn validate(&self, _source: &str, _context: &Context) -> Result<(), ValidationError> {
+        self.matcher.validate(_source, _context)?;
+
+        for (match_pattern, statement) in &self.options {
+            if let MatchPattern::EnumPattern(enum_pattern) = match_pattern {
+                if let Some(identifier) = enum_pattern.inner_identifier() {
+                    self.context.set_type(identifier.clone(), Type::Any)?;
+                }
+            }
+
+            match_pattern.validate(_source, _context)?;
+            statement.validate(_source, &self.context)?;
+        }
+
+        if let Some(statement) = &self.fallback {
+            statement.validate(_source, _context)?;
+        }
+
+        Ok(())
+    }
+
+    fn run(&self, source: &str, context: &Context) -> Result<Value, RuntimeError> {
+        let matcher_value = self.matcher.run(source, context)?;
+
+        for (pattern, statement) in &self.options {
+            if let (Value::Enum(enum_instance), MatchPattern::EnumPattern(enum_pattern)) =
+                (&matcher_value, pattern)
+            {
+                if enum_instance.name() == enum_pattern.name()
+                    && enum_instance.variant() == enum_pattern.variant()
+                {
+                    let statement_context = Context::with_variables_from(context)?;
+
+                    if let (Some(identifier), Some(value)) =
+                        (enum_pattern.inner_identifier(), enum_instance.value())
+                    {
+                        statement_context.set_value(identifier.clone(), value.as_ref().clone())?;
+                    }
+
+                    return statement.run(source, &statement_context);
+                }
+            }
+            let pattern_value = pattern.run(source, context)?;
+
+            if matcher_value == pattern_value {
+                return statement.run(source, context);
+            }
+        }
+
+        if let Some(fallback) = &self.fallback {
+            fallback.run(source, context)
+        } else {
+            Ok(Value::none())
+        }
+    }
+}
+
+impl Format for Match {
+    fn format(&self, output: &mut String, indent_level: u8) {
+        output.push_str("match ");
+        self.matcher.format(output, indent_level);
+        output.push_str(" {");
+
+        for (expression, statement) in &self.options {
+            expression.format(output, indent_level);
+            output.push_str(" => ");
+            statement.format(output, indent_level);
+        }
+
+        if let Some(statement) = &self.fallback {
+            output.push_str("* => ");
+            statement.format(output, indent_level);
+        }
+
+        output.push('}');
+    }
+}

src/abstract_tree/match_pattern.rs

@@ -0,0 +1,64 @@
+use serde::{Deserialize, Serialize};
+use tree_sitter::Node as SyntaxNode;
+
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Context, EnumPattern, Format, Type, Value, ValueNode,
+};
+
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub enum MatchPattern {
+    EnumPattern(EnumPattern),
+    Value(ValueNode),
+    Wildcard,
+}
+
+impl AbstractTree for MatchPattern {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("match_pattern", node)?;
+
+        let child = node.child(0).unwrap();
+        let pattern = match child.kind() {
+            "enum_pattern" => {
+                MatchPattern::EnumPattern(EnumPattern::from_syntax(child, source, context)?)
+            }
+            "value" => MatchPattern::Value(ValueNode::from_syntax(child, source, context)?),
+            "*" => MatchPattern::Wildcard,
+            _ => {
+                return Err(SyntaxError::UnexpectedSyntaxNode {
+                    expected: "enum pattern or value".to_string(),
+                    actual: child.kind().to_string(),
+                    position: node.range().into(),
+                })
+            }
+        };
+
+        Ok(pattern)
+    }
+
+    fn expected_type(&self, _context: &Context) -> Result<Type, ValidationError> {
+        match self {
+            MatchPattern::EnumPattern(enum_pattern) => enum_pattern.expected_type(_context),
+            MatchPattern::Value(value_node) => value_node.expected_type(_context),
+            MatchPattern::Wildcard => todo!(),
+        }
+    }
+
+    fn validate(&self, _source: &str, _context: &Context) -> Result<(), ValidationError> {
+        Ok(())
+    }
+
+    fn run(&self, _source: &str, _context: &Context) -> Result<Value, RuntimeError> {
+        match self {
+            MatchPattern::EnumPattern(enum_pattern) => enum_pattern.run(_source, _context),
+            MatchPattern::Value(value_node) => value_node.run(_source, _context),
+            MatchPattern::Wildcard => todo!(),
+        }
+    }
+}
+
+impl Format for MatchPattern {
+    fn format(&self, _output: &mut String, _indent_level: u8) {
+        todo!()
+    }
+}

src/abstract_tree/math.rs

@@ -0,0 +1,74 @@
+use serde::{Deserialize, Serialize};
+
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Context, Expression, Format, MathOperator, SourcePosition, SyntaxNode, Type,
+    Value,
+};
+
+/// Abstract representation of a math operation.
+///
+/// Dust currently supports the four basic operations and the modulo (or
+/// remainder) operator.
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub struct Math {
+    left: Expression,
+    operator: MathOperator,
+    right: Expression,
+    position: SourcePosition,
+}
+
+impl AbstractTree for Math {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("math", node)?;
+
+        let left_node = node.child(0).unwrap();
+        let left = Expression::from_syntax(left_node, source, context)?;
+
+        let operator_node = node.child(1).unwrap();
+        let operator = MathOperator::from_syntax(operator_node, source, context)?;
+
+        let right_node = node.child(2).unwrap();
+        let right = Expression::from_syntax(right_node, source, context)?;
+
+        Ok(Math {
+            left,
+            operator,
+            right,
+            position: node.range().into(),
+        })
+    }
+
+    fn expected_type(&self, context: &Context) -> Result<Type, ValidationError> {
+        self.left.expected_type(context)
+    }
+
+    fn validate(&self, _source: &str, _context: &Context) -> Result<(), ValidationError> {
+        self.left.validate(_source, _context)?;
+        self.right.validate(_source, _context)
+    }
+
+    fn run(&self, source: &str, context: &Context) -> Result<Value, RuntimeError> {
+        let left = self.left.run(source, context)?;
+        let right = self.right.run(source, context)?;
+        let value = match self.operator {
+            MathOperator::Add => left.add(right, self.position)?,
+            MathOperator::Subtract => left.subtract(right, self.position)?,
+            MathOperator::Multiply => left.multiply(right, self.position)?,
+            MathOperator::Divide => left.divide(right, self.position)?,
+            MathOperator::Modulo => left.modulo(right, self.position)?,
+        };
+
+        Ok(value)
+    }
+}
+
+impl Format for Math {
+    fn format(&self, output: &mut String, indent_level: u8) {
+        self.left.format(output, indent_level);
+        output.push(' ');
+        self.operator.format(output, indent_level);
+        output.push(' ');
+        self.right.format(output, indent_level);
+    }
+}

src/abstract_tree/math_operator.rs

@@ -0,0 +1,69 @@
+use serde::{Deserialize, Serialize};
+
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Context, Format, SyntaxNode, Type, Value,
+};
+
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub enum MathOperator {
+    Add,
+    Subtract,
+    Multiply,
+    Divide,
+    Modulo,
+}
+
+impl AbstractTree for MathOperator {
+    fn from_syntax(
+        node: SyntaxNode,
+        _source: &str,
+        _context: &Context,
+    ) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("math_operator", node)?;
+
+        let operator_node = node.child(0).unwrap();
+        let operator = match operator_node.kind() {
+            "+" => MathOperator::Add,
+            "-" => MathOperator::Subtract,
+            "*" => MathOperator::Multiply,
+            "/" => MathOperator::Divide,
+            "%" => MathOperator::Modulo,
+            _ => {
+                return Err(SyntaxError::UnexpectedSyntaxNode {
+                    expected: "+, -, *, / or %".to_string(),
+                    actual: operator_node.kind().to_string(),
+                    position: node.range().into(),
+                })
+            }
+        };
+
+        Ok(operator)
+    }
+
+    fn expected_type(&self, _context: &Context) -> Result<Type, ValidationError> {
+        Ok(Type::None)
+    }
+
+    fn validate(&self, _source: &str, _context: &Context) -> Result<(), ValidationError> {
+        Ok(())
+    }
+
+    fn run(&self, _source: &str, _context: &Context) -> Result<Value, RuntimeError> {
+        Ok(Value::none())
+    }
+}
+
+impl Format for MathOperator {
+    fn format(&self, output: &mut String, _indent_level: u8) {
+        let char = match self {
+            MathOperator::Add => '+',
+            MathOperator::Subtract => '-',
+            MathOperator::Multiply => '*',
+            MathOperator::Divide => '/',
+            MathOperator::Modulo => '%',
+        };
+
+        output.push(char);
+    }
+}

src/abstract_tree/mod.rs

@@ -0,0 +1,178 @@
+//! Abstract, executable representations of corresponding items found in Dust
+//! source code. The types that implement [AbstractTree] are inteded to be
+//! created by an [Interpreter].
+pub mod r#as;
+pub mod assignment;
+pub mod assignment_operator;
+pub mod block;
+pub mod command;
+pub mod enum_defintion;
+pub mod enum_pattern;
+pub mod expression;
+pub mod r#for;
+pub mod function_call;
+pub mod function_expression;
+pub mod function_node;
+pub mod identifier;
+pub mod if_else;
+pub mod index;
+pub mod index_assignment;
+pub mod index_expression;
+pub mod logic;
+pub mod logic_operator;
+pub mod map_node;
+pub mod r#match;
+pub mod match_pattern;
+pub mod math;
+pub mod math_operator;
+pub mod statement;
+pub mod struct_definition;
+pub mod r#type;
+pub mod type_definition;
+pub mod type_specification;
+pub mod value_node;
+pub mod r#while;
+
+pub use {
+    assignment::*, assignment_operator::*, block::*, command::*, enum_defintion::*,
+    enum_pattern::*, expression::*, function_call::*, function_expression::*, function_node::*,
+    identifier::*, if_else::*, index::*, index_assignment::IndexAssignment, index_expression::*,
+    logic::*, logic_operator::*, map_node::*, match_pattern::*, math::*, math_operator::*, r#as::*,
+    r#for::*, r#match::*, r#type::*, r#while::*, statement::*, struct_definition::*,
+    type_definition::*, type_specification::*, value_node::*,
+};
+
+use serde::{Deserialize, Serialize};
+
+use crate::{
+    context::Context,
+    error::{RuntimeError, SyntaxError, ValidationError},
+    SyntaxNode, Value,
+};
+
+/// A detailed report of a position in the source code string.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Serialize, Deserialize)]
+pub struct SourcePosition {
+    pub start_byte: usize,
+    pub end_byte: usize,
+    pub start_row: usize,
+    pub start_column: usize,
+    pub end_row: usize,
+    pub end_column: usize,
+}
+
+impl From<tree_sitter::Range> for SourcePosition {
+    fn from(range: tree_sitter::Range) -> Self {
+        SourcePosition {
+            start_byte: range.start_byte,
+            end_byte: range.end_byte,
+            start_row: range.start_point.row + 1,
+            start_column: range.start_point.column,
+            end_row: range.end_point.row + 1,
+            end_column: range.end_point.column,
+        }
+    }
+}
+
+/// Abstraction that represents a whole, executable unit of dust code.
+#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Serialize, Deserialize)]
+pub struct Root {
+    statements: Vec<Statement>,
+}
+
+// TODO Change Root to use tree sitter's cursor to traverse the statements
+// instead of indexes. This will be more performant when there are a lot of
+// top-level statements in the tree.
+impl AbstractTree for Root {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("root", node)?;
+
+        let statement_count = node.child_count();
+        let mut statements = Vec::with_capacity(statement_count);
+
+        for index in 0..statement_count {
+            let statement_node = node.child(index).unwrap();
+            let statement = Statement::from_syntax(statement_node, source, context)?;
+
+            statements.push(statement);
+        }
+
+        Ok(Root { statements })
+    }
+
+    fn validate(&self, _source: &str, _context: &Context) -> Result<(), ValidationError> {
+        for statement in &self.statements {
+            statement.validate(_source, _context)?;
+        }
+
+        Ok(())
+    }
+
+    fn run(&self, source: &str, context: &Context) -> Result<Value, RuntimeError> {
+        let mut value = Value::none();
+
+        for statement in &self.statements {
+            value = statement.run(source, context)?;
+
+            if statement.is_return() {
+                return Ok(value);
+            }
+        }
+
+        Ok(value)
+    }
+
+    fn expected_type(&self, context: &Context) -> Result<Type, ValidationError> {
+        self.statements.last().unwrap().expected_type(context)
+    }
+}
+
+impl Format for Root {
+    fn format(&self, output: &mut String, indent_level: u8) {
+        for (index, statement) in self.statements.iter().enumerate() {
+            if index > 0 {
+                output.push('\n');
+            }
+            statement.format(output, indent_level);
+            output.push('\n');
+        }
+    }
+}
+
+/// This trait is implemented by the Evaluator's internal types to form an
+/// executable tree that resolves to a single value.
+pub trait AbstractTree: Sized + Format {
+    /// Interpret the syntax tree at the given node and return the abstraction.
+    /// Returns a syntax error if the source is invalid.
+    ///
+    /// This function is used to convert nodes in the Tree Sitter concrete
+    /// syntax tree into executable nodes in an abstract tree. This function is
+    /// where the tree should be traversed by accessing sibling and child nodes.
+    /// Each node in the CST should be traversed only once.
+    ///
+    /// If necessary, the source code can be accessed directly by getting the
+    /// node's byte range.
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError>;
+
+    /// Return the type of the value that this abstract node will create when
+    /// run. Returns a validation error if the tree is invalid.
+    fn expected_type(&self, context: &Context) -> Result<Type, ValidationError>;
+
+    /// Verify the type integrity of the node. Returns a validation error if the
+    /// tree is invalid.
+    fn validate(&self, source: &str, context: &Context) -> Result<(), ValidationError>;
+
+    /// Execute this node's logic and return a value. Returns a runtime error if
+    /// the node cannot resolve to a value.
+    fn run(&self, source: &str, context: &Context) -> Result<Value, RuntimeError>;
+}
+
+pub trait Format {
+    fn format(&self, output: &mut String, indent_level: u8);
+
+    fn indent(output: &mut String, indent_level: u8) {
+        for _ in 0..indent_level {
+            output.push_str("    ");
+        }
+    }
+}

src/abstract_tree/new.rs

@@ -0,0 +1,45 @@
+use serde::{Deserialize, Serialize};
+use tree_sitter::Node;
+
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Context, Format, Identifier, Type, TypeSpecification, Value, ValueNode,
+};
+
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq, PartialOrd, Ord)]
+pub struct New {
+    identifier: Identifier,
+    properties: Vec<(Identifier, ValueNode, Option<TypeSpecification>)>,
+}
+
+impl AbstractTree for New {
+    fn from_syntax(node: Node, source: &str, context: &Context) -> Result<Self, SyntaxError> {
+        let identifier_node = node.child(1).unwrap();
+        let identifier = Identifier::from_syntax(identifier_node, source, context)?;
+
+        let properties = Vec::new();
+
+        Ok(New {
+            identifier,
+            properties,
+        })
+    }
+
+    fn expected_type(&self, _context: &Context) -> Result<Type, ValidationError> {
+        todo!()
+    }
+
+    fn validate(&self, _source: &str, _context: &Context) -> Result<(), ValidationError> {
+        todo!()
+    }
+
+    fn run(&self, _source: &str, _context: &Context) -> Result<Value, RuntimeError> {
+        todo!()
+    }
+}
+
+impl Format for New {
+    fn format(&self, _output: &mut String, _indent_level: u8) {
+        todo!()
+    }
+}

src/abstract_tree/statement.rs

@@ -0,0 +1,202 @@
+use serde::{Deserialize, Serialize};
+
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Assignment, Block, Context, Expression, For, Format, IfElse, IndexAssignment,
+    Match, SyntaxNode, Type, TypeDefinition, Value, While,
+};
+
+/// Abstract representation of a statement.
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub struct Statement {
+    is_return: bool,
+    statement_kind: StatementKind,
+}
+
+impl Statement {
+    pub fn is_return(&self) -> bool {
+        self.is_return
+    }
+}
+
+impl AbstractTree for Statement {
+    fn from_syntax(
+        node: SyntaxNode,
+        source: &str,
+        _context: &Context,
+    ) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("statement", node)?;
+
+        let first_child = node.child(0).unwrap();
+        let mut is_return = first_child.kind() == "return" || first_child.kind() == "break";
+        let child = if is_return {
+            node.child(1).unwrap()
+        } else {
+            first_child
+        };
+
+        let statement_kind = StatementKind::from_syntax(child, source, _context)?;
+
+        if let StatementKind::Block(block) = &statement_kind {
+            if block.contains_return() {
+                is_return = true;
+            }
+        };
+
+        Ok(Statement {
+            is_return,
+            statement_kind,
+        })
+    }
+
+    fn expected_type(&self, _context: &Context) -> Result<Type, ValidationError> {
+        self.statement_kind.expected_type(_context)
+    }
+
+    fn validate(&self, _source: &str, _context: &Context) -> Result<(), ValidationError> {
+        self.statement_kind.validate(_source, _context)
+    }
+
+    fn run(&self, _source: &str, _context: &Context) -> Result<Value, RuntimeError> {
+        self.statement_kind.run(_source, _context)
+    }
+}
+
+impl Format for Statement {
+    fn format(&self, _output: &mut String, _indent_level: u8) {
+        self.statement_kind.format(_output, _indent_level)
+    }
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+enum StatementKind {
+    Assignment(Box<Assignment>),
+    Expression(Expression),
+    IfElse(Box<IfElse>),
+    Match(Match),
+    While(Box<While>),
+    Block(Box<Block>),
+    For(Box<For>),
+    IndexAssignment(Box<IndexAssignment>),
+    TypeDefinition(TypeDefinition),
+}
+
+impl AbstractTree for StatementKind {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("statement_kind", node)?;
+
+        let child = node.child(0).unwrap();
+
+        match child.kind() {
+            "assignment" => Ok(StatementKind::Assignment(Box::new(
+                Assignment::from_syntax(child, source, context)?,
+            ))),
+            "expression" => Ok(StatementKind::Expression(Expression::from_syntax(
+                child, source, context,
+            )?)),
+            "if_else" => Ok(StatementKind::IfElse(Box::new(IfElse::from_syntax(
+                child, source, context,
+            )?))),
+            "while" => Ok(StatementKind::While(Box::new(While::from_syntax(
+                child, source, context,
+            )?))),
+            "block" => Ok(StatementKind::Block(Box::new(Block::from_syntax(
+                child, source, context,
+            )?))),
+            "for" => Ok(StatementKind::For(Box::new(For::from_syntax(
+                child, source, context,
+            )?))),
+            "index_assignment" => Ok(StatementKind::IndexAssignment(Box::new(
+                IndexAssignment::from_syntax(child, source, context)?,
+            ))),
+            "match" => Ok(StatementKind::Match(Match::from_syntax(
+                child, source, context,
+            )?)),
+            "type_definition" => Ok(StatementKind::TypeDefinition(TypeDefinition::from_syntax(
+                child, source, context
+            )?)),
+            _ => Err(SyntaxError::UnexpectedSyntaxNode {
+                expected:
+                    "assignment, index assignment, expression, type_definition, block, return, if...else, while, for or match".to_string(),
+                actual: child.kind().to_string(),
+                position: node.range().into(),
+            }),
+        }
+    }
+
+    fn expected_type(&self, _context: &Context) -> Result<Type, ValidationError> {
+        match self {
+            StatementKind::Assignment(assignment) => assignment.expected_type(_context),
+            StatementKind::Expression(expression) => expression.expected_type(_context),
+            StatementKind::IfElse(if_else) => if_else.expected_type(_context),
+            StatementKind::Match(r#match) => r#match.expected_type(_context),
+            StatementKind::While(r#while) => r#while.expected_type(_context),
+            StatementKind::Block(block) => block.expected_type(_context),
+            StatementKind::For(r#for) => r#for.expected_type(_context),
+            StatementKind::IndexAssignment(index_assignment) => {
+                index_assignment.expected_type(_context)
+            }
+            StatementKind::TypeDefinition(type_definition) => {
+                type_definition.expected_type(_context)
+            }
+        }
+    }
+
+    fn validate(&self, _source: &str, _context: &Context) -> Result<(), ValidationError> {
+        match self {
+            StatementKind::Assignment(assignment) => assignment.validate(_source, _context),
+            StatementKind::Expression(expression) => expression.validate(_source, _context),
+            StatementKind::IfElse(if_else) => if_else.validate(_source, _context),
+            StatementKind::Match(r#match) => r#match.validate(_source, _context),
+            StatementKind::While(r#while) => r#while.validate(_source, _context),
+            StatementKind::Block(block) => block.validate(_source, _context),
+            StatementKind::For(r#for) => r#for.validate(_source, _context),
+            StatementKind::IndexAssignment(index_assignment) => {
+                index_assignment.validate(_source, _context)
+            }
+            StatementKind::TypeDefinition(type_definition) => {
+                type_definition.validate(_source, _context)
+            }
+        }
+    }
+
+    fn run(&self, _source: &str, _context: &Context) -> Result<Value, RuntimeError> {
+        match self {
+            StatementKind::Assignment(assignment) => assignment.run(_source, _context),
+            StatementKind::Expression(expression) => expression.run(_source, _context),
+            StatementKind::IfElse(if_else) => if_else.run(_source, _context),
+            StatementKind::Match(r#match) => r#match.run(_source, _context),
+            StatementKind::While(r#while) => r#while.run(_source, _context),
+            StatementKind::Block(block) => block.run(_source, _context),
+            StatementKind::For(r#for) => r#for.run(_source, _context),
+            StatementKind::IndexAssignment(index_assignment) => {
+                index_assignment.run(_source, _context)
+            }
+            StatementKind::TypeDefinition(type_definition) => {
+                type_definition.run(_source, _context)
+            }
+        }
+    }
+}
+
+impl Format for StatementKind {
+    fn format(&self, output: &mut String, indent_level: u8) {
+        StatementKind::indent(output, indent_level);
+
+        match self {
+            StatementKind::Assignment(assignment) => assignment.format(output, indent_level),
+            StatementKind::Expression(expression) => expression.format(output, indent_level),
+            StatementKind::IfElse(if_else) => if_else.format(output, indent_level),
+            StatementKind::Match(r#match) => r#match.format(output, indent_level),
+            StatementKind::While(r#while) => r#while.format(output, indent_level),
+            StatementKind::Block(block) => block.format(output, indent_level),
+            StatementKind::For(r#for) => r#for.format(output, indent_level),
+            StatementKind::IndexAssignment(index_assignment) => {
+                index_assignment.format(output, indent_level)
+            }
+            StatementKind::TypeDefinition(type_definition) => {
+                type_definition.format(output, indent_level)
+            }
+        }
+    }
+}

src/abstract_tree/struct_definition.rs

@@ -0,0 +1,120 @@
+use std::collections::BTreeMap;
+
+use serde::{Deserialize, Serialize};
+use tree_sitter::Node as SyntaxNode;
+
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Context, Format, Identifier, Map, MapNode, Statement, StructInstance, Type,
+    TypeDefinition, TypeSpecification, Value,
+};
+
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub struct StructDefinition {
+    name: Identifier,
+    properties: BTreeMap<Identifier, (Option<Statement>, Type)>,
+}
+
+impl StructDefinition {
+    pub fn instantiate(
+        &self,
+        new_properties: &MapNode,
+        source: &str,
+        context: &Context,
+    ) -> Result<StructInstance, RuntimeError> {
+        let mut all_properties = Map::new();
+
+        for (key, (statement_option, _)) in &self.properties {
+            if let Some(statement) = statement_option {
+                let value = statement.run(source, context)?;
+
+                all_properties.set(key.clone(), value);
+            }
+        }
+
+        for (key, (statement, _)) in new_properties.properties() {
+            let value = statement.run(source, context)?;
+
+            all_properties.set(key.clone(), value);
+        }
+
+        Ok(StructInstance::new(self.name.clone(), all_properties))
+    }
+}
+
+impl AbstractTree for StructDefinition {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("struct_definition", node)?;
+
+        let name_node = node.child(1).unwrap();
+        let name = Identifier::from_syntax(name_node, source, context)?;
+
+        let mut properties = BTreeMap::new();
+        let mut current_identifier: Option<Identifier> = None;
+        let mut current_type: Option<Type> = None;
+        let mut current_statement = None;
+
+        for index in 2..node.child_count() - 1 {
+            let child_syntax_node = node.child(index).unwrap();
+
+            if child_syntax_node.kind() == "identifier" {
+                if current_statement.is_none() {
+                    if let (Some(identifier), Some(r#type)) = (&current_identifier, &current_type) {
+                        properties.insert(identifier.clone(), (None, r#type.clone()));
+                    }
+                }
+
+                current_type = None;
+                current_identifier =
+                    Some(Identifier::from_syntax(child_syntax_node, source, context)?);
+            }
+
+            if child_syntax_node.kind() == "type_specification" {
+                current_type = Some(
+                    TypeSpecification::from_syntax(child_syntax_node, source, context)?
+                        .take_inner(),
+                );
+            }
+
+            if child_syntax_node.kind() == "statement" {
+                current_statement =
+                    Some(Statement::from_syntax(child_syntax_node, source, context)?);
+
+                if let Some(identifier) = &current_identifier {
+                    let r#type = if let Some(r#type) = &current_type {
+                        r#type.clone()
+                    } else {
+                        Type::None
+                    };
+
+                    properties.insert(
+                        identifier.clone(),
+                        (current_statement.clone(), r#type.clone()),
+                    );
+                }
+            }
+        }
+
+        Ok(StructDefinition { name, properties })
+    }
+
+    fn expected_type(&self, _context: &Context) -> Result<Type, ValidationError> {
+        Ok(Type::None)
+    }
+
+    fn validate(&self, _source: &str, _context: &Context) -> Result<(), ValidationError> {
+        Ok(())
+    }
+
+    fn run(&self, _source: &str, context: &Context) -> Result<Value, RuntimeError> {
+        context.set_definition(self.name.clone(), TypeDefinition::Struct(self.clone()))?;
+
+        Ok(Value::none())
+    }
+}
+
+impl Format for StructDefinition {
+    fn format(&self, _output: &mut String, _indent_level: u8) {
+        todo!()
+    }
+}

src/abstract_tree/type.rs

@@ -0,0 +1,394 @@
+use std::{
+    collections::BTreeMap,
+    fmt::{self, Display, Formatter},
+};
+
+use serde::{Deserialize, Serialize};
+use tree_sitter::Node as SyntaxNode;
+
+use crate::{
+    built_in_types::BuiltInType,
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Context, Format, Identifier, TypeSpecification, Value,
+};
+
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub enum Type {
+    Any,
+    Boolean,
+    Collection,
+    Custom {
+        name: Identifier,
+        arguments: Vec<Type>,
+    },
+    Float,
+    Function {
+        parameter_types: Vec<Type>,
+        return_type: Box<Type>,
+    },
+    Integer,
+    List,
+    ListOf(Box<Type>),
+    ListExact(Vec<Type>),
+    Map(Option<BTreeMap<Identifier, Type>>),
+    None,
+    Number,
+    String,
+    Range,
+}
+
+impl Type {
+    pub fn custom(name: Identifier, arguments: Vec<Type>) -> Self {
+        Type::Custom { name, arguments }
+    }
+
+    pub fn option(inner_type: Option<Type>) -> Self {
+        BuiltInType::Option(inner_type).get().clone()
+    }
+
+    pub fn list(item_type: Type) -> Self {
+        Type::ListOf(Box::new(item_type))
+    }
+
+    pub fn function(parameter_types: Vec<Type>, return_type: Type) -> Self {
+        Type::Function {
+            parameter_types,
+            return_type: Box::new(return_type),
+        }
+    }
+
+    /// Returns a boolean indicating whether is type is accepting of the other.
+    ///
+    /// The types do not need to match exactly. For example, the Any variant matches all of the
+    /// others and the Number variant accepts Number, Integer and Float.
+    pub fn accepts(&self, other: &Type) -> bool {
+        log::info!("Checking type {self} against {other}.");
+
+        match (self, other) {
+            (Type::Any, _)
+            | (_, Type::Any)
+            | (Type::Boolean, Type::Boolean)
+            | (Type::Collection, Type::Collection)
+            | (Type::Collection, Type::String)
+            | (Type::Collection, Type::List)
+            | (Type::List, Type::Collection)
+            | (Type::Collection, Type::ListExact(_))
+            | (Type::ListExact(_), Type::Collection)
+            | (Type::Collection, Type::ListOf(_))
+            | (Type::ListOf(_), Type::Collection)
+            | (Type::Collection, Type::Map(_))
+            | (Type::Map(_), Type::Collection)
+            | (Type::String, Type::Collection)
+            | (Type::Float, Type::Float)
+            | (Type::Integer, Type::Integer)
+            | (Type::List, Type::List)
+            | (Type::Map(None), Type::Map(None))
+            | (Type::Number, Type::Number)
+            | (Type::Number, Type::Integer)
+            | (Type::Number, Type::Float)
+            | (Type::Integer, Type::Number)
+            | (Type::Float, Type::Number)
+            | (Type::String, Type::String)
+            | (Type::None, Type::None) => true,
+            (Type::Map(left_types), Type::Map(right_types)) => left_types == right_types,
+            (
+                Type::Custom {
+                    name: left_name,
+                    arguments: left_arguments,
+                },
+                Type::Custom {
+                    name: right_name,
+                    arguments: right_arguments,
+                },
+            ) => left_name == right_name && left_arguments == right_arguments,
+            (Type::ListOf(self_item_type), Type::ListOf(other_item_type)) => {
+                self_item_type.accepts(&other_item_type)
+            }
+            (Type::ListExact(self_types), Type::ListExact(other_types)) => {
+                for (left, right) in self_types.iter().zip(other_types.iter()) {
+                    if !left.accepts(right) {
+                        return false;
+                    }
+                }
+
+                true
+            }
+            (Type::ListExact(exact_types), Type::ListOf(of_type))
+            | (Type::ListOf(of_type), Type::ListExact(exact_types)) => {
+                exact_types.iter().all(|r#type| r#type == of_type.as_ref())
+            }
+            (
+                Type::Function {
+                    parameter_types: self_parameter_types,
+                    return_type: self_return_type,
+                },
+                Type::Function {
+                    parameter_types: other_parameter_types,
+                    return_type: other_return_type,
+                },
+            ) => {
+                let parameter_type_pairs = self_parameter_types
+                    .iter()
+                    .zip(other_parameter_types.iter());
+
+                for (self_parameter_type, other_parameter_type) in parameter_type_pairs {
+                    if self_parameter_type == other_parameter_type {
+                        return false;
+                    }
+                }
+
+                self_return_type == other_return_type
+            }
+            _ => false,
+        }
+    }
+
+    pub fn is_function(&self) -> bool {
+        matches!(self, Type::Function { .. })
+    }
+
+    pub fn is_list(&self) -> bool {
+        matches!(self, Type::ListOf(_))
+    }
+
+    pub fn is_map(&self) -> bool {
+        matches!(self, Type::Map(_))
+    }
+}
+
+impl AbstractTree for Type {
+    fn from_syntax(
+        node: SyntaxNode,
+        _source: &str,
+        context: &Context,
+    ) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("type", node)?;
+
+        let type_node = node.child(0).unwrap();
+
+        let r#type = match type_node.kind() {
+            "identifier" => {
+                let name = Identifier::from_syntax(type_node, _source, context)?;
+                let mut arguments = Vec::new();
+
+                for index in 2..node.child_count() - 1 {
+                    let child = node.child(index).unwrap();
+
+                    if child.is_named() {
+                        let r#type = Type::from_syntax(child, _source, context)?;
+
+                        arguments.push(r#type);
+                    }
+                }
+
+                Type::custom(name, arguments)
+            }
+            "{" => {
+                let mut type_map = BTreeMap::new();
+                let mut previous_identifier = None;
+
+                for index in 1..node.child_count() - 1 {
+                    let child = node.child(index).unwrap();
+
+                    if let Some(identifier) = previous_identifier {
+                        let type_specification =
+                            TypeSpecification::from_syntax(child, _source, context)?;
+
+                        type_map.insert(identifier, type_specification.take_inner());
+                        previous_identifier = None;
+                    } else {
+                        previous_identifier =
+                            Some(Identifier::from_syntax(child, _source, context)?)
+                    }
+                }
+
+                Type::Map(Some(type_map))
+            }
+            "[" => {
+                let item_type_node = node.child(1).unwrap();
+                let item_type = Type::from_syntax(item_type_node, _source, context)?;
+
+                Type::ListOf(Box::new(item_type))
+            }
+            "list" => {
+                let item_type_node = node.child(1);
+
+                if let Some(child) = item_type_node {
+                    Type::ListOf(Box::new(Type::from_syntax(child, _source, context)?))
+                } else {
+                    Type::List
+                }
+            }
+            "any" => Type::Any,
+            "bool" => Type::Boolean,
+            "collection" => Type::Collection,
+            "float" => Type::Float,
+            "(" => {
+                let child_count = node.child_count();
+                let mut parameter_types = Vec::new();
+
+                for index in 1..child_count - 2 {
+                    let child = node.child(index).unwrap();
+
+                    if child.is_named() {
+                        let parameter_type = Type::from_syntax(child, _source, context)?;
+
+                        parameter_types.push(parameter_type);
+                    }
+                }
+
+                let final_node = node.child(child_count - 1).unwrap();
+                let return_type = if final_node.is_named() {
+                    Type::from_syntax(final_node, _source, context)?
+                } else {
+                    Type::option(None)
+                };
+
+                Type::Function {
+                    parameter_types,
+                    return_type: Box::new(return_type),
+                }
+            }
+            "int" => Type::Integer,
+            "map" => Type::Map(None),
+            "num" => Type::Number,
+            "none" => Type::None,
+            "str" => Type::String,
+            _ => {
+                return Err(SyntaxError::UnexpectedSyntaxNode {
+                    expected: "any, bool, float, int, num, str, list, map, custom type, (, [ or {"
+                        .to_string(),
+                    actual: type_node.kind().to_string(),
+                    position: node.range().into(),
+                })
+            }
+        };
+
+        Ok(r#type)
+    }
+
+    fn expected_type(&self, _context: &Context) -> Result<Type, ValidationError> {
+        Ok(Type::None)
+    }
+
+    fn validate(&self, _source: &str, _context: &Context) -> Result<(), ValidationError> {
+        Ok(())
+    }
+
+    fn run(&self, _source: &str, _context: &Context) -> Result<Value, RuntimeError> {
+        Ok(Value::none())
+    }
+}
+
+impl Format for Type {
+    fn format(&self, output: &mut String, indent_level: u8) {
+        match self {
+            Type::Any => output.push_str("any"),
+            Type::Boolean => output.push_str("bool"),
+            Type::Collection => output.push_str("collection"),
+            Type::Custom {
+                name: _,
+                arguments: _,
+            } => todo!(),
+            Type::Float => output.push_str("float"),
+            Type::Function {
+                parameter_types,
+                return_type,
+            } => {
+                output.push('(');
+
+                for (index, parameter_type) in parameter_types.iter().enumerate() {
+                    parameter_type.format(output, indent_level);
+
+                    if index != parameter_types.len() - 1 {
+                        output.push(' ');
+                    }
+                }
+
+                output.push_str(") -> ");
+                return_type.format(output, indent_level);
+            }
+            Type::Integer => output.push_str("int"),
+            Type::List => todo!(),
+            Type::ListOf(item_type) => {
+                output.push('[');
+                item_type.format(output, indent_level);
+                output.push(']');
+            }
+            Type::ListExact(_) => todo!(),
+            Type::Map(_) => {
+                output.push_str("map");
+            }
+            Type::None => output.push_str("Option::None"),
+            Type::Number => output.push_str("num"),
+            Type::String => output.push_str("str"),
+            Type::Range => todo!(),
+        }
+    }
+}
+
+impl Display for Type {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        match self {
+            Type::Any => write!(f, "any"),
+            Type::Boolean => write!(f, "bool"),
+            Type::Collection => write!(f, "collection"),
+            Type::Custom { name, arguments } => {
+                if !arguments.is_empty() {
+                    write!(f, "<")?;
+
+                    for (index, r#type) in arguments.into_iter().enumerate() {
+                        if index == arguments.len() - 1 {
+                            write!(f, "{}", r#type)?;
+                        } else {
+                            write!(f, "{}, ", r#type)?;
+                        }
+                    }
+
+                    write!(f, ">")
+                } else {
+                    write!(f, "{name}")
+                }
+            }
+            Type::Float => write!(f, "float"),
+            Type::Function {
+                parameter_types,
+                return_type,
+            } => {
+                write!(f, "(")?;
+
+                for (index, parameter_type) in parameter_types.iter().enumerate() {
+                    write!(f, "{parameter_type}")?;
+
+                    if index != parameter_types.len() - 1 {
+                        write!(f, " ")?;
+                    }
+                }
+
+                write!(f, ")")?;
+                write!(f, " -> {return_type}")
+            }
+            Type::Integer => write!(f, "int"),
+            Type::List => write!(f, "list"),
+            Type::ListOf(item_type) => write!(f, "[{item_type}]"),
+            Type::ListExact(types) => {
+                write!(f, "[")?;
+
+                for (index, r#type) in types.into_iter().enumerate() {
+                    if index == types.len() - 1 {
+                        write!(f, "{}", r#type)?;
+                    } else {
+                        write!(f, "{}, ", r#type)?;
+                    }
+                }
+
+                write!(f, "]")
+            }
+            Type::Map(_) => write!(f, "map"),
+            Type::Number => write!(f, "num"),
+            Type::None => write!(f, "none"),
+            Type::String => write!(f, "str"),
+            Type::Range => todo!(),
+        }
+    }
+}

src/abstract_tree/type_definition.rs

@@ -0,0 +1,73 @@
+use serde::{Deserialize, Serialize};
+use tree_sitter::Node as SyntaxNode;
+
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Context, EnumDefinition, Format, Identifier, StructDefinition, Type, Value,
+};
+
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub enum TypeDefinition {
+    Enum(EnumDefinition),
+    Struct(StructDefinition),
+}
+
+impl TypeDefinition {
+    pub fn identifier(&self) -> &Identifier {
+        match self {
+            TypeDefinition::Enum(enum_definition) => enum_definition.identifier(),
+            TypeDefinition::Struct(_) => todo!(),
+        }
+    }
+}
+
+impl AbstractTree for TypeDefinition {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("type_definition", node)?;
+
+        let child = node.child(0).unwrap();
+
+        match child.kind() {
+            "enum_definition" => Ok(TypeDefinition::Enum(EnumDefinition::from_syntax(
+                child, source, context,
+            )?)),
+            "struct_definition" => Ok(TypeDefinition::Struct(StructDefinition::from_syntax(
+                child, source, context,
+            )?)),
+            _ => Err(SyntaxError::UnexpectedSyntaxNode {
+                expected: "enum or struct definition".to_string(),
+                actual: child.kind().to_string(),
+                position: node.range().into(),
+            }),
+        }
+    }
+
+    fn expected_type(&self, _context: &Context) -> Result<Type, ValidationError> {
+        match self {
+            TypeDefinition::Enum(enum_definition) => enum_definition.expected_type(_context),
+            TypeDefinition::Struct(struct_definition) => struct_definition.expected_type(_context),
+        }
+    }
+
+    fn validate(&self, _source: &str, _context: &Context) -> Result<(), ValidationError> {
+        match self {
+            TypeDefinition::Enum(enum_definition) => enum_definition.validate(_source, _context),
+            TypeDefinition::Struct(struct_definition) => {
+                struct_definition.validate(_source, _context)
+            }
+        }
+    }
+
+    fn run(&self, _source: &str, _context: &Context) -> Result<Value, RuntimeError> {
+        match self {
+            TypeDefinition::Enum(enum_definition) => enum_definition.run(_source, _context),
+            TypeDefinition::Struct(struct_definition) => struct_definition.run(_source, _context),
+        }
+    }
+}
+
+impl Format for TypeDefinition {
+    fn format(&self, _output: &mut String, _indent_level: u8) {
+        todo!()
+    }
+}

src/abstract_tree/type_specification.rs

@@ -0,0 +1,56 @@
+use serde::{Deserialize, Serialize};
+
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Context, Format, SyntaxNode, Type, Value,
+};
+
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub struct TypeSpecification {
+    r#type: Type,
+}
+
+impl TypeSpecification {
+    pub fn new(r#type: Type) -> Self {
+        Self { r#type }
+    }
+
+    pub fn inner(&self) -> &Type {
+        &self.r#type
+    }
+
+    pub fn take_inner(self) -> Type {
+        self.r#type
+    }
+}
+
+impl AbstractTree for TypeSpecification {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("type_specification", node)?;
+
+        let type_node = node.child(1).unwrap();
+        let r#type = Type::from_syntax(type_node, source, context)?;
+
+        Ok(TypeSpecification { r#type })
+    }
+
+    fn expected_type(&self, context: &Context) -> Result<Type, ValidationError> {
+        self.r#type.expected_type(context)
+    }
+
+    fn validate(&self, _source: &str, _context: &Context) -> Result<(), ValidationError> {
+        Ok(())
+    }
+
+    fn run(&self, source: &str, context: &Context) -> Result<Value, RuntimeError> {
+        self.r#type.run(source, context)
+    }
+}
+
+impl Format for TypeSpecification {
+    fn format(&self, output: &mut String, indent_level: u8) {
+        output.push('<');
+        self.r#type.format(output, indent_level);
+        output.push('>');
+    }
+}

src/abstract_tree/value_node.rs

@@ -0,0 +1,357 @@
+use std::{cmp::Ordering, ops::RangeInclusive};
+
+use serde::{Deserialize, Serialize};
+use tree_sitter::Node as SyntaxNode;
+
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Context, Expression, Format, Function, FunctionNode,
+    Identifier, List, Type, Value, TypeDefinition, MapNode,
+};
+
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq)]
+pub enum ValueNode {
+    Boolean(String),
+    Float(String),
+    Function(Function),
+    Integer(String),
+    String(String),
+    List(Vec<Expression>),
+    Map(MapNode),
+    Range(RangeInclusive<i64>),
+    Struct {
+        name: Identifier,
+        properties: MapNode,
+    },
+    Enum {
+        name: Identifier,
+        variant: Identifier,
+        expression: Option<Box<Expression>>,
+    },
+}
+
+impl AbstractTree for ValueNode {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("value", node)?;
+
+        let child = node.child(0).unwrap();
+        let value_node = match child.kind() {
+            "boolean" => ValueNode::Boolean(source[child.byte_range()].to_string()),
+            "float" => ValueNode::Float(source[child.byte_range()].to_string()),
+            "function" => {
+                let function_node = FunctionNode::from_syntax(child, source, context)?;
+
+                ValueNode::Function(Function::ContextDefined(function_node))
+            }
+            "integer" => ValueNode::Integer(source[child.byte_range()].to_string()),
+            "string" => {
+                let without_quotes = child.start_byte() + 1..child.end_byte() - 1;
+
+                ValueNode::String(source[without_quotes].to_string())
+            }
+            "list" => {
+                let mut expressions = Vec::new();
+
+                for index in 1..child.child_count() - 1 {
+                    let current_node = child.child(index).unwrap();
+
+                    if current_node.is_named() {
+                        let expression = Expression::from_syntax(current_node, source, context)?;
+
+                        expressions.push(expression);
+                    }
+                }
+
+                ValueNode::List(expressions)
+            }
+            "map" => {
+                ValueNode::Map(MapNode::from_syntax(child, source, context)?)
+            }
+            "range" => {
+                let mut split = source[child.byte_range()].split("..");
+                let start = split.next().unwrap().parse().unwrap();
+                let end = split.next().unwrap().parse().unwrap();
+
+                ValueNode::Range(start..=end)
+            }
+            "enum_instance" => {
+                let name_node = child.child(0).unwrap();
+                let name = Identifier::from_syntax(name_node, source, context)?;
+                
+                let variant_node = child.child(2).unwrap();
+                let variant = Identifier::from_syntax(variant_node, source, context)?;
+             
+                let expression = if let Some(expression_node) = child.child(4) {                
+                    Some(Box::new(Expression::from_syntax(expression_node, source, context)?))
+                } else {
+                    None
+                };
+
+                ValueNode::Enum { name, variant , expression  }                
+            }
+            "struct_instance" => {
+                let name_node = child.child(0).unwrap();
+                let name = Identifier::from_syntax(name_node, source, context)?;
+
+                let properties_node = child.child(2).unwrap();
+                let properties = MapNode::from_syntax(properties_node, source, context)?;
+
+                ValueNode::Struct
+                {
+                    name,
+                    properties
+                }
+            }
+            _ => {
+                return Err(SyntaxError::UnexpectedSyntaxNode {
+                    expected:
+                        "string, integer, float, boolean, range, list, map, option, function, struct or enum"
+                            .to_string(),
+                    actual: child.kind().to_string(),
+                    position: node.range().into(),
+                })
+            }
+        };
+
+        Ok(value_node)
+    }
+
+    fn expected_type(&self, context: &Context) -> Result<Type, ValidationError> {
+        let r#type = match self {
+            ValueNode::Boolean(_) => Type::Boolean,
+            ValueNode::Float(_) => Type::Float,
+            ValueNode::Function(function) => function.r#type(),
+            ValueNode::Integer(_) => Type::Integer,
+            ValueNode::String(_) => Type::String,
+            ValueNode::List(expressions) => {
+                let mut item_types = Vec::new();
+
+                for expression in expressions {
+                    let expression_type = expression.expected_type(context)?;
+
+                    item_types.push(expression_type);
+                }
+
+                Type::ListExact(item_types)
+            }
+            ValueNode::Map(map_node) => map_node.expected_type(context)?,
+            ValueNode::Struct { name, .. }  => {
+                Type::custom(name.clone(), Vec::with_capacity(0))
+            }
+            ValueNode::Range(_) => Type::Range,
+            ValueNode::Enum { name, variant, expression: _ } => {
+                let types: Vec<Type> = if let Some(type_definition) = context.get_definition(name)? {
+                    if let TypeDefinition::Enum(enum_definition) = type_definition {
+                        let types = enum_definition.variants().into_iter().find_map(|(identifier, types)| {
+                            if identifier == variant {
+                                Some(types.clone())
+                            } else {
+                                None
+                            }
+                        });
+
+                        if let Some(types) = types {
+                            types 
+                        } else {
+                            return Err(ValidationError::VariableIdentifierNotFound(variant.clone()));
+                        }
+                         
+                    } else {
+                        return Err(ValidationError::ExpectedEnumDefintion { actual: type_definition.clone() });
+                    }
+                } else {
+                     return Err(ValidationError::VariableIdentifierNotFound(name.clone()));
+                };
+
+                Type::custom(name.clone(), types.clone())
+                
+            },
+        };
+
+        Ok(r#type)
+    }
+
+    fn validate(&self, _source: &str, context: &Context) -> Result<(), ValidationError> {
+        match self {
+            ValueNode::Function(function) => {
+                if let Function::ContextDefined(function_node) = function {
+                    function_node.validate(_source, context)?;
+                }
+            }
+            ValueNode::Map(map_node) => map_node.validate(_source, context)?,
+            ValueNode::Enum { name, expression, .. } => {
+                name.validate(_source, context)?;
+
+                if let Some(expression) = expression {
+                    expression.validate(_source, context)?;
+                }
+            }
+            _ => {},
+        }
+
+        Ok(())
+    }
+
+    fn run(&self, source: &str, context: &Context) -> Result<Value, RuntimeError> {
+        let value = match self {
+            ValueNode::Boolean(value_source) => Value::Boolean(value_source.parse().unwrap()),
+            ValueNode::Float(value_source) => {
+                let float = value_source.parse()?;
+
+                Value::Float(float)
+            }
+            ValueNode::Function(function) => Value::Function(function.clone()),
+            ValueNode::Integer(value_source) => Value::Integer(value_source.parse().unwrap()),
+            ValueNode::String(value_source) => Value::string(value_source.clone()),
+            ValueNode::List(expressions) => {
+                let mut values = Vec::with_capacity(expressions.len());
+
+                for node in expressions {
+                    let value = node.run(source, context)?;
+
+                    values.push(value);
+                }
+
+                Value::List(List::with_items(values))
+            }
+            ValueNode::Map(map_node) => map_node.run(source, context)?,
+            ValueNode::Range(range) => Value::Range(range.clone()),
+            ValueNode::Struct { name, properties } => {
+                let instance = if let Some(definition) = context.get_definition(name)? {
+                    if let TypeDefinition::Struct(struct_definition) = definition {
+                        struct_definition.instantiate(properties, source, context)?
+                    } else {
+                        return Err(RuntimeError::ValidationFailure(ValidationError::ExpectedStructDefintion { actual: definition.clone() }))
+                    }
+                } else {
+                    return Err(RuntimeError::ValidationFailure(
+                        ValidationError::TypeDefinitionNotFound(name.clone())
+                    ));
+                };
+
+                Value::Struct(instance)
+
+            }
+            ValueNode::Enum { name, variant, expression } => {
+                let value = if let Some(expression) = expression {
+                    expression.run(source, context)?
+                } else {
+                    Value::none()
+                };
+                let instance = if let Some(definition) = context.get_definition(name)? {
+                    if let TypeDefinition::Enum(enum_defintion) = definition {
+                        enum_defintion.instantiate(variant.clone(), Some(value))
+                    } else {
+                        return Err(RuntimeError::ValidationFailure(
+                            ValidationError::ExpectedEnumDefintion {
+                                actual: definition.clone()
+                            }
+                        ));
+                    }
+                } else {
+                    return Err(RuntimeError::ValidationFailure(
+                        ValidationError::TypeDefinitionNotFound(name.clone())
+                    ));
+                };
+
+                Value::Enum(instance)                
+            },
+        };
+
+        Ok(value)
+    }
+}
+
+impl Format for ValueNode {
+    fn format(&self, output: &mut String, indent_level: u8) {
+        match self {
+            ValueNode::Boolean(source) | ValueNode::Float(source) | ValueNode::Integer(source) => {
+                output.push_str(source)
+            }
+            ValueNode::String(source) => {
+                output.push('\'');
+                output.push_str(source);
+                output.push('\'');
+            }
+            ValueNode::Function(function) => function.format(output, indent_level),
+            ValueNode::List(expressions) => {
+                output.push('[');
+
+                for expression in expressions {
+                    expression.format(output, indent_level);
+                }
+
+                output.push(']');
+            }
+            ValueNode::Map(map_node) => map_node.format(output, indent_level),
+            ValueNode::Struct { name, properties } => {
+                name.format(output, indent_level);
+                properties.format(output, indent_level);
+            }
+            ValueNode::Range(_) => todo!(),
+            ValueNode::Enum { ..  } => todo!(),
+        }
+    }
+}
+
+impl Ord for ValueNode {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        match (self, other) {
+            (ValueNode::Boolean(left), ValueNode::Boolean(right)) => left.cmp(right),
+            (ValueNode::Boolean(_), _) => Ordering::Greater,
+            (ValueNode::Float(left), ValueNode::Float(right)) => left.cmp(right),
+            (ValueNode::Float(_), _) => Ordering::Greater,
+            (ValueNode::Function(left), ValueNode::Function(right)) => left.cmp(right),
+            (ValueNode::Function(_), _) => Ordering::Greater,
+            (ValueNode::Integer(left), ValueNode::Integer(right)) => left.cmp(right),
+            (ValueNode::Integer(_), _) => Ordering::Greater,
+            (ValueNode::String(left), ValueNode::String(right)) => left.cmp(right),
+            (ValueNode::String(_), _) => Ordering::Greater,
+            (ValueNode::List(left), ValueNode::List(right)) => left.cmp(right),
+            (ValueNode::List(_), _) => Ordering::Greater,
+            (ValueNode::Map(left), ValueNode::Map(right)) => left.cmp(right),
+            (ValueNode::Map(_), _) => Ordering::Greater,
+            (ValueNode::Struct{ name: left_name, properties: left_properties }, ValueNode::Struct {name: right_name, properties: right_properties} ) => {
+                let name_cmp = left_name.cmp(right_name);
+
+                if name_cmp.is_eq() {
+                    left_properties.cmp(right_properties)
+                } else {
+                    name_cmp
+                }
+            },
+            (ValueNode::Struct {..}, _) => Ordering::Greater,
+            (
+                ValueNode::Enum {
+                    name: left_name, variant: left_variant, expression: left_expression
+                },
+                ValueNode::Enum {
+                    name: right_name, variant: right_variant, expression: right_expression
+                }
+            ) => {
+                let name_cmp = left_name.cmp(right_name);
+
+                if name_cmp.is_eq() {
+                    let variant_cmp = left_variant.cmp(right_variant);
+
+                    if variant_cmp.is_eq() {
+                        left_expression.cmp(right_expression)
+                    } else {
+                        variant_cmp
+                    }
+                } else {
+                    name_cmp
+                }
+            },
+            (ValueNode::Enum { .. }, _) => Ordering::Greater,
+            (ValueNode::Range(left), ValueNode::Range(right)) => left.clone().cmp(right.clone()),
+            (ValueNode::Range(_), _) => Ordering::Less,
+        }
+    }
+}
+
+impl PartialOrd for ValueNode {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}

src/abstract_tree/while.rs

@@ -0,0 +1,64 @@
+use serde::{Deserialize, Serialize};
+
+use crate::{
+    error::{RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Block, Context, Expression, Format, SyntaxNode, Type, Value,
+};
+
+/// Abstract representation of a while loop.
+///
+/// While executes its block repeatedly until its expression evaluates to true.
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+pub struct While {
+    expression: Expression,
+    block: Block,
+}
+
+impl AbstractTree for While {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("while", node)?;
+
+        let expression_node = node.child(1).unwrap();
+        let expression = Expression::from_syntax(expression_node, source, context)?;
+
+        let block_node = node.child(2).unwrap();
+        let block = Block::from_syntax(block_node, source, context)?;
+
+        Ok(While { expression, block })
+    }
+
+    fn expected_type(&self, context: &Context) -> Result<Type, ValidationError> {
+        self.block.expected_type(context)
+    }
+
+    fn validate(&self, _source: &str, context: &Context) -> Result<(), ValidationError> {
+        log::info!("VALIDATE while loop");
+
+        self.expression.validate(_source, context)?;
+        self.block.validate(_source, context)
+    }
+
+    fn run(&self, source: &str, context: &Context) -> Result<Value, RuntimeError> {
+        log::info!("RUN while loop start");
+
+        while self.expression.run(source, context)?.as_boolean()? {
+            self.block.run(source, context)?;
+        }
+
+        log::info!("RUN while loop end");
+
+        Ok(Value::none())
+    }
+}
+
+impl Format for While {
+    fn format(&self, output: &mut String, indent_level: u8) {
+        output.push('\n');
+        While::indent(output, indent_level);
+        output.push_str("while ");
+        self.expression.format(output, indent_level);
+        output.push(' ');
+        self.block.format(output, indent_level);
+        output.push('\n');
+    }
+}

src/bin/dust.rs

@@ -1,168 +0,0 @@
-//! Command line interface for the whale programming language.
-use clap::Parser;
-use rustyline::{
-    completion::FilenameCompleter,
-    error::ReadlineError,
-    highlight::Highlighter,
-    hint::{Hint, Hinter, HistoryHinter},
-    history::DefaultHistory,
-    Completer, Context, Editor, Helper, Validator,
-};
-
-use std::{borrow::Cow, fs::read_to_string};
-
-use dust_lib::{eval, eval_with_context, Value, VariableMap, TOOL_LIST};
-
-/// Command-line arguments to be parsed.
-#[derive(Parser, Debug)]
-#[command(author, version, about, long_about = None)]
-struct Args {
-    /// Whale source code to evaluate.
-    #[arg(short, long)]
-    command: Option<String>,
-
-    /// Location of the file to run.
-    path: Option<String>,
-}
-
-fn main() {
-    let args = Args::parse();
-
-    if args.path.is_none() && args.command.is_none() {
-        return run_cli_shell();
-    }
-
-    let eval_result = if let Some(path) = args.path {
-        let file_contents = read_to_string(path).unwrap();
-
-        eval(&file_contents)
-    } else if let Some(command) = args.command {
-        eval(&command)
-    } else {
-        Ok(Value::Empty)
-    };
-
-    match eval_result {
-        Ok(value) => println!("{value}"),
-        Err(error) => eprintln!("{error}"),
-    }
-}
-
-#[derive(Helper, Completer, Validator)]
-struct DustReadline {
-    #[rustyline(Completer)]
-    completer: FilenameCompleter,
-
-    tool_hints: Vec<ToolHint>,
-
-    #[rustyline(Hinter)]
-    _hinter: HistoryHinter,
-}
-
-impl DustReadline {
-    fn new() -> Self {
-        Self {
-            completer: FilenameCompleter::new(),
-            _hinter: HistoryHinter {},
-            tool_hints: TOOL_LIST
-                .iter()
-                .map(|tool| ToolHint {
-                    display: tool.info().identifier.to_string() + "()",
-                    complete_to: tool.info().identifier.len(),
-                })
-                .collect(),
-        }
-    }
-}
-
-struct ToolHint {
-    display: String,
-    complete_to: usize,
-}
-
-impl Hint for ToolHint {
-    fn display(&self) -> &str {
-        &self.display
-    }
-
-    fn completion(&self) -> Option<&str> {
-        if self.complete_to > 0 {
-            Some(&self.display[..self.complete_to])
-        } else {
-            None
-        }
-    }
-}
-
-impl ToolHint {
-    fn suffix(&self, strip_chars: usize) -> ToolHint {
-        ToolHint {
-            display: self.display[strip_chars..].to_string(),
-            complete_to: self.complete_to.saturating_sub(strip_chars),
-        }
-    }
-}
-
-impl Hinter for DustReadline {
-    type Hint = ToolHint;
-
-    fn hint(&self, line: &str, pos: usize, _ctx: &Context<'_>) -> Option<Self::Hint> {
-        if line.is_empty() || pos < line.len() {
-            return None;
-        }
-
-        self.tool_hints.iter().find_map(|tool_hint| {
-            if tool_hint.display.starts_with(line) {
-                Some(tool_hint.suffix(pos))
-            } else {
-                None
-            }
-        })
-    }
-}
-
-impl Highlighter for DustReadline {
-    fn highlight_hint<'h>(&self, hint: &'h str) -> std::borrow::Cow<'h, str> {
-        let highlighted = ansi_term::Colour::Red.paint(hint).to_string();
-
-        Cow::Owned(highlighted)
-    }
-}
-
-fn run_cli_shell() {
-    let mut context = VariableMap::new();
-    let mut rl: Editor<DustReadline, DefaultHistory> = Editor::new().unwrap();
-
-    rl.set_helper(Some(DustReadline::new()));
-
-    if rl.load_history("target/history.txt").is_err() {
-        println!("No previous history.");
-    }
-
-    loop {
-        let readline = rl.readline("* ");
-        match readline {
-            Ok(line) => {
-                let line = line.as_str();
-
-                rl.add_history_entry(line).unwrap();
-
-                let eval_result = eval_with_context(line, &mut context);
-
-                match eval_result {
-                    Ok(value) => println!("{value}"),
-                    Err(error) => eprintln!("{error}"),
-                }
-            }
-            Err(ReadlineError::Interrupted) => {
-                break;
-            }
-            Err(ReadlineError::Eof) => {
-                break;
-            }
-            Err(error) => eprintln!("{error}"),
-        }
-    }
-
-    rl.save_history("target/history.txt").unwrap();
-}

src/bin/gui.rs

@@ -1,88 +0,0 @@
-use dust_lib::eval;
-use iced::widget::{column, container, text_input, Column};
-use iced::{executor, Application, Command, Element, Settings, Theme};
-use once_cell::sync::Lazy;
-
-static INPUT_ID: Lazy<text_input::Id> = Lazy::new(text_input::Id::unique);
-
-pub fn main() -> iced::Result {
-    DustGui::run(Settings::default())
-}
-
-struct DustGui {
-    text_buffer: String,
-    results: Vec<String>,
-}
-
-impl Application for DustGui {
-    type Executor = executor::Default;
-
-    type Message = Message;
-
-    type Theme = Theme;
-
-    type Flags = ();
-
-    fn new(_flags: Self::Flags) -> (Self, iced::Command<Self::Message>) {
-        (
-            DustGui {
-                text_buffer: String::new(),
-                results: Vec::new(),
-            },
-            Command::none(),
-        )
-    }
-
-    fn title(&self) -> String {
-        "Dust".to_string()
-    }
-
-    fn update(&mut self, message: Self::Message) -> iced::Command<Self::Message> {
-        match message {
-            Message::TextInput(input) => {
-                self.text_buffer = input;
-
-                Command::none()
-            }
-            Message::Evaluate => {
-                let eval_result = eval(&self.text_buffer);
-
-                match eval_result {
-                    Ok(result) => self.results.push(result.to_string()),
-                    Err(error) => self.results.push(error.to_string()),
-                }
-
-                Command::batch(vec![])
-            }
-        }
-    }
-
-    fn view(&self) -> Element<'_, Self::Message, iced::Renderer<Self::Theme>> {
-        let input = text_input("What needs to be done?", &self.text_buffer)
-            .id(INPUT_ID.clone())
-            .on_input(Message::TextInput)
-            .on_submit(Message::Evaluate)
-            .padding(15)
-            .size(30);
-
-        let result_display: Column<Message> = {
-            let mut text_widgets = Vec::new();
-
-            for _result in &self.results {
-                // text_widgets.push(text(result).style().into());
-            }
-
-            text_widgets.reverse();
-
-            Column::with_children(text_widgets)
-        };
-
-        container(column![input, result_display]).into()
-    }
-}
-
-#[derive(Debug, Clone)]
-enum Message {
-    TextInput(String),
-    Evaluate,
-}

src/built_in_functions/fs.rs

@@ -0,0 +1,59 @@
+use std::{fs::File, io::Read};
+
+use enum_iterator::{all, Sequence};
+use serde::{Deserialize, Serialize};
+
+use crate::{error::RuntimeError, Context, Type, Value};
+
+use super::Callable;
+
+pub fn fs_functions() -> impl Iterator<Item = Fs> {
+    all()
+}
+
+#[derive(Sequence, Debug, Copy, Clone, Serialize, Deserialize, PartialEq, Eq, PartialOrd, Ord)]
+pub enum Fs {
+    ReadFile,
+}
+
+impl Callable for Fs {
+    fn name(&self) -> &'static str {
+        match self {
+            Fs::ReadFile => "read_file",
+        }
+    }
+
+    fn description(&self) -> &'static str {
+        match self {
+            Fs::ReadFile => "Read the contents of a file to a string.",
+        }
+    }
+
+    fn r#type(&self) -> Type {
+        match self {
+            Fs::ReadFile => Type::function(vec![Type::String], Type::String),
+        }
+    }
+
+    fn call(
+        &self,
+        arguments: &[Value],
+        _source: &str,
+        _outer_context: &Context,
+    ) -> Result<Value, RuntimeError> {
+        match self {
+            Fs::ReadFile => {
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
+
+                let path = arguments.first().unwrap().as_string()?;
+                let mut file = File::open(path)?;
+                let file_size = file.metadata()?.len() as usize;
+                let mut file_content = String::with_capacity(file_size);
+
+                file.read_to_string(&mut file_content)?;
+
+                Ok(Value::string(file_content))
+            }
+        }
+    }
+}

src/built_in_functions/json.rs

@@ -0,0 +1,77 @@
+use enum_iterator::Sequence;
+use serde::{Deserialize, Serialize};
+
+use crate::{error::RuntimeError, Context, Type, Value};
+
+use super::Callable;
+
+pub fn json_functions() -> impl Iterator<Item = Json> {
+    enum_iterator::all()
+}
+
+#[derive(Sequence, Debug, Copy, Clone, Serialize, Deserialize, PartialEq, Eq, PartialOrd, Ord)]
+pub enum Json {
+    Create,
+    CreatePretty,
+    Parse,
+}
+
+impl Callable for Json {
+    fn name(&self) -> &'static str {
+        match self {
+            Json::Create => "create",
+            Json::CreatePretty => "create_pretty",
+            Json::Parse => "parse",
+        }
+    }
+
+    fn description(&self) -> &'static str {
+        match self {
+            Json::Create => "Convert a value to a JSON string.",
+            Json::CreatePretty => "Convert a value to a formatted JSON string.",
+            Json::Parse => "Convert JSON to a value",
+        }
+    }
+
+    fn r#type(&self) -> Type {
+        match self {
+            Json::Create => Type::function(vec![Type::Any], Type::String),
+            Json::CreatePretty => Type::function(vec![Type::Any], Type::String),
+            Json::Parse => Type::function(vec![Type::String], Type::Any),
+        }
+    }
+
+    fn call(
+        &self,
+        arguments: &[Value],
+        _source: &str,
+        _outer_context: &Context,
+    ) -> Result<Value, RuntimeError> {
+        match self {
+            Json::Create => {
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
+
+                let value = arguments.first().unwrap();
+                let json_string = serde_json::to_string(value)?;
+
+                Ok(Value::String(json_string))
+            }
+            Json::CreatePretty => {
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
+
+                let value = arguments.first().unwrap();
+                let json_string = serde_json::to_string_pretty(value)?;
+
+                Ok(Value::String(json_string))
+            }
+            Json::Parse => {
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
+
+                let json_string = arguments.first().unwrap().as_string()?;
+                let value = serde_json::from_str(json_string)?;
+
+                Ok(value)
+            }
+        }
+    }
+}

src/built_in_functions/mod.rs

@@ -0,0 +1,198 @@
+pub mod fs;
+pub mod json;
+pub mod str;
+
+use std::fmt::{self, Display, Formatter};
+
+use rand::{random, thread_rng, Rng};
+use serde::{Deserialize, Serialize};
+
+use crate::{
+    error::{RuntimeError, ValidationError},
+    Context, EnumInstance, Format, Identifier, Type, Value,
+};
+
+use self::{fs::Fs, json::Json, str::StrFunction};
+
+pub trait Callable {
+    fn name(&self) -> &'static str;
+    fn description(&self) -> &'static str;
+    fn r#type(&self) -> Type;
+    fn call(
+        &self,
+        arguments: &[Value],
+        source: &str,
+        context: &Context,
+    ) -> Result<Value, RuntimeError>;
+}
+
+#[derive(Debug, Copy, Clone, Serialize, Deserialize, PartialEq, Eq, PartialOrd, Ord)]
+pub enum BuiltInFunction {
+    AssertEqual,
+    Fs(Fs),
+    Json(Json),
+    Length,
+    Output,
+    RandomBoolean,
+    RandomFloat,
+    RandomFrom,
+    RandomInteger,
+    String(StrFunction),
+}
+
+impl Callable for BuiltInFunction {
+    fn name(&self) -> &'static str {
+        match self {
+            BuiltInFunction::AssertEqual => "assert_equal",
+            BuiltInFunction::Fs(fs_function) => fs_function.name(),
+            BuiltInFunction::Json(json_function) => json_function.name(),
+            BuiltInFunction::Length => "length",
+            BuiltInFunction::Output => "output",
+            BuiltInFunction::RandomBoolean => "boolean",
+            BuiltInFunction::RandomFloat => "float",
+            BuiltInFunction::RandomFrom => "from",
+            BuiltInFunction::RandomInteger => "integer",
+            BuiltInFunction::String(string_function) => string_function.name(),
+        }
+    }
+
+    fn description(&self) -> &'static str {
+        match self {
+            BuiltInFunction::AssertEqual => "assert_equal",
+            BuiltInFunction::Fs(fs_function) => fs_function.description(),
+            BuiltInFunction::Json(json_function) => json_function.description(),
+            BuiltInFunction::Length => "length",
+            BuiltInFunction::Output => "output",
+            BuiltInFunction::RandomBoolean => "boolean",
+            BuiltInFunction::RandomFloat => "float",
+            BuiltInFunction::RandomFrom => "from",
+            BuiltInFunction::RandomInteger => "integer",
+            BuiltInFunction::String(string_function) => string_function.description(),
+        }
+    }
+
+    fn r#type(&self) -> Type {
+        match self {
+            BuiltInFunction::AssertEqual => Type::function(vec![Type::Any, Type::Any], Type::None),
+            BuiltInFunction::Fs(fs_function) => fs_function.r#type(),
+            BuiltInFunction::Json(json_function) => json_function.r#type(),
+            BuiltInFunction::Length => Type::function(vec![Type::Collection], Type::Integer),
+            BuiltInFunction::Output => Type::function(vec![Type::Any], Type::None),
+            BuiltInFunction::RandomBoolean => Type::function(vec![], Type::Boolean),
+            BuiltInFunction::RandomFloat => Type::function(vec![], Type::Float),
+            BuiltInFunction::RandomFrom => Type::function(vec![Type::Collection], Type::Any),
+            BuiltInFunction::RandomInteger => Type::function(vec![], Type::Integer),
+            BuiltInFunction::String(string_function) => string_function.r#type(),
+        }
+    }
+
+    fn call(
+        &self,
+        arguments: &[Value],
+        _source: &str,
+        context: &Context,
+    ) -> Result<Value, RuntimeError> {
+        match self {
+            BuiltInFunction::AssertEqual => {
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
+
+                let left = arguments.get(0).unwrap();
+                let right = arguments.get(1).unwrap();
+
+                if left == right {
+                    Ok(Value::Enum(EnumInstance::new(
+                        Identifier::new("Result"),
+                        Identifier::new("Ok"),
+                        Some(Value::none()),
+                    )))
+                } else {
+                    Err(RuntimeError::AssertEqualFailed {
+                        left: left.clone(),
+                        right: right.clone(),
+                    })
+                }
+            }
+            BuiltInFunction::Fs(fs_function) => fs_function.call(arguments, _source, context),
+            BuiltInFunction::Json(json_function) => json_function.call(arguments, _source, context),
+            BuiltInFunction::Length => {
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
+
+                let value = arguments.first().unwrap();
+                let length = if let Ok(list) = value.as_list() {
+                    list.items()?.len()
+                } else if let Ok(map) = value.as_map() {
+                    map.inner().len()
+                } else if let Ok(str) = value.as_string() {
+                    str.chars().count()
+                } else {
+                    return Err(RuntimeError::ValidationFailure(
+                        ValidationError::ExpectedCollection {
+                            actual: value.clone(),
+                        },
+                    ));
+                };
+
+                Ok(Value::Integer(length as i64))
+            }
+            BuiltInFunction::Output => {
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
+
+                let value = arguments.first().unwrap();
+
+                println!("{value}");
+
+                Ok(Value::none())
+            }
+            BuiltInFunction::RandomBoolean => {
+                RuntimeError::expect_argument_amount(self.name(), 0, arguments.len())?;
+
+                Ok(Value::Boolean(random()))
+            }
+            BuiltInFunction::RandomFloat => {
+                RuntimeError::expect_argument_amount(self.name(), 0, arguments.len())?;
+
+                Ok(Value::Float(random()))
+            }
+            BuiltInFunction::RandomFrom => {
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
+
+                let value = arguments.first().unwrap();
+
+                if let Ok(list) = value.as_list() {
+                    let items = list.items()?;
+
+                    if items.len() == 0 {
+                        Ok(Value::none())
+                    } else {
+                        let random_index = thread_rng().gen_range(0..items.len());
+                        let random_value = items.get(random_index).cloned().unwrap_or_default();
+
+                        Ok(random_value)
+                    }
+                } else {
+                    todo!()
+                }
+            }
+            BuiltInFunction::RandomInteger => {
+                RuntimeError::expect_argument_amount(self.name(), 0, arguments.len())?;
+
+                Ok(Value::Integer(random()))
+            }
+            BuiltInFunction::String(string_function) => {
+                string_function.call(arguments, _source, context)
+            }
+        }
+    }
+}
+
+impl Format for BuiltInFunction {
+    fn format(&self, output: &mut String, _indent_level: u8) {
+        output.push_str(self.name());
+    }
+}
+
+impl Display for BuiltInFunction {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        write!(f, "{}", self.name())
+    }
+}

src/built_in_functions/str.rs

@@ -0,0 +1,591 @@
+use enum_iterator::Sequence;
+use serde::{Deserialize, Serialize};
+
+use crate::{error::RuntimeError, Context, EnumInstance, Identifier, List, Type, Value};
+
+use super::Callable;
+
+pub fn string_functions() -> impl Iterator<Item = StrFunction> {
+    enum_iterator::all()
+}
+
+#[derive(Sequence, Debug, Copy, Clone, Serialize, Deserialize, PartialEq, Eq, PartialOrd, Ord)]
+pub enum StrFunction {
+    AsBytes,
+    EndsWith,
+    Find,
+    Insert,
+    IsAscii,
+    IsEmpty,
+    Lines,
+    Matches,
+    Parse,
+    Remove,
+    ReplaceRange,
+    Retain,
+    Split,
+    SplitAt,
+    SplitInclusive,
+    SplitN,
+    SplitOnce,
+    SplitTerminator,
+    SplitWhitespace,
+    StartsWith,
+    StripPrefix,
+    ToLowercase,
+    ToUppercase,
+    Trim,
+    TrimEnd,
+    TrimEndMatches,
+    TrimMatches,
+    TrimStart,
+    TrimStartMatches,
+    Truncate,
+}
+
+impl Callable for StrFunction {
+    fn name(&self) -> &'static str {
+        match self {
+            StrFunction::AsBytes => "as_bytes",
+            StrFunction::EndsWith => "ends_with",
+            StrFunction::Find => "find",
+            StrFunction::Insert => "insert",
+            StrFunction::IsAscii => "is_ascii",
+            StrFunction::IsEmpty => "is_empty",
+            StrFunction::Lines => "lines",
+            StrFunction::Matches => "matches",
+            StrFunction::Parse => "parse",
+            StrFunction::Remove => "remove",
+            StrFunction::ReplaceRange => "replace_range",
+            StrFunction::Retain => "retain",
+            StrFunction::Split => "split",
+            StrFunction::SplitAt => "split_at",
+            StrFunction::SplitInclusive => "split_inclusive",
+            StrFunction::SplitN => "split_n",
+            StrFunction::SplitOnce => "split_once",
+            StrFunction::SplitTerminator => "split_terminator",
+            StrFunction::SplitWhitespace => "split_whitespace",
+            StrFunction::StartsWith => "starts_with",
+            StrFunction::StripPrefix => "strip_prefix",
+            StrFunction::ToLowercase => "to_lowercase",
+            StrFunction::ToUppercase => "to_uppercase",
+            StrFunction::Trim => "trim",
+            StrFunction::TrimEnd => "trim_end",
+            StrFunction::TrimEndMatches => "trim_end_matches",
+            StrFunction::TrimMatches => "trim_matches",
+            StrFunction::TrimStart => "trim_start",
+            StrFunction::TrimStartMatches => "trim_start_matches",
+            StrFunction::Truncate => "truncate",
+        }
+    }
+
+    fn description(&self) -> &'static str {
+        match self {
+            StrFunction::AsBytes => "TODO",
+            StrFunction::EndsWith => "TODO",
+            StrFunction::Find => "TODO",
+            StrFunction::Insert => "TODO",
+            StrFunction::IsAscii => "TODO",
+            StrFunction::IsEmpty => "TODO",
+            StrFunction::Lines => "TODO",
+            StrFunction::Matches => "TODO",
+            StrFunction::Parse => "TODO",
+            StrFunction::Remove => "TODO",
+            StrFunction::ReplaceRange => "TODO",
+            StrFunction::Retain => "TODO",
+            StrFunction::Split => "TODO",
+            StrFunction::SplitAt => "TODO",
+            StrFunction::SplitInclusive => "TODO",
+            StrFunction::SplitN => "TODO",
+            StrFunction::SplitOnce => "TODO",
+            StrFunction::SplitTerminator => "TODO",
+            StrFunction::SplitWhitespace => "TODO",
+            StrFunction::StartsWith => "TODO",
+            StrFunction::StripPrefix => "TODO",
+            StrFunction::ToLowercase => "TODO",
+            StrFunction::ToUppercase => "TODO",
+            StrFunction::Trim => "TODO",
+            StrFunction::TrimEnd => "TODO",
+            StrFunction::TrimEndMatches => "TODO",
+            StrFunction::TrimMatches => "TODO",
+            StrFunction::TrimStart => "TODO",
+            StrFunction::TrimStartMatches => "TODO",
+            StrFunction::Truncate => "TODO",
+        }
+    }
+
+    fn r#type(&self) -> Type {
+        match self {
+            StrFunction::AsBytes => Type::function(vec![Type::String], Type::list(Type::Integer)),
+            StrFunction::EndsWith => {
+                Type::function(vec![Type::String, Type::String], Type::Boolean)
+            }
+            StrFunction::Find => Type::function(
+                vec![Type::String, Type::String],
+                Type::option(Some(Type::Integer)),
+            ),
+            StrFunction::Insert => Type::function(
+                vec![Type::String, Type::Integer, Type::String],
+                Type::String,
+            ),
+            StrFunction::IsAscii => Type::function(vec![Type::String], Type::Boolean),
+            StrFunction::IsEmpty => Type::function(vec![Type::String], Type::Boolean),
+            StrFunction::Lines => Type::function(vec![Type::String], Type::list(Type::String)),
+            StrFunction::Matches => {
+                Type::function(vec![Type::String, Type::String], Type::list(Type::String))
+            }
+            StrFunction::Parse => Type::function(vec![Type::String], Type::Any),
+            StrFunction::Remove => Type::function(
+                vec![Type::String, Type::Integer],
+                Type::option(Some(Type::String)),
+            ),
+            StrFunction::ReplaceRange => Type::function(
+                vec![Type::String, Type::list(Type::Integer), Type::String],
+                Type::String,
+            ),
+            StrFunction::Retain => Type::function(
+                vec![
+                    Type::String,
+                    Type::function(vec![Type::String], Type::Boolean),
+                ],
+                Type::String,
+            ),
+            StrFunction::Split => {
+                Type::function(vec![Type::String, Type::String], Type::list(Type::String))
+            }
+            StrFunction::SplitAt => {
+                Type::function(vec![Type::String, Type::Integer], Type::list(Type::String))
+            }
+            StrFunction::SplitInclusive => {
+                Type::function(vec![Type::String, Type::String], Type::list(Type::String))
+            }
+            StrFunction::SplitN => Type::function(
+                vec![Type::String, Type::Integer, Type::String],
+                Type::list(Type::String),
+            ),
+            StrFunction::SplitOnce => {
+                Type::function(vec![Type::String, Type::String], Type::list(Type::String))
+            }
+            StrFunction::SplitTerminator => {
+                Type::function(vec![Type::String, Type::String], Type::list(Type::String))
+            }
+            StrFunction::SplitWhitespace => {
+                Type::function(vec![Type::String], Type::list(Type::String))
+            }
+            StrFunction::StartsWith => {
+                Type::function(vec![Type::String, Type::String], Type::Boolean)
+            }
+            StrFunction::StripPrefix => Type::function(
+                vec![Type::String, Type::String],
+                Type::option(Some(Type::String)),
+            ),
+            StrFunction::ToLowercase => Type::function(vec![Type::String], Type::String),
+            StrFunction::ToUppercase => Type::function(vec![Type::String], Type::String),
+            StrFunction::Truncate => {
+                Type::function(vec![Type::String, Type::Integer], Type::String)
+            }
+            StrFunction::Trim => Type::function(vec![Type::String], Type::String),
+            StrFunction::TrimEnd => Type::function(vec![Type::String], Type::String),
+            StrFunction::TrimEndMatches => {
+                Type::function(vec![Type::String, Type::String], Type::String)
+            }
+            StrFunction::TrimMatches => {
+                Type::function(vec![Type::String, Type::String], Type::String)
+            }
+            StrFunction::TrimStart => Type::function(vec![Type::String], Type::String),
+            StrFunction::TrimStartMatches => {
+                Type::function(vec![Type::String, Type::String], Type::String)
+            }
+        }
+    }
+
+    fn call(
+        &self,
+        arguments: &[Value],
+        _source: &str,
+        _context: &Context,
+    ) -> Result<Value, RuntimeError> {
+        let value = match self {
+            StrFunction::AsBytes => {
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
+
+                let string = arguments.first().unwrap().as_string()?;
+                let bytes = string
+                    .bytes()
+                    .map(|byte| Value::Integer(byte as i64))
+                    .collect();
+
+                Value::List(List::with_items(bytes))
+            }
+            StrFunction::EndsWith => {
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
+
+                let string = arguments.first().unwrap().as_string()?;
+                let pattern_string = arguments.get(1).unwrap().as_string()?;
+                let pattern = pattern_string.as_str();
+
+                Value::Boolean(string.ends_with(pattern))
+            }
+            StrFunction::Find => {
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
+
+                let string = arguments.first().unwrap().as_string()?;
+                let pattern_string = arguments.get(1).unwrap().as_string()?;
+                let pattern = pattern_string.as_str();
+                let find = string
+                    .find(pattern)
+                    .map(|index| Value::Integer(index as i64));
+
+                if let Some(index) = find {
+                    Value::Enum(EnumInstance::new(
+                        Identifier::new("Option"),
+                        Identifier::new("Some"),
+                        Some(index),
+                    ))
+                } else {
+                    Value::Enum(EnumInstance::new(
+                        Identifier::new("Option"),
+                        Identifier::new("None"),
+                        Some(Value::none()),
+                    ))
+                }
+            }
+            StrFunction::IsAscii => {
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
+
+                let string = arguments.first().unwrap().as_string()?;
+
+                Value::Boolean(string.is_ascii())
+            }
+            StrFunction::IsEmpty => {
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
+
+                let string = arguments.first().unwrap().as_string()?;
+
+                Value::Boolean(string.is_empty())
+            }
+            StrFunction::Insert => {
+                RuntimeError::expect_argument_amount(self.name(), 3, arguments.len())?;
+
+                let mut string = arguments.first().unwrap().as_string()?.clone();
+                let index = arguments.get(1).unwrap().as_integer()? as usize;
+                let insertion = arguments.get(2).unwrap().as_string()?;
+
+                string.insert_str(index, insertion);
+
+                Value::String(string)
+            }
+            StrFunction::Lines => {
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
+
+                let string = arguments.first().unwrap().as_string()?;
+                let lines = string
+                    .lines()
+                    .map(|line| Value::string(line.to_string()))
+                    .collect();
+
+                Value::List(List::with_items(lines))
+            }
+            StrFunction::Matches => {
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
+
+                let string = arguments.first().unwrap().as_string()?;
+                let pattern_string = arguments.get(1).unwrap().as_string()?;
+                let pattern = pattern_string.as_str();
+                let matches = string
+                    .matches(pattern)
+                    .map(|pattern| Value::string(pattern.to_string()))
+                    .collect();
+
+                Value::List(List::with_items(matches))
+            }
+            StrFunction::Parse => {
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
+
+                let string = arguments.first().unwrap().as_string()?;
+
+                if let Ok(integer) = string.parse::<i64>() {
+                    Value::Integer(integer)
+                } else if let Ok(float) = string.parse::<f64>() {
+                    Value::Float(float)
+                } else {
+                    Value::none()
+                }
+            }
+            StrFunction::Remove => {
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
+
+                let string = arguments.first().unwrap().as_string()?;
+                let index = arguments.get(1).unwrap().as_integer()? as usize;
+                let chars = string.chars().collect::<Vec<char>>();
+
+                if index < chars.len() {
+                    let new_string = chars
+                        .iter()
+                        .map(|char| char.to_string())
+                        .collect::<String>();
+
+                    Value::some(Value::string(new_string))
+                } else {
+                    Value::none()
+                }
+            }
+            StrFunction::ReplaceRange => {
+                RuntimeError::expect_argument_amount(self.name(), 3, arguments.len())?;
+
+                let mut string = arguments.first().unwrap().as_string()?.clone();
+                let range = arguments.get(1).unwrap().as_list()?.items()?;
+                let start = range[0].as_integer()? as usize;
+                let end = range[1].as_integer()? as usize;
+                let pattern = arguments.get(2).unwrap().as_string()?;
+
+                string.replace_range(start..end, pattern);
+
+                Value::String(string)
+            }
+            StrFunction::Retain => {
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
+
+                todo!();
+
+                // let mut string = arguments.first().unwrap().as_string()?.clone();
+                // let predicate = arguments.get(1).unwrap().as_function()?;
+
+                // string.retain(|char| {
+                //     predicate
+                //         .call(&[Value::string(char)], _source, _outer_context)
+                //         .unwrap()
+                //         .as_boolean()
+                //         .unwrap()
+                // });
+
+                // Value::String(string)
+            }
+            StrFunction::Split => {
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
+
+                let string = arguments.first().unwrap().as_string()?;
+                let pattern_string = arguments.get(1).unwrap().as_string()?;
+                let pattern = pattern_string.as_str();
+                let sections = string
+                    .split(pattern)
+                    .map(|section| Value::string(section.to_string()))
+                    .collect();
+
+                Value::List(List::with_items(sections))
+            }
+            StrFunction::SplitAt => {
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
+
+                let string = arguments.first().unwrap().as_string()?;
+                let index = arguments.get(1).unwrap().as_integer()?;
+                let (left, right) = string.split_at(index as usize);
+
+                Value::List(List::with_items(vec![
+                    Value::string(left.to_string()),
+                    Value::string(right.to_string()),
+                ]))
+            }
+            StrFunction::SplitInclusive => {
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
+
+                let string = arguments.first().unwrap().as_string()?;
+                let pattern_string = arguments.get(1).unwrap().as_string()?;
+                let pattern = pattern_string.as_str();
+                let sections = string
+                    .split(pattern)
+                    .map(|pattern| Value::string(pattern.to_string()))
+                    .collect();
+
+                Value::List(List::with_items(sections))
+            }
+            StrFunction::SplitN => {
+                RuntimeError::expect_argument_amount(self.name(), 3, arguments.len())?;
+
+                let string = arguments.first().unwrap().as_string()?;
+                let count = arguments.get(1).unwrap().as_integer()?;
+                let pattern_string = arguments.get(2).unwrap().as_string()?;
+                let pattern = pattern_string.as_str();
+                let sections = string
+                    .splitn(count as usize, pattern)
+                    .map(|pattern| Value::string(pattern.to_string()))
+                    .collect();
+
+                Value::List(List::with_items(sections))
+            }
+            StrFunction::SplitOnce => {
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
+
+                let string = arguments.first().unwrap().as_string()?;
+                let pattern_string = arguments.get(1).unwrap().as_string()?;
+                let pattern = pattern_string.as_str();
+                let sections = string.split_once(pattern).map(|(left, right)| {
+                    Value::List(List::with_items(vec![
+                        Value::string(left.to_string()),
+                        Value::string(right.to_string()),
+                    ]))
+                });
+
+                if let Some(sections) = sections {
+                    Value::some(sections)
+                } else {
+                    Value::none()
+                }
+            }
+            StrFunction::SplitTerminator => {
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
+
+                let string = arguments.first().unwrap().as_string()?;
+                let pattern_string = arguments.get(1).unwrap().as_string()?;
+                let pattern = pattern_string.as_str();
+                let sections = string
+                    .split_terminator(pattern)
+                    .map(|section| Value::string(section.to_string()))
+                    .collect();
+
+                Value::List(List::with_items(sections))
+            }
+            StrFunction::SplitWhitespace => {
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
+
+                let string = arguments.first().unwrap().as_string()?;
+                let sections = string
+                    .split_whitespace()
+                    .map(|section| Value::string(section.to_string()))
+                    .collect();
+
+                Value::List(List::with_items(sections))
+            }
+            StrFunction::StartsWith => {
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
+
+                let string = arguments.first().unwrap().as_string()?;
+                let pattern_string = arguments.get(1).unwrap().as_string()?;
+                let pattern = pattern_string.as_str();
+
+                Value::Boolean(string.starts_with(pattern))
+            }
+            StrFunction::StripPrefix => {
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
+
+                let string = arguments.first().unwrap().as_string()?;
+                let prefix_string = arguments.get(1).unwrap().as_string()?;
+                let prefix = prefix_string.as_str();
+                let stripped = string
+                    .strip_prefix(prefix)
+                    .map(|remainder| Value::string(remainder.to_string()));
+
+                if let Some(value) = stripped {
+                    Value::Enum(EnumInstance::new(
+                        Identifier::new("Option"),
+                        Identifier::new("Some"),
+                        Some(value),
+                    ))
+                } else {
+                    Value::none()
+                }
+            }
+            StrFunction::ToLowercase => {
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
+
+                let string = arguments.first().unwrap().as_string()?;
+                let lowercase = string.to_lowercase();
+
+                Value::string(lowercase)
+            }
+            StrFunction::ToUppercase => {
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
+
+                let string = arguments.first().unwrap().as_string()?;
+                let uppercase = string.to_uppercase();
+
+                Value::string(uppercase)
+            }
+            StrFunction::Trim => {
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
+
+                let trimmed = arguments.first().unwrap().as_string()?.trim().to_string();
+
+                Value::string(trimmed)
+            }
+            StrFunction::TrimEnd => {
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
+
+                let trimmed = arguments
+                    .first()
+                    .unwrap()
+                    .as_string()?
+                    .trim_end()
+                    .to_string();
+
+                Value::string(trimmed)
+            }
+            StrFunction::TrimEndMatches => {
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
+
+                let string = arguments.first().unwrap().as_string()?;
+                let pattern_string = arguments.get(1).unwrap().as_string()?;
+                let pattern = pattern_string.as_str();
+                let trimmed = string.trim_end_matches(pattern).to_string();
+
+                Value::string(trimmed)
+            }
+            StrFunction::TrimMatches => {
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
+
+                let string = arguments.first().unwrap().as_string()?;
+                let pattern = arguments
+                    .get(1)
+                    .unwrap()
+                    .as_string()?
+                    .chars()
+                    .collect::<Vec<char>>();
+                let trimmed = string.trim_matches(pattern.as_slice()).to_string();
+
+                Value::string(trimmed)
+            }
+            StrFunction::TrimStart => {
+                RuntimeError::expect_argument_amount(self.name(), 1, arguments.len())?;
+
+                let trimmed = arguments
+                    .first()
+                    .unwrap()
+                    .as_string()?
+                    .trim_start()
+                    .to_string();
+
+                Value::string(trimmed)
+            }
+            StrFunction::TrimStartMatches => {
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
+
+                let string = arguments.first().unwrap().as_string()?;
+                let pattern = arguments
+                    .get(1)
+                    .unwrap()
+                    .as_string()?
+                    .chars()
+                    .collect::<Vec<char>>();
+                let trimmed = string.trim_start_matches(pattern.as_slice()).to_string();
+
+                Value::string(trimmed)
+            }
+            StrFunction::Truncate => {
+                RuntimeError::expect_argument_amount(self.name(), 2, arguments.len())?;
+
+                let input_string = arguments.first().unwrap().as_string()?;
+                let new_length = arguments.get(1).unwrap().as_integer()? as usize;
+
+                let new_string = input_string
+                    .chars()
+                    .take(new_length)
+                    .map(|char| char.to_string())
+                    .collect();
+
+                Value::String(new_string)
+            }
+        };
+
+        Ok(value)
+    }
+}

src/built_in_identifiers.rs

@@ -0,0 +1,51 @@
+use std::sync::{Arc, OnceLock};
+
+use enum_iterator::{all, Sequence};
+
+use crate::Identifier;
+
+pub fn all_built_in_identifiers() -> impl Iterator<Item = BuiltInIdentifier> {
+    all()
+}
+
+static OPTION: OnceLock<Identifier> = OnceLock::new();
+static NONE: OnceLock<Identifier> = OnceLock::new();
+static SOME: OnceLock<Identifier> = OnceLock::new();
+static RESULT: OnceLock<Identifier> = OnceLock::new();
+static OK: OnceLock<Identifier> = OnceLock::new();
+static ERROR: OnceLock<Identifier> = OnceLock::new();
+
+#[derive(Sequence, Debug)]
+pub enum BuiltInIdentifier {
+    Option,
+    None,
+    Some,
+    Result,
+    Ok,
+    Error,
+}
+
+impl BuiltInIdentifier {
+    pub fn get(&self) -> &Identifier {
+        match self {
+            BuiltInIdentifier::Option => {
+                OPTION.get_or_init(|| Identifier::from_raw_parts(Arc::new("Option".to_string())))
+            }
+            BuiltInIdentifier::None => {
+                NONE.get_or_init(|| Identifier::from_raw_parts(Arc::new("None".to_string())))
+            }
+            BuiltInIdentifier::Some => {
+                SOME.get_or_init(|| Identifier::from_raw_parts(Arc::new("Some".to_string())))
+            }
+            BuiltInIdentifier::Result => {
+                RESULT.get_or_init(|| Identifier::from_raw_parts(Arc::new("Result".to_string())))
+            }
+            BuiltInIdentifier::Ok => {
+                OK.get_or_init(|| Identifier::from_raw_parts(Arc::new("Ok".to_string())))
+            }
+            BuiltInIdentifier::Error => {
+                ERROR.get_or_init(|| Identifier::from_raw_parts(Arc::new("Error".to_string())))
+            }
+        }
+    }
+}

src/built_in_type_definitions.rs

@@ -0,0 +1,56 @@
+use std::sync::OnceLock;
+
+use enum_iterator::{all, Sequence};
+
+use crate::{
+    error::rw_lock_error::RwLockError, Context, EnumDefinition, Identifier, Type, TypeDefinition,
+};
+
+static OPTION: OnceLock<Result<TypeDefinition, RwLockError>> = OnceLock::new();
+static RESULT: OnceLock<Result<TypeDefinition, RwLockError>> = OnceLock::new();
+
+pub fn all_built_in_type_definitions() -> impl Iterator<Item = BuiltInTypeDefinition> {
+    all()
+}
+
+#[derive(Sequence)]
+pub enum BuiltInTypeDefinition {
+    Option,
+    Result,
+}
+
+impl BuiltInTypeDefinition {
+    pub fn name(&self) -> &'static str {
+        match self {
+            BuiltInTypeDefinition::Option => "Option",
+            BuiltInTypeDefinition::Result => "Result",
+        }
+    }
+
+    pub fn get(&self, _context: &Context) -> &Result<TypeDefinition, RwLockError> {
+        match self {
+            BuiltInTypeDefinition::Option => OPTION.get_or_init(|| {
+                let definition = TypeDefinition::Enum(EnumDefinition::new(
+                    Identifier::new(self.name()),
+                    vec![
+                        (Identifier::new("Some"), vec![Type::Any]),
+                        (Identifier::new("None"), Vec::with_capacity(0)),
+                    ],
+                ));
+
+                Ok(definition)
+            }),
+            BuiltInTypeDefinition::Result => RESULT.get_or_init(|| {
+                let definition = TypeDefinition::Enum(EnumDefinition::new(
+                    Identifier::new(self.name()),
+                    vec![
+                        (Identifier::new("Ok"), vec![Type::Any]),
+                        (Identifier::new("Error"), vec![Type::Any]),
+                    ],
+                ));
+
+                Ok(definition)
+            }),
+        }
+    }
+}

src/built_in_types.rs

@@ -0,0 +1,29 @@
+use std::sync::OnceLock;
+
+use crate::{Identifier, Type};
+
+static OPTION: OnceLock<Type> = OnceLock::new();
+
+pub enum BuiltInType {
+    Option(Option<Type>),
+}
+
+impl BuiltInType {
+    pub fn name(&self) -> &'static str {
+        match self {
+            BuiltInType::Option(_) => "Option",
+        }
+    }
+
+    pub fn get(&self) -> &Type {
+        match self {
+            BuiltInType::Option(content_type) => OPTION.get_or_init(|| {
+                if let Some(content_type) = content_type {
+                    Type::custom(Identifier::new("Option"), vec![content_type.clone()])
+                } else {
+                    Type::custom(Identifier::new("Option"), Vec::with_capacity(0))
+                }
+            }),
+        }
+    }
+}

src/built_in_values.rs

@@ -0,0 +1,180 @@
+use std::{env::args, sync::OnceLock};
+
+use enum_iterator::{all, Sequence};
+use serde::{Deserialize, Serialize};
+
+use crate::{
+    built_in_functions::{fs::fs_functions, json::json_functions, str::string_functions, Callable},
+    BuiltInFunction, EnumInstance, Function, Identifier, List, Map, Value,
+};
+
+static ARGS: OnceLock<Value> = OnceLock::new();
+static FS: OnceLock<Value> = OnceLock::new();
+static JSON: OnceLock<Value> = OnceLock::new();
+static NONE: OnceLock<Value> = OnceLock::new();
+static RANDOM: OnceLock<Value> = OnceLock::new();
+static STR: OnceLock<Value> = OnceLock::new();
+
+/// Returns the entire built-in value API.
+pub fn all_built_in_values() -> impl Iterator<Item = BuiltInValue> {
+    all()
+}
+
+/// A variable with a hard-coded key that is globally available.
+#[derive(Sequence, Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Serialize, Deserialize)]
+pub enum BuiltInValue {
+    /// The arguments used to launch the current program.
+    Args,
+
+    /// Create an error if two values are not equal.
+    AssertEqual,
+
+    /// File system tools.
+    Fs,
+
+    /// JSON format tools.
+    Json,
+
+    /// Get the length of a collection.
+    Length,
+
+    /// The absence of a value.
+    None,
+
+    /// Print a value to stdout.
+    Output,
+
+    /// Random value generators.
+    Random,
+
+    /// String utilities.
+    Str,
+}
+
+impl BuiltInValue {
+    /// Returns the hard-coded key used to identify the value.
+    pub fn name(&self) -> &'static str {
+        match self {
+            BuiltInValue::Args => "args",
+            BuiltInValue::AssertEqual => "assert_equal",
+            BuiltInValue::Fs => "fs",
+            BuiltInValue::Json => "json",
+            BuiltInValue::Length => BuiltInFunction::Length.name(),
+            BuiltInValue::None => "None",
+            BuiltInValue::Output => "output",
+            BuiltInValue::Random => "random",
+            BuiltInValue::Str => "str",
+        }
+    }
+
+    /// Returns a brief description of the value's features.
+    ///
+    /// This is used by the shell when suggesting completions.
+    pub fn description(&self) -> &'static str {
+        match self {
+            BuiltInValue::Args => "The command line arguments sent to this program.",
+            BuiltInValue::AssertEqual => "Error if the two values are not equal.",
+            BuiltInValue::Fs => "File and directory tools.",
+            BuiltInValue::Json => "JSON formatting tools.",
+            BuiltInValue::Length => BuiltInFunction::Length.description(),
+            BuiltInValue::None => "The absence of a value.",
+            BuiltInValue::Output => "output",
+            BuiltInValue::Random => "random",
+            BuiltInValue::Str => "string",
+        }
+    }
+
+    /// Returns the value by creating it or, if it has already been accessed, retrieving it from its
+    /// [OnceLock][].
+    pub fn get(&self) -> Value {
+        match self {
+            BuiltInValue::Args => ARGS
+                .get_or_init(|| {
+                    let args = args().map(|arg| Value::string(arg.to_string())).collect();
+
+                    Value::List(List::with_items(args))
+                })
+                .clone(),
+            BuiltInValue::AssertEqual => {
+                Value::Function(Function::BuiltIn(BuiltInFunction::AssertEqual))
+            }
+            BuiltInValue::Fs => FS
+                .get_or_init(|| {
+                    let mut fs_map = Map::new();
+
+                    for fs_function in fs_functions() {
+                        let key = fs_function.name();
+                        let value =
+                            Value::Function(Function::BuiltIn(BuiltInFunction::Fs(fs_function)));
+
+                        fs_map.set(Identifier::new(key), value);
+                    }
+
+                    Value::Map(fs_map)
+                })
+                .clone(),
+            BuiltInValue::Json => JSON
+                .get_or_init(|| {
+                    let mut json_map = Map::new();
+
+                    for json_function in json_functions() {
+                        let key = json_function.name();
+                        let value = Value::Function(Function::BuiltIn(BuiltInFunction::Json(
+                            json_function,
+                        )));
+
+                        json_map.set(Identifier::new(key), value);
+                    }
+
+                    Value::Map(json_map)
+                })
+                .clone(),
+            BuiltInValue::Length => Value::Function(Function::BuiltIn(BuiltInFunction::Length)),
+            BuiltInValue::None => NONE
+                .get_or_init(|| {
+                    Value::Enum(EnumInstance::new(
+                        Identifier::new("Option"),
+                        Identifier::new("None"),
+                        None,
+                    ))
+                })
+                .clone(),
+            BuiltInValue::Output => Value::Function(Function::BuiltIn(BuiltInFunction::Output)),
+            BuiltInValue::Random => RANDOM
+                .get_or_init(|| {
+                    let mut random_map = Map::new();
+
+                    for built_in_function in [
+                        BuiltInFunction::RandomBoolean,
+                        BuiltInFunction::RandomFloat,
+                        BuiltInFunction::RandomFrom,
+                        BuiltInFunction::RandomInteger,
+                    ] {
+                        let identifier = Identifier::new(built_in_function.name());
+                        let value = Value::Function(Function::BuiltIn(built_in_function));
+
+                        random_map.set(identifier, value);
+                    }
+
+                    Value::Map(random_map)
+                })
+                .clone(),
+            BuiltInValue::Str => STR
+                .get_or_init(|| {
+                    let mut str_map = Map::new();
+
+                    for string_function in string_functions() {
+                        let identifier = Identifier::new(string_function.name());
+                        let value = Value::Function(Function::BuiltIn(BuiltInFunction::String(
+                            string_function,
+                        )));
+
+                        str_map.set(identifier, value);
+                    }
+
+                    Value::Map(str_map)
+                })
+                .clone(),
+        }
+    }
+}

src/context/mod.rs

@@ -0,0 +1,395 @@
+//! A garbage-collecting execution context that stores variables and type data
+//! during the [Interpreter][crate::Interpreter]'s abstraction and execution
+//! process.
+//!
+//! ## Setting values
+//!
+//! When data is stored in a context, it can be accessed by dust source code.
+//! This allows you to insert values and type definitions before any code is
+//! interpreted.
+//!
+//! ```
+//! # use dust_lang::*;
+//! let context = Context::default();
+//!
+//! context.set_value(
+//!     "foobar".into(),
+//!     Value::String("FOOBAR".to_string())
+//! ).unwrap();
+//!
+//! interpret_with_context("output foobar", context);
+//!
+//! // Stdout: "FOOBAR"
+//! ```
+//!
+//! ## Built-in values and type definitions
+//!
+//! When looking up values and definitions, the Context will try to use one that
+//! has been explicitly set. If nothing is found, it will then check the built-
+//! in values and type definitions for a match. This means that the user can
+//! override the built-ins.
+//!
+//! ## Garbage Collection
+//!
+//! To disable garbage collection, run a Context in AllowGarbage mode.
+//!
+//! ```
+//! # use dust_lang::*;
+//! let context = Context::new(ContextMode::AllowGarbage);   
+//! ```
+//!
+//!
+//! Every item stored in a Context has a counter attached to it. You must use
+//! [Context::add_allowance][] to let the Context know not to drop the value.
+//! Every time you use [Context::get_value][] it checks the number of times it
+//! has been used and compares it to the number of allowances. If the limit
+//! has been reached, the value will be removed from the context and can no
+//! longer be found.
+mod usage_counter;
+mod value_data;
+
+pub use usage_counter::UsageCounter;
+pub use value_data::ValueData;
+
+use std::{
+    cmp::Ordering,
+    collections::BTreeMap,
+    fmt::Display,
+    sync::{Arc, RwLock, RwLockReadGuard},
+};
+
+use crate::{
+    built_in_type_definitions::all_built_in_type_definitions, built_in_values::all_built_in_values,
+    error::rw_lock_error::RwLockError, Identifier, Type, TypeDefinition, Value,
+};
+
+#[derive(Clone, Debug, PartialEq)]
+pub enum ContextMode {
+    AllowGarbage,
+    RemoveGarbage,
+}
+
+/// An execution context stores that variable and type data during the
+/// [Interpreter]'s abstraction and execution process.
+///
+/// See the [module-level docs][self] for more info.
+#[derive(Clone, Debug)]
+pub struct Context {
+    mode: ContextMode,
+    inner: Arc<RwLock<BTreeMap<Identifier, (ValueData, UsageCounter)>>>,
+}
+
+impl Context {
+    /// Return a new, empty Context.
+    pub fn new(mode: ContextMode) -> Self {
+        Self {
+            mode,
+            inner: Arc::new(RwLock::new(BTreeMap::new())),
+        }
+    }
+
+    /// Return a lock guard to the inner BTreeMap.
+    pub fn inner(
+        &self,
+    ) -> Result<RwLockReadGuard<BTreeMap<Identifier, (ValueData, UsageCounter)>>, RwLockError> {
+        Ok(self.inner.read()?)
+    }
+
+    /// Create a new context with all of the data from an existing context.
+    pub fn with_variables_from(other: &Context) -> Result<Context, RwLockError> {
+        let mut new_variables = BTreeMap::new();
+
+        for (identifier, (value_data, counter)) in other.inner.read()?.iter() {
+            let (allowances, _runtime_uses) = counter.get_counts()?;
+            let new_counter = UsageCounter::with_counts(allowances, 0);
+
+            new_variables.insert(identifier.clone(), (value_data.clone(), new_counter));
+        }
+
+        Ok(Context {
+            mode: other.mode.clone(),
+            inner: Arc::new(RwLock::new(new_variables)),
+        })
+    }
+
+    /// Modify a context to take the functions and type definitions of another.
+    ///
+    /// In the case of the conflict, the inherited value will override the previous
+    /// value.
+    pub fn inherit_from(&self, other: &Context) -> Result<(), RwLockError> {
+        let mut self_variables = self.inner.write()?;
+
+        for (identifier, (value_data, counter)) in other.inner.read()?.iter() {
+            let (allowances, _runtime_uses) = counter.get_counts()?;
+            let new_counter = UsageCounter::with_counts(allowances, 0);
+
+            if let ValueData::Value(value) = value_data {
+                if value.is_function() {
+                    self_variables.insert(identifier.clone(), (value_data.clone(), new_counter));
+                }
+            } else if let ValueData::TypeHint(r#type) = value_data {
+                if r#type.is_function() {
+                    self_variables.insert(identifier.clone(), (value_data.clone(), new_counter));
+                }
+            } else if let ValueData::TypeDefinition(_) = value_data {
+                self_variables.insert(identifier.clone(), (value_data.clone(), new_counter));
+            }
+        }
+
+        Ok(())
+    }
+
+    /// Modify a context to take all the information of another.
+    ///
+    /// In the case of the conflict, the inherited value will override the previous
+    /// value.
+    ///
+    /// ```
+    /// # use dust_lang::*;
+    /// let first_context = Context::default();
+    /// let second_context = Context::default();
+    ///
+    /// second_context.set_value(
+    ///     "Foo".into(),
+    ///     Value::String("Bar".to_string())
+    /// );
+    ///
+    /// first_context.inherit_all_from(&second_context).unwrap();
+    ///
+    /// assert_eq!(first_context, second_context);
+    /// ```
+    pub fn inherit_all_from(&self, other: &Context) -> Result<(), RwLockError> {
+        let mut self_variables = self.inner.write()?;
+
+        for (identifier, (value_data, _counter)) in other.inner.read()?.iter() {
+            self_variables.insert(
+                identifier.clone(),
+                (value_data.clone(), UsageCounter::new()),
+            );
+        }
+
+        Ok(())
+    }
+
+    /// Increment the number of allowances a variable has. Return a boolean
+    /// representing whether or not the variable was found.
+    pub fn add_allowance(&self, identifier: &Identifier) -> Result<bool, RwLockError> {
+        if let Some((_value_data, counter)) = self.inner.read()?.get(identifier) {
+            log::debug!("Adding allowance for {identifier}.");
+
+            counter.add_allowance()?;
+
+            Ok(true)
+        } else {
+            Ok(false)
+        }
+    }
+
+    /// Get a [Value] from the context.
+    pub fn get_value(&self, identifier: &Identifier) -> Result<Option<Value>, RwLockError> {
+        let (value, counter) =
+            if let Some((value_data, counter)) = self.inner.read()?.get(identifier) {
+                if let ValueData::Value(value) = value_data {
+                    (value.clone(), counter.clone())
+                } else {
+                    return Ok(None);
+                }
+            } else {
+                for built_in_value in all_built_in_values() {
+                    if built_in_value.name() == identifier.inner().as_ref() {
+                        return Ok(Some(built_in_value.get().clone()));
+                    }
+                }
+
+                return Ok(None);
+            };
+
+        counter.add_runtime_use()?;
+
+        log::debug!("Adding runtime use for {identifier}.");
+
+        let (allowances, runtime_uses) = counter.get_counts()?;
+
+        if self.mode == ContextMode::RemoveGarbage && allowances == runtime_uses {
+            self.unset(identifier)?;
+        }
+
+        Ok(Some(value))
+    }
+
+    /// Get a [Type] from the context.
+    ///
+    /// If the key matches a stored [Value], its type will be returned. It if
+    /// matches a type hint, the type hint will be returned.
+    pub fn get_type(&self, identifier: &Identifier) -> Result<Option<Type>, RwLockError> {
+        if let Some((value_data, _counter)) = self.inner.read()?.get(identifier) {
+            match value_data {
+                ValueData::Value(value) => return Ok(Some(value.r#type()?)),
+                ValueData::TypeHint(r#type) => return Ok(Some(r#type.clone())),
+                ValueData::TypeDefinition(_) => todo!(),
+            }
+        }
+
+        for built_in_value in all_built_in_values() {
+            if built_in_value.name() == identifier.inner().as_ref() {
+                return Ok(Some(built_in_value.get().r#type()?));
+            }
+        }
+
+        Ok(None)
+    }
+
+    /// Get a [TypeDefinition] from the context.
+    ///
+    /// This will also return a built-in type definition if one matches the key.
+    /// See the [module-level docs][self] for more info.
+    pub fn get_definition(
+        &self,
+        identifier: &Identifier,
+    ) -> Result<Option<TypeDefinition>, RwLockError> {
+        if let Some((value_data, _counter)) = self.inner.read()?.get(identifier) {
+            if let ValueData::TypeDefinition(definition) = value_data {
+                return Ok(Some(definition.clone()));
+            }
+        }
+
+        for built_in_definition in all_built_in_type_definitions() {
+            if built_in_definition.name() == identifier.inner().as_ref() {
+                return Ok(Some(built_in_definition.get(self).clone()?));
+            }
+        }
+
+        Ok(None)
+    }
+
+    /// Set a value to a key.
+    pub fn set_value(&self, key: Identifier, value: Value) -> Result<(), RwLockError> {
+        let mut map = self.inner.write()?;
+        let old_data = map.remove(&key);
+
+        if let Some((_, old_counter)) = old_data {
+            map.insert(key, (ValueData::Value(value), old_counter));
+        } else {
+            map.insert(key, (ValueData::Value(value), UsageCounter::new()));
+        }
+
+        Ok(())
+    }
+
+    /// Set a type hint.
+    ///
+    /// This allows the interpreter to check a value's type before the value
+    /// actually exists by predicting what the abstract tree will produce.
+    pub fn set_type(&self, key: Identifier, r#type: Type) -> Result<(), RwLockError> {
+        self.inner
+            .write()?
+            .insert(key, (ValueData::TypeHint(r#type), UsageCounter::new()));
+
+        Ok(())
+    }
+
+    /// Set a type definition.
+    ///
+    /// This allows defined types (i.e. structs and enums) to be instantiated
+    /// later while running the interpreter using this context.
+    pub fn set_definition(
+        &self,
+        key: Identifier,
+        definition: TypeDefinition,
+    ) -> Result<(), RwLockError> {
+        self.inner.write()?.insert(
+            key,
+            (ValueData::TypeDefinition(definition), UsageCounter::new()),
+        );
+
+        Ok(())
+    }
+
+    /// Remove a key-value pair.
+    pub fn unset(&self, key: &Identifier) -> Result<(), RwLockError> {
+        log::debug!("Dropping variable {key}.");
+
+        self.inner.write()?.remove(key);
+
+        Ok(())
+    }
+}
+
+impl Default for Context {
+    fn default() -> Self {
+        Context::new(ContextMode::RemoveGarbage)
+    }
+}
+
+impl Eq for Context {}
+
+impl PartialEq for Context {
+    fn eq(&self, other: &Self) -> bool {
+        let self_variables = self.inner().unwrap();
+        let other_variables = other.inner().unwrap();
+
+        if self_variables.len() != other_variables.len() {
+            return false;
+        }
+
+        for ((left_key, left_value_data), (right_key, right_value_data)) in
+            self_variables.iter().zip(other_variables.iter())
+        {
+            if left_key != right_key || left_value_data != right_value_data {
+                return false;
+            }
+        }
+
+        true
+    }
+}
+
+impl PartialOrd for Context {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl Ord for Context {
+    fn cmp(&self, other: &Self) -> Ordering {
+        let left = self.inner().unwrap();
+        let right = other.inner().unwrap();
+
+        left.cmp(&right)
+    }
+}
+
+impl Display for Context {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        writeln!(f, "{{")?;
+
+        for (identifier, value_data) in self.inner.read().unwrap().iter() {
+            writeln!(f, "{identifier} {value_data:?}")?;
+        }
+
+        writeln!(f, "}}")
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::*;
+
+    #[test]
+    fn drops_variables() {
+        let context = Context::default();
+
+        interpret_with_context(
+            "
+                x = 1
+                y = 2
+
+                z = x + y
+            ",
+            context.clone(),
+        )
+        .unwrap();
+
+        assert_eq!(context.inner.read().unwrap().len(), 1);
+    }
+}

src/context/usage_counter.rs

@@ -0,0 +1,74 @@
+use std::{
+    cmp::Ordering,
+    sync::{Arc, RwLock},
+};
+
+use crate::error::rw_lock_error::RwLockError;
+
+#[derive(Clone, Debug)]
+pub struct UsageCounter(Arc<RwLock<UsageCounterInner>>);
+
+impl UsageCounter {
+    pub fn new() -> UsageCounter {
+        UsageCounter(Arc::new(RwLock::new(UsageCounterInner {
+            allowances: 0,
+            runtime_uses: 0,
+        })))
+    }
+
+    pub fn with_counts(allowances: usize, runtime_uses: usize) -> UsageCounter {
+        UsageCounter(Arc::new(RwLock::new(UsageCounterInner {
+            allowances,
+            runtime_uses,
+        })))
+    }
+
+    pub fn get_counts(&self) -> Result<(usize, usize), RwLockError> {
+        let inner = self.0.read()?;
+        Ok((inner.allowances, inner.runtime_uses))
+    }
+
+    pub fn add_allowance(&self) -> Result<(), RwLockError> {
+        self.0.write()?.allowances += 1;
+
+        Ok(())
+    }
+
+    pub fn add_runtime_use(&self) -> Result<(), RwLockError> {
+        self.0.write()?.runtime_uses += 1;
+
+        Ok(())
+    }
+}
+
+impl Eq for UsageCounter {}
+
+impl PartialEq for UsageCounter {
+    fn eq(&self, other: &Self) -> bool {
+        let left = self.0.read().unwrap();
+        let right = other.0.read().unwrap();
+
+        *left == *right
+    }
+}
+
+impl PartialOrd for UsageCounter {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl Ord for UsageCounter {
+    fn cmp(&self, other: &Self) -> Ordering {
+        let left = self.0.read().unwrap();
+        let right = other.0.read().unwrap();
+
+        left.cmp(&right)
+    }
+}
+
+#[derive(Clone, Debug, Eq, PartialEq, PartialOrd, Ord)]
+struct UsageCounterInner {
+    pub allowances: usize,
+    pub runtime_uses: usize,
+}

src/context/value_data.rs

@@ -0,0 +1,8 @@
+use crate::{Type, TypeDefinition, Value};
+
+#[derive(Clone, Debug, Eq, PartialEq, PartialOrd, Ord)]
+pub enum ValueData {
+    Value(Value),
+    TypeHint(Type),
+    TypeDefinition(TypeDefinition),
+}

src/error.rs

@@ -1,633 +0,0 @@
-//! Error and Result types.
-//!
-//! To deal with errors from dependencies, either create a new error variant
-//! or use the MacroFailure variant if the error can only occur inside a macro.
-use crate::{
-    operator::Operator, token::PartialToken, value::value_type::ValueType, value::Value, Node,
-    ToolInfo,
-};
-
-use std::{fmt, io, time::SystemTimeError};
-
-pub type Result<T> = std::result::Result<T, Error>;
-
-#[derive(Debug, Clone, PartialEq)]
-#[non_exhaustive]
-pub enum Error {
-    /// Dust's internal type checking failed to identify a type mismatch. This should never happen,      /// the error prompts the user to report the bug.
-    TypeCheckFailure {
-        tool_info: ToolInfo<'static>,
-        argument: Value,
-    },
-
-    /// The 'assert' macro did not resolve successfully.
-    AssertEqualFailed {
-        expected: Value,
-        actual: Value,
-    },
-
-    /// The 'assert' macro did not resolve successfully.
-    AssertFailed,
-
-    /// A row was inserted to a table with the wrong amount of values.
-    WrongColumnAmount {
-        expected: usize,
-        actual: usize,
-    },
-
-    /// An operator was called with the wrong amount of arguments.
-    ExpectedOperatorArgumentAmount {
-        expected: usize,
-        actual: usize,
-    },
-
-    /// A function was called with the wrong amount of arguments.
-    ExpectedFunctionArgumentAmount {
-        identifier: String,
-        expected: usize,
-        actual: usize,
-    },
-
-    /// A function was called with the wrong amount of arguments.
-    ExpectedAtLeastFunctionArgumentAmount {
-        identifier: String,
-        minimum: usize,
-        actual: usize,
-    },
-
-    ExpectedString {
-        actual: Value,
-    },
-
-    ExpectedInt {
-        actual: Value,
-    },
-
-    ExpectedFloat {
-        actual: Value,
-    },
-
-    /// An integer, floating point or value was expected.
-    ExpectedNumber {
-        actual: Value,
-    },
-
-    /// An integer, floating point or string value was expected.
-    ExpectedNumberOrString {
-        actual: Value,
-    },
-
-    ExpectedBoolean {
-        actual: Value,
-    },
-
-    ExpectedList {
-        actual: Value,
-    },
-
-    ExpectedFixedLenList {
-        expected_len: usize,
-        actual: Value,
-    },
-
-    ExpectedEmpty {
-        actual: Value,
-    },
-
-    ExpectedMap {
-        actual: Value,
-    },
-
-    ExpectedTable {
-        actual: Value,
-    },
-
-    ExpectedFunction {
-        actual: Value,
-    },
-
-    /// A string, list, map or table value was expected.
-    ExpectedCollection {
-        actual: Value,
-    },
-
-    /// Tried to append a child to a leaf node.
-    /// Leaf nodes cannot have children.
-    AppendedToLeafNode(Node),
-
-    /// Tried to append a child to a node such that the precedence of the child
-    /// is not higher. This error should never occur. If it does, please file a
-    /// bug report.
-    PrecedenceViolation,
-
-    /// A `VariableIdentifier` operation did not find its value in the context.
-    VariableIdentifierNotFound(String),
-
-    /// A `FunctionIdentifier` operation did not find its value in the context.
-    FunctionIdentifierNotFound(String),
-
-    /// A value has the wrong type.
-    /// Only use this if there is no other error that describes the expected and
-    /// provided types in more detail.
-    TypeError {
-        /// The expected types.
-        expected: &'static [ValueType],
-        /// The actual value.
-        actual: Value,
-    },
-
-    /// A macro or function was called with the wrong type of input.
-    MacroArgumentType {
-        /// The macro that was called.
-        macro_info: ToolInfo<'static>,
-        /// The actual value.
-        actual: Value,
-    },
-
-    /// An operator is used with a wrong combination of types.
-    WrongTypeCombination {
-        /// The operator that whose evaluation caused the error.
-        operator: Operator,
-        /// The types that were used in the operator causing it to fail.
-        actual: Vec<ValueType>,
-    },
-
-    /// An opening brace without a matching closing brace was found.
-    UnmatchedLBrace,
-
-    /// A closing brace without a matching opening brace was found.
-    UnmatchedRBrace,
-
-    /// Left of an opening brace or right of a closing brace is a token that does not expect the brace next to it.
-    /// For example, writing `4(5)` would yield this error, as the `4` does not have any operands.
-    MissingOperatorOutsideOfBrace,
-
-    /// A `PartialToken` is unmatched, such that it cannot be combined into a full `Token`.
-    /// This happens if for example a single `=` is found, surrounded by whitespace.
-    /// It is not a token, but it is part of the string representation of some tokens.
-    UnmatchedPartialToken {
-        /// The unmatched partial token.
-        first: PartialToken,
-        /// The token that follows the unmatched partial token and that cannot be matched to the partial token, or `None`, if `first` is the last partial token in the stream.
-        second: Option<PartialToken>,
-    },
-
-    /// An addition operation performed by Rust failed.
-    AdditionError {
-        /// The first argument of the addition.
-        augend: Value,
-        /// The second argument of the addition.
-        addend: Value,
-    },
-
-    /// A subtraction operation performed by Rust failed.
-    SubtractionError {
-        /// The first argument of the subtraction.
-        minuend: Value,
-        /// The second argument of the subtraction.
-        subtrahend: Value,
-    },
-
-    /// A negation operation performed by Rust failed.
-    NegationError {
-        /// The argument of the negation.
-        argument: Value,
-    },
-
-    /// A multiplication operation performed by Rust failed.
-    MultiplicationError {
-        /// The first argument of the multiplication.
-        multiplicand: Value,
-        /// The second argument of the multiplication.
-        multiplier: Value,
-    },
-
-    /// A division operation performed by Rust failed.
-    DivisionError {
-        /// The first argument of the division.
-        dividend: Value,
-        /// The second argument of the division.
-        divisor: Value,
-    },
-
-    /// A modulation operation performed by Rust failed.
-    ModulationError {
-        /// The first argument of the modulation.
-        dividend: Value,
-        /// The second argument of the modulation.
-        divisor: Value,
-    },
-
-    /// A regular expression could not be parsed
-    InvalidRegex {
-        /// The invalid regular expression
-        regex: String,
-        /// Failure message from the regex engine
-        message: String,
-    },
-
-    /// A modification was attempted on a `Context` that does not allow modifications.
-    ContextNotMutable,
-
-    /// An escape sequence within a string literal is illegal.
-    IllegalEscapeSequence(String),
-
-    /// This context does not allow enabling builtin functions.
-    BuiltinFunctionsCannotBeEnabled,
-
-    /// This context does not allow disabling builtin functions.
-    BuiltinFunctionsCannotBeDisabled,
-
-    /// The function failed due to an external error.
-    MacroFailure(String),
-
-    /// A custom error explained by its message.
-    CustomMessage(String),
-}
-
-impl From<csv::Error> for Error {
-    fn from(value: csv::Error) -> Self {
-        Error::MacroFailure(value.to_string())
-    }
-}
-
-impl From<json::Error> for Error {
-    fn from(value: json::Error) -> Self {
-        Error::MacroFailure(value.to_string())
-    }
-}
-
-impl From<io::Error> for Error {
-    fn from(value: std::io::Error) -> Self {
-        Error::MacroFailure(value.to_string())
-    }
-}
-
-impl From<git2::Error> for Error {
-    fn from(value: git2::Error) -> Self {
-        Error::MacroFailure(value.to_string())
-    }
-}
-
-impl From<reqwest::Error> for Error {
-    fn from(value: reqwest::Error) -> Self {
-        Error::MacroFailure(value.to_string())
-    }
-}
-
-impl From<serde_json::Error> for Error {
-    fn from(value: serde_json::Error) -> Self {
-        Error::MacroFailure(value.to_string())
-    }
-}
-
-impl From<SystemTimeError> for Error {
-    fn from(value: SystemTimeError) -> Self {
-        Error::MacroFailure(value.to_string())
-    }
-}
-
-impl From<trash::Error> for Error {
-    fn from(value: trash::Error) -> Self {
-        Error::MacroFailure(value.to_string())
-    }
-}
-
-impl From<toml::de::Error> for Error {
-    fn from(value: toml::de::Error) -> Self {
-        Error::MacroFailure(value.to_string())
-    }
-}
-
-impl Error {
-    pub(crate) fn expect_operator_argument_amount(actual: usize, expected: usize) -> Result<()> {
-        if actual == expected {
-            Ok(())
-        } else {
-            Err(Error::ExpectedOperatorArgumentAmount { expected, actual })
-        }
-    }
-
-    pub(crate) fn expect_function_argument_amount(
-        identifier: &str,
-        actual: usize,
-        expected: usize,
-    ) -> Result<()> {
-        if actual == expected {
-            Ok(())
-        } else {
-            Err(Error::ExpectedFunctionArgumentAmount {
-                identifier: identifier.to_string(),
-                expected,
-                actual,
-            })
-        }
-    }
-
-    pub(crate) fn expected_minimum_function_argument_amount(
-        identifier: &str,
-        actual: usize,
-        minimum: usize,
-    ) -> Result<()> {
-        if actual >= minimum {
-            Ok(())
-        } else {
-            Err(Error::ExpectedAtLeastFunctionArgumentAmount {
-                identifier: identifier.to_string(),
-                minimum,
-                actual,
-            })
-        }
-    }
-
-    pub fn type_error(actual: Value, expected: &'static [ValueType]) -> Self {
-        Error::TypeError { actual, expected }
-    }
-
-    pub fn wrong_type_combination(operator: Operator, actual: Vec<ValueType>) -> Self {
-        Error::WrongTypeCombination { operator, actual }
-    }
-
-    pub fn expected_string(actual: Value) -> Self {
-        Error::ExpectedString { actual }
-    }
-
-    pub fn expected_int(actual: Value) -> Self {
-        Error::ExpectedInt { actual }
-    }
-
-    pub fn expected_float(actual: Value) -> Self {
-        Error::ExpectedFloat { actual }
-    }
-
-    pub fn expected_number(actual: Value) -> Self {
-        Error::ExpectedNumber { actual }
-    }
-
-    pub fn expected_number_or_string(actual: Value) -> Self {
-        Error::ExpectedNumberOrString { actual }
-    }
-
-    pub fn expected_boolean(actual: Value) -> Self {
-        Error::ExpectedBoolean { actual }
-    }
-
-    pub fn expected_list(actual: Value) -> Self {
-        Error::ExpectedList { actual }
-    }
-
-    pub fn expected_fixed_len_list(expected_len: usize, actual: Value) -> Self {
-        Error::ExpectedFixedLenList {
-            expected_len,
-            actual,
-        }
-    }
-
-    pub fn expected_empty(actual: Value) -> Self {
-        Error::ExpectedEmpty { actual }
-    }
-
-    pub fn expected_map(actual: Value) -> Self {
-        Error::ExpectedMap { actual }
-    }
-
-    pub fn expected_table(actual: Value) -> Self {
-        Error::ExpectedTable { actual }
-    }
-
-    pub fn expected_function(actual: Value) -> Self {
-        Error::ExpectedFunction { actual }
-    }
-
-    pub fn expected_collection(actual: Value) -> Self {
-        Error::ExpectedCollection { actual }
-    }
-
-    pub(crate) fn unmatched_partial_token(
-        first: PartialToken,
-        second: Option<PartialToken>,
-    ) -> Self {
-        Error::UnmatchedPartialToken { first, second }
-    }
-
-    pub(crate) fn addition_error(augend: Value, addend: Value) -> Self {
-        Error::AdditionError { augend, addend }
-    }
-
-    pub(crate) fn subtraction_error(minuend: Value, subtrahend: Value) -> Self {
-        Error::SubtractionError {
-            minuend,
-            subtrahend,
-        }
-    }
-
-    pub(crate) fn negation_error(argument: Value) -> Self {
-        Error::NegationError { argument }
-    }
-
-    pub(crate) fn multiplication_error(multiplicand: Value, multiplier: Value) -> Self {
-        Error::MultiplicationError {
-            multiplicand,
-            multiplier,
-        }
-    }
-
-    pub(crate) fn division_error(dividend: Value, divisor: Value) -> Self {
-        Error::DivisionError { dividend, divisor }
-    }
-
-    pub(crate) fn modulation_error(dividend: Value, divisor: Value) -> Self {
-        Error::ModulationError { dividend, divisor }
-    }
-
-    /// Constructs `EvalexprError::InvalidRegex(regex)`
-    pub fn invalid_regex(regex: String, message: String) -> Self {
-        Error::InvalidRegex { regex, message }
-    }
-}
-
-/// Returns `Ok(())` if the given value is a string or a numeric.
-pub fn expect_number_or_string(actual: &Value) -> Result<()> {
-    match actual {
-        Value::String(_) | Value::Float(_) | Value::Integer(_) => Ok(()),
-        _ => Err(Error::expected_number_or_string(actual.clone())),
-    }
-}
-
-/// Returns `Ok(())` if the given value is a String, List, Map or Table.
-pub fn _expect_collection(actual: &Value) -> Result<()> {
-    match actual {
-        Value::String(_) | Value::List(_) | Value::Map(_) | Value::Table(_) => Ok(()),
-        _ => Err(Error::expected_collection(actual.clone())),
-    }
-}
-
-impl std::error::Error for Error {}
-
-impl fmt::Display for Error {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        use Error::*;
-
-        match self {
-            TypeCheckFailure { tool_info, argument } => write!(f, "Type check failure. This is a bug with the tool or with Dust's internal type checking. Please report this bug and include this error message.\nToolInfo = {tool_info:?}\nargument = {argument}"),
-            AssertEqualFailed {expected, actual } => write!(f, "Equality assertion failed. {expected} does not equal {actual}."),
-            AssertFailed => write!(f, "Assertion failed. A false value was passed to \"assert\"."),
-            ExpectedOperatorArgumentAmount { expected, actual } => write!(
-                f,
-                "An operator expected {} arguments, but got {}.",
-                expected, actual
-            ),
-            ExpectedFunctionArgumentAmount {
-                expected,
-                actual,
-                identifier,
-            } => write!(
-                f,
-                "{identifier} expected {expected} arguments, but got {actual}.",
-            ),
-            ExpectedAtLeastFunctionArgumentAmount {
-                minimum,
-                actual,
-                identifier,
-            } => write!(
-                f,
-                "{identifier} expected a minimum of {minimum} arguments, but got {actual}.",
-            ),
-            ExpectedString { actual } => {
-                write!(f, "Expected a Value::String, but got {:?}.", actual)
-            }
-            ExpectedInt { actual } => write!(f, "Expected a Value::Int, but got {:?}.", actual),
-            ExpectedFloat { actual } => write!(f, "Expected a Value::Float, but got {:?}.", actual),
-            ExpectedNumber { actual } => write!(
-                f,
-                "Expected a Value::Float or Value::Int, but got {:?}.",
-                actual
-            ),
-            ExpectedNumberOrString { actual } => write!(
-                f,
-                "Expected a Value::Number or a Value::String, but got {:?}.",
-                actual
-            ),
-            ExpectedBoolean { actual } => {
-                write!(f, "Expected a Value::Boolean, but got {:?}.", actual)
-            }
-            ExpectedList { actual } => write!(f, "Expected a Value::Tuple, but got {:?}.", actual),
-            ExpectedFixedLenList {
-                expected_len,
-                actual,
-            } => write!(
-                f,
-                "Expected a Value::Tuple of len {}, but got {:?}.",
-                expected_len, actual
-            ),
-            ExpectedEmpty { actual } => write!(f, "Expected a Value::Empty, but got {:?}.", actual),
-            ExpectedMap { actual } => write!(f, "Expected a Value::Map, but got {:?}.", actual),
-            ExpectedTable { actual } => write!(f, "Expected a Value::Table, but got {:?}.", actual),
-            ExpectedFunction { actual } => {
-                write!(f, "Expected Value::Function, but got {:?}.", actual)
-            }
-            ExpectedCollection { actual } => {
-                write!(
-                    f,
-                    "Expected a string, list, map or table, but got {:?}.",
-                    actual
-                )
-            }
-            AppendedToLeafNode(node) => write!(f, "Syntax error at \"{node}\"."),
-            PrecedenceViolation => write!(
-                f,
-                "Tried to append a node to another node with higher precedence."
-            ),
-            VariableIdentifierNotFound(identifier) => write!(
-                f,
-                "Variable identifier is not bound to anything by context: {:?}.",
-                identifier
-            ),
-            FunctionIdentifierNotFound(identifier) => write!(
-                f,
-                "Function identifier is not bound to anything by context: {:?}.",
-                identifier
-            ),
-            TypeError { expected, actual } => {
-                write!(
-                    f,
-                    "Type Error. The value {actual} is not one of the following: {expected:?}.",
-                )
-            }
-            WrongTypeCombination { operator, actual } => write!(
-                f,
-                "The operator {:?} was called with a wrong combination of types: {:?}",
-                operator, actual
-            ),
-            UnmatchedLBrace => write!(f, "Found an unmatched opening parenthesis '('."),
-            UnmatchedRBrace => write!(f, "Found an unmatched closing parenthesis ')'."),
-            MissingOperatorOutsideOfBrace { .. } => write!(
-                f,
-                "Found an opening parenthesis that is preceded by something that does not take \
-                 any arguments on the right, or found a closing parenthesis that is succeeded by \
-                 something that does not take any arguments on the left."
-            ),
-            UnmatchedPartialToken { first, second } => {
-                if let Some(second) = second {
-                    write!(
-                        f,
-                        "Found a partial token '{}' that should not be followed by '{}'.",
-                        first, second
-                    )
-                } else {
-                    write!(
-                        f,
-                        "Found a partial token '{}' that should be followed by another partial \
-                         token.",
-                        first
-                    )
-                }
-            }
-            AdditionError { augend, addend } => write!(f, "Error adding {} + {}", augend, addend),
-            SubtractionError {
-                minuend,
-                subtrahend,
-            } => write!(f, "Error subtracting {} - {}", minuend, subtrahend),
-            NegationError { argument } => write!(f, "Error negating -{}", argument),
-            MultiplicationError {
-                multiplicand,
-                multiplier,
-            } => write!(f, "Error multiplying {} * {}", multiplicand, multiplier),
-            DivisionError { dividend, divisor } => {
-                write!(f, "Error dividing {} / {}", dividend, divisor)
-            }
-            ModulationError { dividend, divisor } => {
-                write!(f, "Error modulating {} % {}", dividend, divisor)
-            }
-            InvalidRegex { regex, message } => write!(
-                f,
-                "Regular expression {:?} is invalid: {:?}",
-                regex, message
-            ),
-            ContextNotMutable => write!(f, "Cannot manipulate context"),
-            BuiltinFunctionsCannotBeEnabled => {
-                write!(f, "This context does not allow enabling builtin functions")
-            }
-            BuiltinFunctionsCannotBeDisabled => {
-                write!(f, "This context does not allow disabling builtin functions")
-            }
-            IllegalEscapeSequence(string) => write!(f, "Illegal escape sequence: {}", string),
-            MacroFailure(message) => write!(f, "Function failure: {}", message),
-            CustomMessage(message) => write!(f, "Error: {}", message),
-            WrongColumnAmount { expected, actual } => write!(
-                f,
-                "Wrong number of columns for this table. Expected {expected}, found {actual}."
-            ),
-            MacroArgumentType {
-                macro_info,
-                actual,
-            } => write!(
-                f,
-                "Wrong argument of type {:?} was passed to {}. Expected one of the following types: {:?}.",
-                actual.value_type(),
-                macro_info.identifier,
-                macro_info.inputs
-            ),
-        }
-    }
-}

src/error/mod.rs

@@ -0,0 +1,110 @@
+//! Error and Result types.
+//!
+//! To deal with errors from dependencies, either create a new error variant
+//! or use the ToolFailure variant if the error can only occur inside a tool.
+mod runtime_error;
+pub(crate) mod rw_lock_error;
+mod syntax_error;
+mod validation_error;
+
+use colored::Colorize;
+pub use runtime_error::RuntimeError;
+pub use syntax_error::SyntaxError;
+pub use validation_error::ValidationError;
+
+use tree_sitter::LanguageError;
+
+use std::fmt::{self, Formatter};
+
+#[derive(Debug, PartialEq)]
+pub enum Error {
+    Syntax(SyntaxError),
+
+    Validation(ValidationError),
+
+    Runtime(RuntimeError),
+
+    ParserCancelled,
+
+    Language(LanguageError),
+}
+
+impl Error {
+    /// Create a pretty error report with `lyneate`.
+    ///
+    /// The `source` argument should be the full source code document that was
+    /// used to create this error.
+    pub fn create_report(&self, source: &str) -> String {
+        match self {
+            Error::Syntax(syntax_error) => {
+                let report = syntax_error.create_report(source);
+
+                format!(
+                    "{}\n{}\n{report}",
+                    "Syntax Error".bold().yellow().underline(),
+                    "Dust does not recognize this syntax.".dimmed()
+                )
+            }
+            Error::Validation(validation_error) => {
+                let report = validation_error.create_report(source);
+
+                format!(
+                    "{}\n{}\n{report}",
+                    "Validation Error".bold().yellow().underline(),
+                    "Dust prevented the program from running.".dimmed()
+                )
+            }
+            Error::Runtime(runtime_error) => {
+                let report = runtime_error.create_report(source);
+
+                format!(
+                    "{}\n{}\n{report}",
+                    "Runtime Error".bold().red().underline(),
+                    "This error occured while the program was running.".dimmed()
+                )
+            }
+            Error::ParserCancelled => todo!(),
+            Error::Language(_) => todo!(),
+        }
+    }
+}
+
+impl From<SyntaxError> for Error {
+    fn from(error: SyntaxError) -> Self {
+        Error::Syntax(error)
+    }
+}
+
+impl From<ValidationError> for Error {
+    fn from(error: ValidationError) -> Self {
+        Error::Validation(error)
+    }
+}
+
+impl From<RuntimeError> for Error {
+    fn from(error: RuntimeError) -> Self {
+        Error::Runtime(error)
+    }
+}
+
+impl From<LanguageError> for Error {
+    fn from(error: LanguageError) -> Self {
+        Error::Language(error)
+    }
+}
+
+impl std::error::Error for Error {}
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        use Error::*;
+
+        match self {
+            Syntax(error) => write!(f, "{error}"),
+            Validation(error) => write!(f, "{error}"),
+            Runtime(error) => write!(f, "{error}"),
+            ParserCancelled => write!(f, "Parsing was cancelled because the parser took too long."),
+            Language(_error) => write!(f, "Parser failed to load language grammar."),
+        }
+    }
+}

src/error/runtime_error.rs

@@ -0,0 +1,193 @@
+use std::{
+    fmt::{self, Debug, Display, Formatter},
+    io,
+    num::ParseFloatError,
+    string::FromUtf8Error,
+    sync::PoisonError,
+    time,
+};
+
+use lyneate::Report;
+
+use crate::{SourcePosition, Type, Value};
+
+use super::{rw_lock_error::RwLockError, ValidationError};
+
+#[derive(Debug, PartialEq)]
+pub enum RuntimeError {
+    /// The 'assert' macro did not resolve successfully.
+    AssertEqualFailed {
+        left: Value,
+        right: Value,
+    },
+
+    /// The 'assert' macro did not resolve successfully.
+    AssertFailed {
+        assertion: Value,
+    },
+
+    /// The attempted conversion is impossible.
+    ConversionImpossible {
+        from: Type,
+        to: Type,
+        position: SourcePosition,
+    },
+
+    Csv(String),
+
+    Io(String),
+
+    Reqwest(String),
+
+    Json(String),
+
+    SystemTime(String),
+
+    Toml(toml::de::Error),
+
+    /// Failed to read or write a map.
+    ///
+    /// See the [MapError] docs for more info.
+    RwLock(RwLockError),
+
+    ParseFloat(ParseFloatError),
+
+    Utf8(FromUtf8Error),
+
+    /// A built-in function was called with the wrong amount of arguments.
+    ExpectedBuiltInFunctionArgumentAmount {
+        function_name: String,
+        expected: usize,
+        actual: usize,
+    },
+
+    ValidationFailure(ValidationError),
+}
+
+impl RuntimeError {
+    pub fn create_report(&self, source: &str) -> String {
+        let messages = match self {
+            RuntimeError::AssertEqualFailed {
+                left: expected,
+                right: actual,
+            } => {
+                vec![(
+                    0..source.len(),
+                    format!("\"assert_equal\" failed. {} != {}", expected, actual),
+                    (200, 0, 0),
+                )]
+            }
+            RuntimeError::AssertFailed { assertion: _ } => todo!(),
+            RuntimeError::ConversionImpossible { from, to, position } => vec![(
+                position.start_byte..position.end_byte,
+                format!("Cannot convert from {from} to {to}."),
+                (255, 64, 112),
+            )],
+            RuntimeError::Csv(_) => todo!(),
+            RuntimeError::Io(_) => todo!(),
+            RuntimeError::Reqwest(_) => todo!(),
+            RuntimeError::Json(_) => todo!(),
+            RuntimeError::SystemTime(_) => todo!(),
+            RuntimeError::Toml(_) => todo!(),
+            RuntimeError::RwLock(_) => todo!(),
+            RuntimeError::ParseFloat(_) => todo!(),
+            RuntimeError::Utf8(_) => todo!(),
+            RuntimeError::ExpectedBuiltInFunctionArgumentAmount {
+                function_name: _,
+                expected: _,
+                actual: _,
+            } => todo!(),
+            RuntimeError::ValidationFailure(_) => todo!(),
+        };
+
+        Report::new_byte_spanned(source, messages).display_str()
+    }
+
+    pub fn expect_argument_amount(
+        function_name: &str,
+        expected: usize,
+        actual: usize,
+    ) -> Result<(), Self> {
+        if expected == actual {
+            Ok(())
+        } else {
+            Err(RuntimeError::ExpectedBuiltInFunctionArgumentAmount {
+                function_name: function_name.to_string(),
+                expected,
+                actual,
+            })
+        }
+    }
+}
+
+impl From<ValidationError> for RuntimeError {
+    fn from(error: ValidationError) -> Self {
+        RuntimeError::ValidationFailure(error)
+    }
+}
+
+impl From<csv::Error> for RuntimeError {
+    fn from(error: csv::Error) -> Self {
+        RuntimeError::Csv(error.to_string())
+    }
+}
+
+impl From<io::Error> for RuntimeError {
+    fn from(error: std::io::Error) -> Self {
+        RuntimeError::Io(error.to_string())
+    }
+}
+
+impl From<reqwest::Error> for RuntimeError {
+    fn from(error: reqwest::Error) -> Self {
+        RuntimeError::Reqwest(error.to_string())
+    }
+}
+
+impl From<serde_json::Error> for RuntimeError {
+    fn from(error: serde_json::Error) -> Self {
+        RuntimeError::Json(error.to_string())
+    }
+}
+
+impl From<time::SystemTimeError> for RuntimeError {
+    fn from(error: time::SystemTimeError) -> Self {
+        RuntimeError::SystemTime(error.to_string())
+    }
+}
+
+impl From<toml::de::Error> for RuntimeError {
+    fn from(error: toml::de::Error) -> Self {
+        RuntimeError::Toml(error)
+    }
+}
+
+impl From<ParseFloatError> for RuntimeError {
+    fn from(error: ParseFloatError) -> Self {
+        RuntimeError::ParseFloat(error)
+    }
+}
+
+impl From<FromUtf8Error> for RuntimeError {
+    fn from(error: FromUtf8Error) -> Self {
+        RuntimeError::Utf8(error)
+    }
+}
+
+impl From<RwLockError> for RuntimeError {
+    fn from(error: RwLockError) -> Self {
+        RuntimeError::RwLock(error)
+    }
+}
+
+impl<T> From<PoisonError<T>> for RuntimeError {
+    fn from(_: PoisonError<T>) -> Self {
+        RuntimeError::RwLock(RwLockError)
+    }
+}
+
+impl Display for RuntimeError {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        write!(f, "{self:?}")
+    }
+}

src/error/rw_lock_error.rs

@@ -0,0 +1,30 @@
+use std::{
+    fmt::{self, Debug, Display, Formatter},
+    sync::PoisonError,
+};
+
+use serde::{Deserialize, Serialize};
+
+#[derive(Clone, PartialEq, Serialize, Deserialize)]
+pub struct RwLockError;
+
+impl Display for RwLockError {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "Map error: failed to acquire a read/write lock because another thread has panicked."
+        )
+    }
+}
+
+impl Debug for RwLockError {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        write!(f, "{self}")
+    }
+}
+
+impl<T> From<PoisonError<T>> for RwLockError {
+    fn from(_: PoisonError<T>) -> Self {
+        RwLockError
+    }
+}

src/error/syntax_error.rs

@@ -0,0 +1,126 @@
+use std::fmt::{self, Display, Formatter};
+
+use colored::Colorize;
+use lyneate::Report;
+use serde::{Deserialize, Serialize};
+use tree_sitter::Node as SyntaxNode;
+
+use crate::SourcePosition;
+
+use super::rw_lock_error::RwLockError;
+
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub enum SyntaxError {
+    /// Invalid user input.
+    InvalidSource {
+        expected: String,
+        actual: String,
+        position: SourcePosition,
+    },
+
+    RwLock(RwLockError),
+
+    UnexpectedSyntaxNode {
+        expected: String,
+        actual: String,
+        position: SourcePosition,
+    },
+}
+
+impl SyntaxError {
+    pub fn create_report(&self, source: &str) -> String {
+        let messages = match self {
+            SyntaxError::InvalidSource { position, .. } => self
+                .to_string()
+                .split_inclusive(".")
+                .map(|message_part| {
+                    (
+                        position.start_byte..position.end_byte,
+                        message_part.to_string(),
+                        (255, 200, 100),
+                    )
+                })
+                .collect(),
+            SyntaxError::RwLock(_) => todo!(),
+            SyntaxError::UnexpectedSyntaxNode { position, .. } => {
+                vec![(
+                    position.start_byte..position.end_byte,
+                    self.to_string(),
+                    (255, 200, 100),
+                )]
+            }
+        };
+
+        Report::new_byte_spanned(source, messages).display_str()
+    }
+
+    pub fn expect_syntax_node(expected: &str, actual: SyntaxNode) -> Result<(), SyntaxError> {
+        log::trace!("Converting {} to abstract node", actual.kind());
+
+        if expected == actual.kind() {
+            Ok(())
+        } else if actual.is_error() {
+            Err(SyntaxError::InvalidSource {
+                expected: expected.to_owned(),
+                actual: actual.kind().to_string(),
+                position: SourcePosition::from(actual.range()),
+            })
+        } else {
+            Err(SyntaxError::UnexpectedSyntaxNode {
+                expected: expected.to_string(),
+                actual: actual.kind().to_string(),
+                position: SourcePosition::from(actual.range()),
+            })
+        }
+    }
+}
+
+impl From<RwLockError> for SyntaxError {
+    fn from(error: RwLockError) -> Self {
+        SyntaxError::RwLock(error)
+    }
+}
+
+impl Display for SyntaxError {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        match self {
+            SyntaxError::InvalidSource {
+                expected,
+                actual,
+                position,
+            } => {
+                let actual = if actual == "ERROR" {
+                    "unrecognized characters"
+                } else {
+                    actual
+                };
+
+                write!(
+                    f,
+                    "Invalid syntax from ({}, {}) to ({}, {}). Exected {} but found {}.",
+                    position.start_row,
+                    position.start_column,
+                    position.end_row,
+                    position.end_column,
+                    expected.bold().green(),
+                    actual.bold().red(),
+                )
+            }
+            SyntaxError::RwLock(_) => todo!(),
+            SyntaxError::UnexpectedSyntaxNode {
+                expected,
+                actual,
+                position,
+            } => {
+                write!(
+                        f,
+                        "Interpreter Error. Tried to parse {actual} as {expected} from ({}, {}) to ({}, {}).",
+                        position.start_row,
+                        position.start_column,
+                        position.end_row,
+                        position.end_column,
+                    )
+            }
+        }
+    }
+}

src/error/validation_error.rs

@@ -0,0 +1,274 @@
+use std::fmt::{self, Display, Formatter};
+
+use colored::Colorize;
+use lyneate::Report;
+use serde::{Deserialize, Serialize};
+
+use crate::{Identifier, SourcePosition, Type, TypeDefinition, Value};
+
+use super::rw_lock_error::RwLockError;
+
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub enum ValidationError {
+    /// Two value are incompatible for addition.
+    CannotAdd {
+        left: Value,
+        right: Value,
+        position: SourcePosition,
+    },
+
+    /// Two value are incompatible for subtraction.
+    CannotSubtract {
+        left: Value,
+        right: Value,
+        position: SourcePosition,
+    },
+
+    /// Two value are incompatible for multiplication.
+    CannotMultiply {
+        left: Value,
+        right: Value,
+        position: SourcePosition,
+    },
+
+    /// Two value are incompatible for dividing.
+    CannotDivide {
+        left: Value,
+        right: Value,
+        position: SourcePosition,
+    },
+
+    /// The attempted conversion is impossible.
+    ConversionImpossible {
+        initial_type: Type,
+        target_type: Type,
+    },
+
+    ExpectedString {
+        actual: Value,
+    },
+
+    ExpectedInteger {
+        actual: Value,
+    },
+
+    ExpectedFloat {
+        actual: Value,
+    },
+
+    /// An integer, floating point or value was expected.
+    ExpectedNumber {
+        actual: Value,
+    },
+
+    /// An integer, floating point or string value was expected.
+    ExpectedNumberOrString {
+        actual: Value,
+    },
+
+    ExpectedBoolean {
+        actual: Value,
+    },
+
+    ExpectedList {
+        actual: Value,
+    },
+
+    ExpectedMinLengthList {
+        minimum_len: usize,
+        actual_len: usize,
+    },
+
+    ExpectedFixedLenList {
+        expected_len: usize,
+        actual: Value,
+    },
+
+    ExpectedMap {
+        actual: Value,
+    },
+
+    ExpectedFunction {
+        actual: Value,
+    },
+
+    /// A string, list, map or table value was expected.
+    ExpectedCollection {
+        actual: Value,
+    },
+
+    /// A built-in function was called with the wrong amount of arguments.
+    ExpectedBuiltInFunctionArgumentAmount {
+        function_name: String,
+        expected: usize,
+        actual: usize,
+    },
+
+    /// A function was called with the wrong amount of arguments.
+    ExpectedFunctionArgumentAmount {
+        expected: usize,
+        actual: usize,
+        position: SourcePosition,
+    },
+
+    /// A function was called with the wrong amount of arguments.
+    ExpectedFunctionArgumentMinimum {
+        minumum_expected: usize,
+        actual: usize,
+        position: SourcePosition,
+    },
+
+    /// Failed to read or write a map.
+    ///
+    /// See the [MapError] docs for more info.
+    RwLock(RwLockError),
+
+    TypeCheck {
+        expected: Type,
+        actual: Type,
+        position: SourcePosition,
+    },
+
+    TypeCheckExpectedFunction {
+        actual: Type,
+        position: SourcePosition,
+    },
+
+    /// Failed to find a value with this key.
+    VariableIdentifierNotFound(Identifier),
+
+    /// Failed to find a type definition with this key.
+    TypeDefinitionNotFound(Identifier),
+
+    /// Failed to find an enum definition with this key.
+    ExpectedEnumDefintion {
+        actual: TypeDefinition,
+    },
+
+    /// Failed to find a struct definition with this key.
+    ExpectedStructDefintion {
+        actual: TypeDefinition,
+    },
+}
+
+impl ValidationError {
+    pub fn create_report(&self, source: &str) -> String {
+        let messages = match self {
+            ValidationError::CannotAdd {
+                left: _,
+                right: _,
+                position,
+            } => vec![
+                ((
+                    position.start_byte..position.end_byte,
+                    format!(""),
+                    (255, 159, 64),
+                )),
+            ],
+            ValidationError::CannotSubtract {
+                left: _,
+                right: _,
+                position: _,
+            } => todo!(),
+            ValidationError::CannotMultiply {
+                left: _,
+                right: _,
+                position: _,
+            } => todo!(),
+            ValidationError::CannotDivide {
+                left: _,
+                right: _,
+                position: _,
+            } => todo!(),
+            ValidationError::ConversionImpossible {
+                initial_type: _,
+                target_type: _,
+            } => todo!(),
+            ValidationError::ExpectedString { actual: _ } => todo!(),
+            ValidationError::ExpectedInteger { actual: _ } => todo!(),
+            ValidationError::ExpectedFloat { actual: _ } => todo!(),
+            ValidationError::ExpectedNumber { actual: _ } => todo!(),
+            ValidationError::ExpectedNumberOrString { actual: _ } => todo!(),
+            ValidationError::ExpectedBoolean { actual: _ } => todo!(),
+            ValidationError::ExpectedList { actual: _ } => todo!(),
+            ValidationError::ExpectedMinLengthList {
+                minimum_len: _,
+                actual_len: _,
+            } => todo!(),
+            ValidationError::ExpectedFixedLenList {
+                expected_len: _,
+                actual: _,
+            } => todo!(),
+            ValidationError::ExpectedMap { actual: _ } => todo!(),
+            ValidationError::ExpectedFunction { actual: _ } => todo!(),
+            ValidationError::ExpectedCollection { actual: _ } => todo!(),
+            ValidationError::ExpectedBuiltInFunctionArgumentAmount {
+                function_name: _,
+                expected: _,
+                actual: _,
+            } => todo!(),
+            ValidationError::ExpectedFunctionArgumentAmount {
+                expected: _,
+                actual: _,
+                position: _,
+            } => todo!(),
+            ValidationError::ExpectedFunctionArgumentMinimum {
+                minumum_expected: _,
+                actual: _,
+                position: _,
+            } => todo!(),
+            ValidationError::RwLock(_) => todo!(),
+            ValidationError::TypeCheck {
+                expected,
+                actual,
+                position,
+            } => vec![(
+                position.start_byte..position.end_byte,
+                format!(
+                    "Type {} is incompatible with {}.",
+                    actual.to_string().bold().red(),
+                    expected.to_string().bold().green()
+                ),
+                (200, 200, 200),
+            )],
+            ValidationError::TypeCheckExpectedFunction {
+                actual: _,
+                position: _,
+            } => todo!(),
+            ValidationError::VariableIdentifierNotFound(_) => todo!(),
+            ValidationError::TypeDefinitionNotFound(_) => todo!(),
+            ValidationError::ExpectedEnumDefintion { actual: _ } => todo!(),
+            ValidationError::ExpectedStructDefintion { actual: _ } => todo!(),
+        };
+
+        Report::new_byte_spanned(source, messages).display_str()
+    }
+
+    pub fn expect_argument_amount(
+        function_name: &str,
+        expected: usize,
+        actual: usize,
+    ) -> Result<(), Self> {
+        if expected == actual {
+            Ok(())
+        } else {
+            Err(ValidationError::ExpectedBuiltInFunctionArgumentAmount {
+                function_name: function_name.to_string(),
+                expected,
+                actual,
+            })
+        }
+    }
+}
+
+impl From<RwLockError> for ValidationError {
+    fn from(_error: RwLockError) -> Self {
+        ValidationError::RwLock(RwLockError)
+    }
+}
+
+impl Display for ValidationError {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        write!(f, "{self:?}")
+    }
+}

src/interface.rs

@@ -1,61 +0,0 @@
-//! The top level of Dust's API with functions in interpret Dust code.
-
-use crate::{token, tree, Result, Value, VariableMap};
-
-/// Evaluate the given expression string.
-///
-/// # Examples
-///
-/// ```rust
-/// # use dust_lib::*;
-/// assert_eq!(eval("1 + 2 + 3"), Ok(Value::from(6)));
-/// ```
-///
-/// *See the [crate doc](index.html) for more examples and explanations of the expression format.*
-pub fn eval(string: &str) -> Result<Value> {
-    let mut context = VariableMap::new();
-    eval_with_context(string, &mut context)
-}
-
-/// Evaluate the given expression string with the given context.
-///
-/// # Examples
-///
-/// ```rust
-/// # use dust_lib::*;
-/// let mut context = VariableMap::new();
-/// context.set_value("one".into(), 1.into()).unwrap(); // Do proper error handling here
-/// context.set_value("two".into(), 2.into()).unwrap(); // Do proper error handling here
-/// context.set_value("three".into(), 3.into()).unwrap(); // Do proper error handling here
-/// assert_eq!(eval_with_context("one + two + three", &mut context), Ok(Value::from(6)));
-/// ```
-pub fn eval_with_context(input: &str, context: &mut VariableMap) -> Result<Value> {
-    let without_comments = input
-        .lines()
-        .map(|line| {
-            let split = line.split_once('#');
-
-            if let Some((code, _comment)) = split {
-                code
-            } else {
-                line
-            }
-        })
-        .collect::<String>();
-
-    let split = without_comments.split_once("->");
-
-    if let Some((left, right)) = split {
-        let left_result = tree::tokens_to_operator_tree(token::tokenize(left)?)?
-            .eval_with_context_mut(context)?;
-
-        context.set_value("input", left_result)?;
-
-        let right_result = eval_with_context(right, context)?;
-
-        Ok(right_result)
-    } else {
-        tree::tokens_to_operator_tree(token::tokenize(&without_comments)?)?
-            .eval_with_context_mut(context)
-    }
-}

src/interpret.rs

@@ -0,0 +1,157 @@
+//! Tools to interpret dust source code.
+//!
+//! This module has three tools to run Dust code.
+//!
+//! - [interpret] is the simplest way to run Dust code inside of an application or library
+//! - [interpret_with_context] allows you to set variables on the execution context
+//! - [Interpreter] is an advanced tool that can parse, validate, run and format Dust code
+//!
+//! # Examples
+//!
+//! Run some Dust and get the result.
+//!
+//! ```rust
+//! # use dust_lang::*;
+//! assert_eq!(
+//!     interpret("1 + 2 + 3"),
+//!     Ok(Value::Integer(6))
+//! );
+//! ```
+//!
+//! Create a custom context with variables you can use in your code.
+//!
+//! ```rust
+//! # use dust_lang::*;
+//! let context = Context::default();
+//!
+//! context.set_value("one".into(), 1.into()).unwrap();
+//! context.set_value("two".into(), 2.into()).unwrap();
+//! context.set_value("three".into(), 3.into()).unwrap();
+//!
+//! let dust_code = "four = 4; one + two + three + four";
+//!
+//! assert_eq!(
+//!     interpret_with_context(dust_code, context),
+//!     Ok(Value::Integer(10))
+//! );
+//! ```
+use tree_sitter::{Parser, Tree as SyntaxTree};
+
+use crate::{language, AbstractTree, Context, ContextMode, Error, Format, Root, Value};
+
+/// Interpret the given source code. Returns the value of last statement or the
+/// first error encountered.
+///
+/// See the [module-level docs][self] for more info.
+pub fn interpret(source: &str) -> Result<Value, Error> {
+    interpret_with_context(source, Context::new(ContextMode::RemoveGarbage))
+}
+
+/// Interpret the given source code with the given context.
+///
+/// See the [module-level docs][self] for more info.
+pub fn interpret_with_context(source: &str, context: Context) -> Result<Value, Error> {
+    let mut interpreter = Interpreter::new(context);
+    let value = interpreter.run(source)?;
+
+    Ok(value)
+}
+
+/// A source code interpreter for the Dust language.
+///
+/// The interpreter's most important functions are used to parse dust source
+/// code, verify it is safe and run it. They are written in a way that forces
+/// them to be used safely: each step in this process contains the prior
+/// steps, meaning that the same code is always used to create the syntax tree,
+/// abstract tree and final evaluation. This avoids a critical logic error.
+///
+/// ```
+/// # use dust_lang::*;
+/// let context = Context::default();
+/// let mut interpreter = Interpreter::new(context);
+/// let result = interpreter.run("2 + 2");
+///
+/// assert_eq!(result, Ok(Value::Integer(4)));
+/// ```
+pub struct Interpreter {
+    parser: Parser,
+    context: Context,
+}
+
+impl Interpreter {
+    /// Create a new interpreter with the given context.
+    pub fn new(context: Context) -> Self {
+        let mut parser = Parser::new();
+
+        parser
+            .set_language(language())
+            .expect("Language version is incompatible with tree sitter version.");
+
+        parser.set_logger(Some(Box::new(|_log_type, message| {
+            log::trace!("{}", message)
+        })));
+
+        Interpreter { parser, context }
+    }
+
+    /// Generate a syntax tree from the source. Returns an error if the the
+    /// parser is cancelled for taking too long. The syntax tree may contain
+    /// error nodes, which represent syntax errors.
+    ///
+    /// Tree sitter is designed to be run on every keystroke, so this is
+    /// generally a lightweight function to call.
+    pub fn parse(&mut self, source: &str) -> Result<SyntaxTree, Error> {
+        if let Some(tree) = self.parser.parse(source, None) {
+            Ok(tree)
+        } else {
+            Err(Error::ParserCancelled)
+        }
+    }
+
+    /// Check the source for errors and generate an abstract tree.
+    ///
+    /// The order in which this function works is:
+    ///
+    /// - parse the source into a syntax tree
+    /// - generate an abstract tree from the source and syntax tree
+    /// - check the abstract tree for errors
+    pub fn validate(&mut self, source: &str) -> Result<Root, Error> {
+        let syntax_tree = self.parse(source)?;
+        let abstract_tree = Root::from_syntax(syntax_tree.root_node(), source, &self.context)?;
+
+        abstract_tree.validate(source, &self.context)?;
+
+        Ok(abstract_tree)
+    }
+
+    /// Run the source, returning the final statement's value or first error.
+    ///
+    /// This function [parses][Self::parse], [validates][Self::validate] and
+    /// [runs][Root::run] using the same source code.
+    pub fn run(&mut self, source: &str) -> Result<Value, Error> {
+        let final_value = self.validate(source)?.run(source, &self.context)?;
+
+        Ok(final_value)
+    }
+
+    /// Return an s-expression displaying a syntax tree of the source or an
+    /// error.
+    pub fn syntax_tree(&mut self, source: &str) -> Result<String, Error> {
+        Ok(self.parse(source)?.root_node().to_sexp())
+    }
+
+    /// Return a formatted version of the source.
+    pub fn format(&mut self, source: &str) -> Result<String, Error> {
+        let mut formatted_output = String::new();
+
+        self.validate(source)?.format(&mut formatted_output, 0);
+
+        Ok(formatted_output)
+    }
+}
+
+impl Default for Interpreter {
+    fn default() -> Self {
+        Interpreter::new(Context::default())
+    }
+}

src/lib.rs

@@ -1,28 +1,47 @@
-//! The Dust library is used to implement the Dust language, `src/main.rs` implements the command
-//! line binary.
+//! The Dust library is used to parse, format and run dust source code.
 //!
-//! Using this library is simple and straightforward, see the [inferface] module for instructions on
-//! interpreting Dust code. Most of the language's features are implemented in the [tools] module.
-#![forbid(unsafe_code)]
-
+//! See the [interpret] module for more information.
+//!
+//! You can use this library externally by calling either of the "interpret"
+//! functions or by constructing your own Interpreter.
 pub use crate::{
-    error::*,
-    interface::*,
-    operator::Operator,
-    token::PartialToken,
-    tools::{Tool, ToolInfo, TOOL_LIST},
-    tree::Node,
-    value::{
-        function::Function, table::Table, time::Time, value_type::ValueType,
-        variable_map::VariableMap, Value,
-    },
+    abstract_tree::*, built_in_functions::BuiltInFunction, context::*, error::Error, interpret::*,
+    value::*,
 };
 
-pub mod tools;
+pub use tree_sitter::Node as SyntaxNode;
 
-mod error;
-mod interface;
-mod operator;
-mod token;
-mod tree;
-mod value;
+pub mod abstract_tree;
+pub mod built_in_functions;
+pub mod built_in_identifiers;
+pub mod built_in_type_definitions;
+pub mod built_in_types;
+pub mod built_in_values;
+pub mod context;
+pub mod error;
+pub mod interpret;
+pub mod value;
+
+use tree_sitter::Language;
+
+extern "C" {
+    fn tree_sitter_dust() -> Language;
+}
+
+/// Get the tree-sitter [Language][] for this grammar.
+///
+/// [Language]: https://docs.rs/tree-sitter/*/tree_sitter/struct.Language.html
+pub fn language() -> Language {
+    unsafe { tree_sitter_dust() }
+}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn test_can_load_grammar() {
+        let mut parser = tree_sitter::Parser::new();
+        parser
+            .set_language(super::language())
+            .expect("Error loading dust language");
+    }
+}

src/main.rs

@@ -0,0 +1,407 @@
+//! Command line interface for the dust programming language.
+
+use clap::{Parser, Subcommand};
+use colored::Colorize;
+use crossterm::event::{KeyCode, KeyModifiers};
+use nu_ansi_term::{Color, Style};
+use reedline::{
+    default_emacs_keybindings, ColumnarMenu, Completer, DefaultHinter, EditCommand, Emacs, Prompt,
+    Reedline, ReedlineEvent, ReedlineMenu, Signal, Span, SqliteBackedHistory, Suggestion,
+};
+
+use std::{borrow::Cow, fs::read_to_string, io::Write, path::PathBuf, process::Command};
+
+use dust_lang::{
+    built_in_values::all_built_in_values, Context, ContextMode, Error, Interpreter, Value,
+    ValueData,
+};
+
+/// Command-line arguments to be parsed.
+#[derive(Parser, Debug)]
+#[command(author, version, about, long_about = None)]
+struct Args {
+    /// Dust source code to evaluate.
+    #[arg(short, long)]
+    command: Option<String>,
+
+    /// Command for alternate functionality besides running the source.
+    #[command(subcommand)]
+    cli_command: Option<CliCommand>,
+
+    /// Location of the file to run.
+    path: Option<String>,
+}
+
+#[derive(Subcommand, Debug)]
+pub enum CliCommand {
+    /// Output a formatted version of the input.
+    Format,
+
+    /// Output a concrete syntax tree of the input.
+    Syntax { path: String },
+}
+
+fn main() {
+    env_logger::Builder::from_env("DUST_LOG")
+        .format(|buffer, record| {
+            let args = record.args();
+            let log_level = record.level().to_string().bold();
+            let timestamp = buffer.timestamp_seconds().to_string().dimmed();
+
+            writeln!(buffer, "[{log_level} {timestamp}] {args}")
+        })
+        .init();
+
+    let args = Args::parse();
+    let context = Context::new(ContextMode::AllowGarbage);
+
+    if args.path.is_none() && args.command.is_none() {
+        let run_shell_result = run_shell(context);
+
+        match run_shell_result {
+            Ok(_) => {}
+            Err(error) => eprintln!("{error}"),
+        }
+
+        return;
+    }
+
+    let source = if let Some(path) = &args.path {
+        read_to_string(path).unwrap()
+    } else if let Some(command) = args.command {
+        command
+    } else {
+        String::with_capacity(0)
+    };
+
+    let mut interpreter = Interpreter::new(context);
+
+    if let Some(CliCommand::Syntax { path }) = args.cli_command {
+        let source = read_to_string(path).unwrap();
+        let syntax_tree_sexp = interpreter.syntax_tree(&source).unwrap();
+
+        println!("{syntax_tree_sexp}");
+
+        return;
+    }
+
+    if let Some(CliCommand::Format) = args.cli_command {
+        let formatted = interpreter.format(&source).unwrap();
+
+        println!("{formatted}");
+
+        return;
+    }
+
+    let eval_result = interpreter.run(&source);
+
+    match eval_result {
+        Ok(value) => {
+            if !value.is_none() {
+                println!("{value}")
+            }
+        }
+        Err(error) => eprintln!("{}", error.create_report(&source)),
+    }
+}
+
+// struct DustHighlighter {
+//     context: Context,
+// }
+
+// impl DustHighlighter {
+//     fn new(context: Context) -> Self {
+//         Self { context }
+//     }
+// }
+
+// const HIGHLIGHT_TERMINATORS: [char; 8] = [' ', ':', '(', ')', '{', '}', '[', ']'];
+
+// impl Highlighter for DustHighlighter {
+//     fn highlight(&self, line: &str, _cursor: usize) -> reedline::StyledText {
+//         let mut styled = StyledText::new();
+
+//         for word in line.split_inclusive(&HIGHLIGHT_TERMINATORS) {
+//             let mut word_is_highlighted = false;
+
+//             for key in self.context.inner().unwrap().keys() {
+//                 if key == &word {
+//                     styled.push((Style::new().bold(), word.to_string()));
+//                 }
+
+//                 word_is_highlighted = true;
+//             }
+
+//             for built_in_value in built_in_values() {
+//                 if built_in_value.name() == word {
+//                     styled.push((Style::new().bold(), word.to_string()));
+//                 }
+
+//                 word_is_highlighted = true;
+//             }
+
+//             if word_is_highlighted {
+//                 let final_char = word.chars().last().unwrap();
+
+//                 if HIGHLIGHT_TERMINATORS.contains(&final_char) {
+//                     let mut terminator_style = Style::new();
+
+//                     terminator_style.foreground = Some(Color::Cyan);
+
+//                     styled.push((terminator_style, final_char.to_string()));
+//                 }
+//             } else {
+//                 styled.push((Style::new(), word.to_string()));
+//             }
+//         }
+
+//         styled
+//     }
+// }
+
+struct StarshipPrompt {
+    left: String,
+    right: String,
+}
+
+impl StarshipPrompt {
+    fn new() -> Self {
+        Self {
+            left: String::new(),
+            right: String::new(),
+        }
+    }
+
+    fn reload(&mut self) {
+        let run_starship_left = Command::new("starship").arg("prompt").output();
+        let run_starship_right = Command::new("starship")
+            .args(["prompt", "--right"])
+            .output();
+        let left_prompt = if let Ok(output) = &run_starship_left {
+            String::from_utf8_lossy(&output.stdout).trim().to_string()
+        } else {
+            ">".to_string()
+        };
+        let right_prompt = if let Ok(output) = &run_starship_right {
+            String::from_utf8_lossy(&output.stdout).trim().to_string()
+        } else {
+            "".to_string()
+        };
+
+        self.left = left_prompt;
+        self.right = right_prompt;
+    }
+}
+
+impl Prompt for StarshipPrompt {
+    fn render_prompt_left(&self) -> Cow<str> {
+        Cow::Borrowed(&self.left)
+    }
+
+    fn render_prompt_right(&self) -> Cow<str> {
+        Cow::Borrowed(&self.right)
+    }
+
+    fn render_prompt_indicator(&self, _prompt_mode: reedline::PromptEditMode) -> Cow<str> {
+        Cow::Borrowed(" ")
+    }
+
+    fn render_prompt_multiline_indicator(&self) -> Cow<str> {
+        Cow::Borrowed("")
+    }
+
+    fn render_prompt_history_search_indicator(
+        &self,
+        _history_search: reedline::PromptHistorySearch,
+    ) -> Cow<str> {
+        Cow::Borrowed("")
+    }
+}
+
+pub struct DustCompleter {
+    context: Context,
+}
+
+impl DustCompleter {
+    fn new(context: Context) -> Self {
+        DustCompleter { context }
+    }
+}
+
+impl Completer for DustCompleter {
+    fn complete(&mut self, line: &str, pos: usize) -> Vec<Suggestion> {
+        let mut suggestions = Vec::new();
+        let last_word = if let Some(word) = line.rsplit([' ', ':']).next() {
+            word
+        } else {
+            line
+        };
+
+        if let Ok(path) = PathBuf::try_from(last_word) {
+            if let Ok(read_dir) = path.read_dir() {
+                for entry in read_dir {
+                    if let Ok(entry) = entry {
+                        let description = if let Ok(file_type) = entry.file_type() {
+                            if file_type.is_dir() {
+                                "directory"
+                            } else if file_type.is_file() {
+                                "file"
+                            } else if file_type.is_symlink() {
+                                "symlink"
+                            } else {
+                                "unknown"
+                            }
+                        } else {
+                            "unknown"
+                        };
+
+                        suggestions.push(Suggestion {
+                            value: entry.path().to_string_lossy().to_string(),
+                            description: Some(description.to_string()),
+                            extra: None,
+                            span: Span::new(pos - last_word.len(), pos),
+                            append_whitespace: false,
+                        });
+                    }
+                }
+            }
+        }
+
+        for built_in_value in all_built_in_values() {
+            let name = built_in_value.name();
+            let description = built_in_value.description();
+
+            if built_in_value.name().contains(last_word) {
+                suggestions.push(Suggestion {
+                    value: name.to_string(),
+                    description: Some(description.to_string()),
+                    extra: None,
+                    span: Span::new(pos - last_word.len(), pos),
+                    append_whitespace: false,
+                });
+            }
+
+            if let Value::Map(map) = built_in_value.get() {
+                for (key, value) in map.inner() {
+                    if key.contains(last_word) {
+                        suggestions.push(Suggestion {
+                            value: format!("{name}:{key}"),
+                            description: Some(value.to_string()),
+                            extra: None,
+                            span: Span::new(pos - last_word.len(), pos),
+                            append_whitespace: false,
+                        });
+                    }
+                }
+            }
+        }
+
+        for (key, (value_data, _counter)) in self.context.inner().unwrap().iter() {
+            let value = match value_data {
+                ValueData::Value(value) => value,
+                ValueData::TypeHint(_) => continue,
+                ValueData::TypeDefinition(_) => continue,
+            };
+
+            if key.contains(last_word) {
+                suggestions.push(Suggestion {
+                    value: key.to_string(),
+                    description: Some(value.to_string()),
+                    extra: None,
+                    span: Span::new(pos - last_word.len(), pos),
+                    append_whitespace: false,
+                });
+            }
+        }
+
+        suggestions
+    }
+}
+
+fn run_shell(context: Context) -> Result<(), Error> {
+    let mut interpreter = Interpreter::new(context.clone());
+    let mut keybindings = default_emacs_keybindings();
+
+    keybindings.add_binding(
+        KeyModifiers::CONTROL,
+        KeyCode::Char(' '),
+        ReedlineEvent::Edit(vec![EditCommand::InsertNewline]),
+    );
+    keybindings.add_binding(
+        KeyModifiers::NONE,
+        KeyCode::Enter,
+        ReedlineEvent::SubmitOrNewline,
+    );
+    keybindings.add_binding(
+        KeyModifiers::NONE,
+        KeyCode::Tab,
+        ReedlineEvent::Edit(vec![EditCommand::InsertString("    ".to_string())]),
+    );
+    keybindings.add_binding(
+        KeyModifiers::NONE,
+        KeyCode::Tab,
+        ReedlineEvent::Multiple(vec![
+            ReedlineEvent::Menu("context menu".to_string()),
+            ReedlineEvent::MenuNext,
+        ]),
+    );
+
+    let edit_mode = Box::new(Emacs::new(keybindings));
+    let history = Box::new(
+        SqliteBackedHistory::with_file(PathBuf::from("target/history"), None, None)
+            .expect("Error loading history."),
+    );
+    let hinter = Box::new(DefaultHinter::default().with_style(Style::new().dimmed()));
+    let completer = DustCompleter::new(context.clone());
+
+    let mut line_editor = Reedline::create()
+        .with_edit_mode(edit_mode)
+        .with_history(history)
+        .with_hinter(hinter)
+        .use_kitty_keyboard_enhancement(true)
+        .with_completer(Box::new(completer))
+        .with_menu(ReedlineMenu::EngineCompleter(Box::new(
+            ColumnarMenu::default()
+                .with_name("context menu")
+                .with_text_style(Style::new().fg(Color::White))
+                .with_columns(1)
+                .with_column_padding(10),
+        )));
+    let mut prompt = StarshipPrompt::new();
+
+    prompt.reload();
+
+    loop {
+        let sig = line_editor.read_line(&prompt);
+
+        match sig {
+            Ok(Signal::Success(buffer)) => {
+                if buffer.trim().is_empty() {
+                    continue;
+                }
+
+                let run_result = interpreter.run(&buffer);
+
+                match run_result {
+                    Ok(value) => {
+                        if !value.is_none() {
+                            println!("{value}")
+                        }
+                    }
+                    Err(error) => println!("{error}"),
+                }
+
+                prompt.reload();
+            }
+            Ok(Signal::CtrlD) | Ok(Signal::CtrlC) => {
+                println!("\nLeaving the Dust shell.");
+                break;
+            }
+            x => {
+                println!("Unknown event: {:?}", x);
+            }
+        }
+    }
+
+    Ok(())
+}

src/operator.rs

@@ -1,547 +0,0 @@
-use std::fmt::{self, Display, Formatter};
-
-use crate::{error::*, value::Value, Result, VariableMap};
-
-/// An enum that represents operators in the operator tree.
-#[derive(Debug, PartialEq, Clone)]
-pub enum Operator {
-    /// A root node in the operator tree.
-    /// The whole expression is stored under a root node, as well as each subexpression surrounded by parentheses.
-    RootNode,
-
-    /// A binary addition operator.
-    Add,
-    /// A binary subtraction operator.
-    Sub,
-    /// A unary negation operator.
-    Neg,
-    /// A binary multiplication operator.
-    Mul,
-    /// A binary division operator.
-    Div,
-    /// A binary modulo operator.
-    Mod,
-    /// A binary exponentiation operator.
-    Exp,
-
-    /// A binary equality comparator.
-    Eq,
-    /// A binary inequality comparator.
-    Neq,
-    /// A binary greater-than comparator.
-    Gt,
-    /// A binary lower-than comparator.
-    Lt,
-    /// A binary greater-than-or-equal comparator.
-    Geq,
-    /// A binary lower-than-or-equal comparator.
-    Leq,
-    /// A binary logical and operator.
-    And,
-    /// A binary logical or operator.
-    Or,
-    /// A binary logical not operator.
-    Not,
-
-    /// A binary assignment operator.
-    Assign,
-    /// A binary add-assign operator.
-    AddAssign,
-    /// A binary subtract-assign operator.
-    SubAssign,
-    /// A binary multiply-assign operator.
-    MulAssign,
-    /// A binary divide-assign operator.
-    DivAssign,
-    /// A binary modulo-assign operator.
-    ModAssign,
-    /// A binary exponentiate-assign operator.
-    ExpAssign,
-    /// A binary and-assign operator.
-    AndAssign,
-    /// A binary or-assign operator.
-    OrAssign,
-
-    /// An n-ary tuple constructor.
-    Tuple,
-    /// An n-ary subexpression chain.
-    Chain,
-
-    /// A constant value.
-    Const {
-        /** The value of the constant. */
-        value: Value,
-    },
-    /// A write to a variable identifier.
-    VariableIdentifierWrite {
-        /// The identifier of the variable.
-        identifier: String,
-    },
-    /// A read from a variable identifier.
-    VariableIdentifierRead {
-        /// The identifier of the variable.
-        identifier: String,
-    },
-    /// A function identifier.
-    FunctionIdentifier {
-        /// The identifier of the function.
-        identifier: String,
-    },
-}
-
-impl Operator {
-    pub(crate) fn value(value: Value) -> Self {
-        Operator::Const { value }
-    }
-
-    pub(crate) fn variable_identifier_write(identifier: String) -> Self {
-        Operator::VariableIdentifierWrite { identifier }
-    }
-
-    pub(crate) fn variable_identifier_read(identifier: String) -> Self {
-        Operator::VariableIdentifierRead { identifier }
-    }
-
-    pub(crate) fn function_identifier(identifier: String) -> Self {
-        Operator::FunctionIdentifier { identifier }
-    }
-
-    /// Returns the precedence of the operator.
-    /// A high precedence means that the operator has priority to be deeper in the tree.
-    pub(crate) const fn precedence(&self) -> i32 {
-        use crate::operator::Operator::*;
-        match self {
-            RootNode => 200,
-
-            Add | Sub => 95,
-            Neg => 110,
-            Mul | Div | Mod => 100,
-            Exp => 120,
-
-            Eq | Neq | Gt | Lt | Geq | Leq => 80,
-            And => 75,
-            Or => 70,
-            Not => 110,
-
-            Assign | AddAssign | SubAssign | MulAssign | DivAssign | ModAssign | ExpAssign
-            | AndAssign | OrAssign => 50,
-
-            Tuple => 40,
-            Chain => 0,
-
-            Const { .. } => 200,
-            VariableIdentifierWrite { .. } | VariableIdentifierRead { .. } => 200,
-            FunctionIdentifier { .. } => 190,
-        }
-    }
-
-    /// Returns true if chains of operators with the same precedence as this one should be evaluated left-to-right,
-    /// and false if they should be evaluated right-to-left.
-    /// Left-to-right chaining has priority if operators with different order but same precedence are chained.
-    pub(crate) const fn is_left_to_right(&self) -> bool {
-        use crate::operator::Operator::*;
-        !matches!(self, Assign | FunctionIdentifier { .. })
-    }
-
-    /// Returns true if chains of this operator should be flattened into one operator with many arguments.
-    pub(crate) const fn is_sequence(&self) -> bool {
-        use crate::operator::Operator::*;
-        matches!(self, Tuple | Chain)
-    }
-
-    /// True if this operator is a leaf, meaning it accepts no arguments.
-    // Make this a const fn as soon as whatever is missing gets stable (issue #57563)
-    pub(crate) fn is_leaf(&self) -> bool {
-        self.max_argument_amount() == Some(0)
-    }
-
-    /// Returns the maximum amount of arguments required by this operator.
-    pub(crate) const fn max_argument_amount(&self) -> Option<usize> {
-        use crate::operator::Operator::*;
-        match self {
-            Add | Sub | Mul | Div | Mod | Exp | Eq | Neq | Gt | Lt | Geq | Leq | And | Or
-            | Assign | AddAssign | SubAssign | MulAssign | DivAssign | ModAssign | ExpAssign
-            | AndAssign | OrAssign => Some(2),
-            Tuple | Chain => None,
-            Not | Neg | RootNode => Some(1),
-            Const { .. } => Some(0),
-            VariableIdentifierWrite { .. } | VariableIdentifierRead { .. } => Some(0),
-            FunctionIdentifier { .. } => Some(1),
-        }
-    }
-
-    /// Returns true if this operator is unary, i.e. it requires exactly one argument.
-    pub(crate) fn is_unary(&self) -> bool {
-        self.max_argument_amount() == Some(1) && *self != Operator::RootNode
-    }
-
-    /// Evaluates the operator with the given arguments and context.
-    pub(crate) fn eval(&self, arguments: &[Value], context: &VariableMap) -> Result<Value> {
-        use crate::operator::Operator::*;
-
-        match self {
-            RootNode => {
-                if let Some(first) = arguments.first() {
-                    Ok(first.clone())
-                } else {
-                    Ok(Value::Empty)
-                }
-            }
-            Add => {
-                Error::expect_operator_argument_amount(arguments.len(), 2)?;
-                expect_number_or_string(&arguments[0])?;
-                expect_number_or_string(&arguments[1])?;
-
-                if let (Ok(a), Ok(b)) = (arguments[0].as_string(), arguments[1].as_string()) {
-                    let mut result = String::with_capacity(a.len() + b.len());
-                    result.push_str(a);
-                    result.push_str(b);
-                    Ok(Value::String(result))
-                } else if let (Ok(a), Ok(b)) = (arguments[0].as_int(), arguments[1].as_int()) {
-                    let result = a.checked_add(b);
-                    if let Some(result) = result {
-                        Ok(Value::Integer(result))
-                    } else {
-                        Err(Error::addition_error(
-                            arguments[0].clone(),
-                            arguments[1].clone(),
-                        ))
-                    }
-                } else if let (Ok(a), Ok(b)) = (arguments[0].as_number(), arguments[1].as_number())
-                {
-                    Ok(Value::Float(a + b))
-                } else {
-                    Err(Error::wrong_type_combination(
-                        self.clone(),
-                        vec![
-                            arguments.get(0).unwrap().into(),
-                            arguments.get(1).unwrap().into(),
-                        ],
-                    ))
-                }
-            }
-            Sub => {
-                Error::expect_operator_argument_amount(arguments.len(), 2)?;
-                arguments[0].as_number()?;
-                arguments[1].as_number()?;
-
-                if let (Ok(a), Ok(b)) = (arguments[0].as_int(), arguments[1].as_int()) {
-                    let result = a.checked_sub(b);
-                    if let Some(result) = result {
-                        Ok(Value::Integer(result))
-                    } else {
-                        Err(Error::subtraction_error(
-                            arguments[0].clone(),
-                            arguments[1].clone(),
-                        ))
-                    }
-                } else {
-                    Ok(Value::Float(
-                        arguments[0].as_number()? - arguments[1].as_number()?,
-                    ))
-                }
-            }
-            Neg => {
-                Error::expect_operator_argument_amount(arguments.len(), 1)?;
-                arguments[0].as_number()?;
-
-                if let Ok(a) = arguments[0].as_int() {
-                    let result = a.checked_neg();
-                    if let Some(result) = result {
-                        Ok(Value::Integer(result))
-                    } else {
-                        Err(Error::negation_error(arguments[0].clone()))
-                    }
-                } else {
-                    Ok(Value::Float(-arguments[0].as_number()?))
-                }
-            }
-            Mul => {
-                Error::expect_operator_argument_amount(arguments.len(), 2)?;
-                arguments[0].as_number()?;
-                arguments[1].as_number()?;
-
-                if let (Ok(a), Ok(b)) = (arguments[0].as_int(), arguments[1].as_int()) {
-                    let result = a.checked_mul(b);
-                    if let Some(result) = result {
-                        Ok(Value::Integer(result))
-                    } else {
-                        Err(Error::multiplication_error(
-                            arguments[0].clone(),
-                            arguments[1].clone(),
-                        ))
-                    }
-                } else {
-                    Ok(Value::Float(
-                        arguments[0].as_number()? * arguments[1].as_number()?,
-                    ))
-                }
-            }
-            Div => {
-                Error::expect_operator_argument_amount(arguments.len(), 2)?;
-                arguments[0].as_number()?;
-                arguments[1].as_number()?;
-
-                if let (Ok(a), Ok(b)) = (arguments[0].as_int(), arguments[1].as_int()) {
-                    let result = a.checked_div(b);
-                    if let Some(result) = result {
-                        Ok(Value::Integer(result))
-                    } else {
-                        Err(Error::division_error(
-                            arguments[0].clone(),
-                            arguments[1].clone(),
-                        ))
-                    }
-                } else {
-                    Ok(Value::Float(
-                        arguments[0].as_number()? / arguments[1].as_number()?,
-                    ))
-                }
-            }
-            Mod => {
-                Error::expect_operator_argument_amount(arguments.len(), 2)?;
-                arguments[0].as_number()?;
-                arguments[1].as_number()?;
-
-                if let (Ok(a), Ok(b)) = (arguments[0].as_int(), arguments[1].as_int()) {
-                    let result = a.checked_rem(b);
-                    if let Some(result) = result {
-                        Ok(Value::Integer(result))
-                    } else {
-                        Err(Error::modulation_error(
-                            arguments[0].clone(),
-                            arguments[1].clone(),
-                        ))
-                    }
-                } else {
-                    Ok(Value::Float(
-                        arguments[0].as_number()? % arguments[1].as_number()?,
-                    ))
-                }
-            }
-            Exp => {
-                Error::expect_operator_argument_amount(arguments.len(), 2)?;
-                arguments[0].as_number()?;
-                arguments[1].as_number()?;
-
-                Ok(Value::Float(
-                    arguments[0].as_number()?.powf(arguments[1].as_number()?),
-                ))
-            }
-            Eq => {
-                Error::expect_operator_argument_amount(arguments.len(), 2)?;
-
-                Ok(Value::Boolean(arguments[0] == arguments[1]))
-            }
-            Neq => {
-                Error::expect_operator_argument_amount(arguments.len(), 2)?;
-
-                Ok(Value::Boolean(arguments[0] != arguments[1]))
-            }
-            Gt => {
-                Error::expect_operator_argument_amount(arguments.len(), 2)?;
-                expect_number_or_string(&arguments[0])?;
-                expect_number_or_string(&arguments[1])?;
-
-                if let (Ok(a), Ok(b)) = (arguments[0].as_string(), arguments[1].as_string()) {
-                    Ok(Value::Boolean(a > b))
-                } else if let (Ok(a), Ok(b)) = (arguments[0].as_int(), arguments[1].as_int()) {
-                    Ok(Value::Boolean(a > b))
-                } else {
-                    Ok(Value::Boolean(
-                        arguments[0].as_number()? > arguments[1].as_number()?,
-                    ))
-                }
-            }
-            Lt => {
-                Error::expect_operator_argument_amount(arguments.len(), 2)?;
-                expect_number_or_string(&arguments[0])?;
-                expect_number_or_string(&arguments[1])?;
-
-                if let (Ok(a), Ok(b)) = (arguments[0].as_string(), arguments[1].as_string()) {
-                    Ok(Value::Boolean(a < b))
-                } else if let (Ok(a), Ok(b)) = (arguments[0].as_int(), arguments[1].as_int()) {
-                    Ok(Value::Boolean(a < b))
-                } else {
-                    Ok(Value::Boolean(
-                        arguments[0].as_number()? < arguments[1].as_number()?,
-                    ))
-                }
-            }
-            Geq => {
-                Error::expect_operator_argument_amount(arguments.len(), 2)?;
-                expect_number_or_string(&arguments[0])?;
-                expect_number_or_string(&arguments[1])?;
-
-                if let (Ok(a), Ok(b)) = (arguments[0].as_string(), arguments[1].as_string()) {
-                    Ok(Value::Boolean(a >= b))
-                } else if let (Ok(a), Ok(b)) = (arguments[0].as_int(), arguments[1].as_int()) {
-                    Ok(Value::Boolean(a >= b))
-                } else {
-                    Ok(Value::Boolean(
-                        arguments[0].as_number()? >= arguments[1].as_number()?,
-                    ))
-                }
-            }
-            Leq => {
-                Error::expect_operator_argument_amount(arguments.len(), 2)?;
-                expect_number_or_string(&arguments[0])?;
-                expect_number_or_string(&arguments[1])?;
-
-                if let (Ok(a), Ok(b)) = (arguments[0].as_string(), arguments[1].as_string()) {
-                    Ok(Value::Boolean(a <= b))
-                } else if let (Ok(a), Ok(b)) = (arguments[0].as_int(), arguments[1].as_int()) {
-                    Ok(Value::Boolean(a <= b))
-                } else {
-                    Ok(Value::Boolean(
-                        arguments[0].as_number()? <= arguments[1].as_number()?,
-                    ))
-                }
-            }
-            And => {
-                Error::expect_operator_argument_amount(arguments.len(), 2)?;
-                let a = arguments[0].as_boolean()?;
-                let b = arguments[1].as_boolean()?;
-
-                Ok(Value::Boolean(a && b))
-            }
-            Or => {
-                Error::expect_operator_argument_amount(arguments.len(), 2)?;
-                let a = arguments[0].as_boolean()?;
-                let b = arguments[1].as_boolean()?;
-
-                Ok(Value::Boolean(a || b))
-            }
-            Not => {
-                Error::expect_operator_argument_amount(arguments.len(), 1)?;
-                let a = arguments[0].as_boolean()?;
-
-                Ok(Value::Boolean(!a))
-            }
-            Assign | AddAssign | SubAssign | MulAssign | DivAssign | ModAssign | ExpAssign
-            | AndAssign | OrAssign => Err(Error::ContextNotMutable),
-            Tuple => Ok(Value::List(arguments.into())),
-            Chain => {
-                if arguments.is_empty() {
-                    return Err(Error::expect_operator_argument_amount(0, 1).unwrap_err());
-                }
-
-                Ok(arguments.last().cloned().unwrap_or(Value::Empty))
-            }
-            Const { value } => {
-                Error::expect_operator_argument_amount(arguments.len(), 0)?;
-
-                Ok(value.clone())
-            }
-            VariableIdentifierWrite { identifier } => {
-                Error::expect_operator_argument_amount(arguments.len(), 0)?;
-
-                Ok(identifier.clone().into())
-            }
-            VariableIdentifierRead { identifier } => {
-                Error::expect_operator_argument_amount(arguments.len(), 0)?;
-
-                if let Some(value) = context.get_value(identifier)? {
-                    Ok(value)
-                } else {
-                    Err(Error::VariableIdentifierNotFound(identifier.clone()))
-                }
-            }
-            FunctionIdentifier { identifier } => {
-                Error::expect_operator_argument_amount(arguments.len(), 1)?;
-                let arguments = &arguments[0];
-
-                context.call_function(identifier, arguments)
-            }
-        }
-    }
-
-    /// Evaluates the operator with the given arguments and mutable context.
-    pub(crate) fn eval_mut(&self, arguments: &[Value], context: &mut VariableMap) -> Result<Value> {
-        use crate::operator::Operator::*;
-        match self {
-            Assign => {
-                Error::expect_operator_argument_amount(arguments.len(), 2)?;
-                let target = arguments[0].as_string()?;
-                context.set_value(target, arguments[1].clone())?;
-
-                Ok(Value::Empty)
-            }
-            AddAssign | SubAssign | MulAssign | DivAssign | ModAssign | ExpAssign | AndAssign
-            | OrAssign => {
-                Error::expect_operator_argument_amount(arguments.len(), 2)?;
-
-                let target = arguments[0].as_string()?;
-                let left_value = Operator::VariableIdentifierRead {
-                    identifier: target.clone(),
-                }
-                .eval(&Vec::new(), context)?;
-                let arguments = vec![left_value, arguments[1].clone()];
-
-                let result = match self {
-                    AddAssign => Operator::Add.eval(&arguments, context),
-                    SubAssign => Operator::Sub.eval(&arguments, context),
-                    MulAssign => Operator::Mul.eval(&arguments, context),
-                    DivAssign => Operator::Div.eval(&arguments, context),
-                    ModAssign => Operator::Mod.eval(&arguments, context),
-                    ExpAssign => Operator::Exp.eval(&arguments, context),
-                    AndAssign => Operator::And.eval(&arguments, context),
-                    OrAssign => Operator::Or.eval(&arguments, context),
-                    _ => unreachable!(
-                        "Forgot to add a match arm for an assign operation: {}",
-                        self
-                    ),
-                }?;
-                context.set_value(target, result)?;
-
-                Ok(Value::Empty)
-            }
-            _ => self.eval(arguments, context),
-        }
-    }
-}
-
-impl Display for Operator {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        use crate::operator::Operator::*;
-        match self {
-            RootNode => Ok(()),
-            Add => write!(f, "+"),
-            Sub => write!(f, "-"),
-            Neg => write!(f, "-"),
-            Mul => write!(f, "*"),
-            Div => write!(f, "/"),
-            Mod => write!(f, "%"),
-            Exp => write!(f, "^"),
-
-            Eq => write!(f, "=="),
-            Neq => write!(f, "!="),
-            Gt => write!(f, ">"),
-            Lt => write!(f, "<"),
-            Geq => write!(f, ">="),
-            Leq => write!(f, "<="),
-            And => write!(f, "&&"),
-            Or => write!(f, "||"),
-            Not => write!(f, "!"),
-
-            Assign => write!(f, " = "),
-            AddAssign => write!(f, " += "),
-            SubAssign => write!(f, " -= "),
-            MulAssign => write!(f, " *= "),
-            DivAssign => write!(f, " /= "),
-            ModAssign => write!(f, " %= "),
-            ExpAssign => write!(f, " ^= "),
-            AndAssign => write!(f, " &&= "),
-            OrAssign => write!(f, " ||= "),
-
-            Tuple => write!(f, ", "),
-            Chain => write!(f, "; "),
-
-            Const { value } => write!(f, "{}", value),
-            VariableIdentifierWrite { identifier } | VariableIdentifierRead { identifier } => {
-                write!(f, "{}", identifier)
-            }
-            FunctionIdentifier { identifier } => write!(f, "{}", identifier),
-        }
-    }
-}

src/token.rs

@@ -1,562 +0,0 @@
-use std::fmt::{self, Display, Formatter};
-
-use crate::error::{Error, Result};
-
-#[derive(Clone, PartialEq, Debug)]
-pub enum Token {
-    // Arithmetic
-    Plus,
-    Minus,
-    Star,
-    Slash,
-    Percent,
-    Hat,
-
-    // Logic
-    Eq,
-    Neq,
-    Gt,
-    Lt,
-    Geq,
-    Leq,
-    And,
-    Or,
-    Not,
-
-    // Precedence
-    LBrace,
-    RBrace,
-
-    // Assignment
-    Assign,
-    PlusAssign,
-    MinusAssign,
-    StarAssign,
-    SlashAssign,
-    PercentAssign,
-    HatAssign,
-    AndAssign,
-    OrAssign,
-
-    // Special
-    Comma,
-    Semicolon,
-    Yield(String, String),
-
-    // Values, Variables and Functions
-    Identifier(String),
-    Float(f64),
-    Int(i64),
-    Boolean(bool),
-    String(String),
-    Function(String),
-}
-
-/// A partial token is an input character whose meaning depends on the characters around it.
-#[derive(Clone, Debug, PartialEq)]
-pub enum PartialToken {
-    /// A partial token that unambiguously maps to a single token.
-    Token(Token),
-    /// A partial token that is a literal.
-    Literal(String),
-    /// A plus character '+'.
-    Plus,
-    /// A minus character '-'.
-    Minus,
-    /// A star character '*'.
-    Star,
-    /// A slash character '/'.
-    Slash,
-    /// A percent character '%'.
-    Percent,
-    /// A hat character '^'.
-    Hat,
-    /// A whitespace character, e.g. ' '.
-    Whitespace,
-    /// An equal-to character '='.
-    Eq,
-    /// An exclamation mark character '!'.
-    ExclamationMark,
-    /// A greater-than character '>'.
-    Gt,
-    /// A lower-than character '<'.
-    Lt,
-    /// An ampersand character '&'.
-    Ampersand,
-    /// A vertical bar character '|'.
-    VerticalBar,
-}
-
-// Make this a const fn as soon as is_whitespace and to_string get stable (issue #57563)
-fn char_to_partial_token(c: char) -> PartialToken {
-    match c {
-        '+' => PartialToken::Plus,
-        '-' => PartialToken::Minus,
-        '*' => PartialToken::Star,
-        '/' => PartialToken::Slash,
-        '%' => PartialToken::Percent,
-        '^' => PartialToken::Hat,
-
-        '(' => PartialToken::Token(Token::LBrace),
-        ')' => PartialToken::Token(Token::RBrace),
-
-        ',' => PartialToken::Token(Token::Comma),
-        ';' => PartialToken::Token(Token::Semicolon),
-
-        '=' => PartialToken::Eq,
-        '!' => PartialToken::ExclamationMark,
-        '>' => PartialToken::Gt,
-        '<' => PartialToken::Lt,
-        '&' => PartialToken::Ampersand,
-        '|' => PartialToken::VerticalBar,
-
-        c => {
-            if c.is_whitespace() {
-                PartialToken::Whitespace
-            } else {
-                PartialToken::Literal(c.to_string())
-            }
-        }
-    }
-}
-
-impl Token {
-    #[cfg(not(tarpaulin_include))]
-    pub(crate) const fn is_leftsided_value(&self) -> bool {
-        match self {
-            Token::Plus => false,
-            Token::Minus => false,
-            Token::Star => false,
-            Token::Slash => false,
-            Token::Percent => false,
-            Token::Hat => false,
-
-            Token::Eq => false,
-            Token::Neq => false,
-            Token::Gt => false,
-            Token::Lt => false,
-            Token::Geq => false,
-            Token::Leq => false,
-            Token::And => false,
-            Token::Or => false,
-            Token::Not => false,
-
-            Token::LBrace => true,
-            Token::RBrace => false,
-
-            Token::Comma => false,
-            Token::Semicolon => false,
-            Token::Yield(_, _) => false,
-
-            Token::Assign => false,
-            Token::PlusAssign => false,
-            Token::MinusAssign => false,
-            Token::StarAssign => false,
-            Token::SlashAssign => false,
-            Token::PercentAssign => false,
-            Token::HatAssign => false,
-            Token::AndAssign => false,
-            Token::OrAssign => false,
-
-            Token::Identifier(_) => true,
-            Token::Float(_) => true,
-            Token::Int(_) => true,
-            Token::Boolean(_) => true,
-            Token::String(_) => true,
-            Token::Function(_) => true,
-        }
-    }
-
-    #[cfg(not(tarpaulin_include))]
-    pub(crate) const fn is_rightsided_value(&self) -> bool {
-        match self {
-            Token::Plus => false,
-            Token::Minus => false,
-            Token::Star => false,
-            Token::Slash => false,
-            Token::Percent => false,
-            Token::Hat => false,
-
-            Token::Eq => false,
-            Token::Neq => false,
-            Token::Gt => false,
-            Token::Lt => false,
-            Token::Geq => false,
-            Token::Leq => false,
-            Token::And => false,
-            Token::Or => false,
-            Token::Not => false,
-
-            Token::LBrace => false,
-            Token::RBrace => true,
-
-            Token::Comma => false,
-            Token::Semicolon => false,
-            Token::Yield(_, _) => false,
-
-            Token::Assign => false,
-            Token::PlusAssign => false,
-            Token::MinusAssign => false,
-            Token::StarAssign => false,
-            Token::SlashAssign => false,
-            Token::PercentAssign => false,
-            Token::HatAssign => false,
-            Token::AndAssign => false,
-            Token::OrAssign => false,
-
-            Token::Identifier(_) => true,
-            Token::Float(_) => true,
-            Token::Int(_) => true,
-            Token::Boolean(_) => true,
-            Token::String(_) => true,
-            Token::Function(_) => true,
-        }
-    }
-
-    #[cfg(not(tarpaulin_include))]
-    pub(crate) fn is_assignment(&self) -> bool {
-        use Token::*;
-        matches!(
-            self,
-            Assign
-                | PlusAssign
-                | MinusAssign
-                | StarAssign
-                | SlashAssign
-                | PercentAssign
-                | HatAssign
-                | AndAssign
-                | OrAssign
-        )
-    }
-}
-
-/// Parses an escape sequence within a string literal.
-fn parse_escape_sequence<Iter: Iterator<Item = char>>(iter: &mut Iter) -> Result<char> {
-    match iter.next() {
-        Some('"') => Ok('"'),
-        Some('\\') => Ok('\\'),
-        Some(c) => Err(Error::IllegalEscapeSequence(format!("\\{}", c))),
-        None => Err(Error::IllegalEscapeSequence("\\".to_string())),
-    }
-}
-
-/// Parses a string value from the given character iterator.
-///
-/// The first character from the iterator is interpreted as first character of the string.
-/// The string is terminated by a double quote `"`.
-/// Occurrences of `"` within the string can be escaped with `\`.
-/// The backslash needs to be escaped with another backslash `\`.
-fn parse_string_literal<Iter: Iterator<Item = char>>(mut iter: &mut Iter) -> Result<PartialToken> {
-    let mut result = String::new();
-
-    while let Some(c) = iter.next() {
-        match c {
-            '"' => break,
-            '\\' => result.push(parse_escape_sequence(&mut iter)?),
-            c => result.push(c),
-        }
-    }
-
-    Ok(PartialToken::Token(Token::String(result)))
-}
-
-fn parse_function<Iter: Iterator<Item = char>>(mut iter: &mut Iter) -> Result<PartialToken> {
-    let mut result = String::new();
-
-    while let Some(c) = iter.next() {
-        match c {
-            '\'' => break,
-            '\\' => result.push(parse_escape_sequence(&mut iter)?),
-            c => result.push(c),
-        }
-    }
-
-    Ok(PartialToken::Token(Token::Function(result)))
-}
-
-/// Converts a string to a vector of partial tokens.
-fn str_to_partial_tokens(string: &str) -> Result<Vec<PartialToken>> {
-    let mut result = Vec::new();
-    let mut iter = string.chars().peekable();
-
-    while let Some(c) = iter.next() {
-        if c == '"' {
-            result.push(parse_string_literal(&mut iter)?);
-        } else if c == '\'' {
-            result.push(parse_function(&mut iter)?)
-        } else {
-            let partial_token = char_to_partial_token(c);
-
-            let if_let_successful =
-                if let (Some(PartialToken::Literal(last)), PartialToken::Literal(literal)) =
-                    (result.last_mut(), &partial_token)
-                {
-                    last.push_str(literal);
-                    true
-                } else {
-                    false
-                };
-
-            if !if_let_successful {
-                result.push(partial_token);
-            }
-        }
-    }
-    Ok(result)
-}
-
-/// Resolves all partial tokens by converting them to complex tokens.
-fn partial_tokens_to_tokens(mut tokens: &[PartialToken]) -> Result<Vec<Token>> {
-    let mut result = Vec::new();
-    while !tokens.is_empty() {
-        let first = tokens[0].clone();
-        let second = tokens.get(1).cloned();
-        let third = tokens.get(2).cloned();
-        let mut cutoff = 2;
-
-        result.extend(
-            match first {
-                PartialToken::Token(token) => {
-                    cutoff = 1;
-                    Some(token)
-                }
-                PartialToken::Plus => match second {
-                    Some(PartialToken::Eq) => Some(Token::PlusAssign),
-                    _ => {
-                        cutoff = 1;
-                        Some(Token::Plus)
-                    }
-                },
-                PartialToken::Minus => match second {
-                    Some(PartialToken::Eq) => Some(Token::MinusAssign),
-                    _ => {
-                        cutoff = 1;
-                        Some(Token::Minus)
-                    }
-                },
-                PartialToken::Star => match second {
-                    Some(PartialToken::Eq) => Some(Token::StarAssign),
-                    _ => {
-                        cutoff = 1;
-                        Some(Token::Star)
-                    }
-                },
-                PartialToken::Slash => match second {
-                    Some(PartialToken::Eq) => Some(Token::SlashAssign),
-                    _ => {
-                        cutoff = 1;
-                        Some(Token::Slash)
-                    }
-                },
-                PartialToken::Percent => match second {
-                    Some(PartialToken::Eq) => Some(Token::PercentAssign),
-                    _ => {
-                        cutoff = 1;
-                        Some(Token::Percent)
-                    }
-                },
-                PartialToken::Hat => match second {
-                    Some(PartialToken::Eq) => Some(Token::HatAssign),
-                    _ => {
-                        cutoff = 1;
-                        Some(Token::Hat)
-                    }
-                },
-                PartialToken::Literal(literal) => {
-                    cutoff = 1;
-                    if let Ok(number) = parse_dec_or_hex(&literal) {
-                        Some(Token::Int(number))
-                    } else if let Ok(number) = literal.parse::<f64>() {
-                        Some(Token::Float(number))
-                    } else if let Ok(boolean) = literal.parse::<bool>() {
-                        Some(Token::Boolean(boolean))
-                    } else {
-                        // If there are two tokens following this one, check if the next one is
-                        // a plus or a minus. If so, then attempt to parse all three tokens as a
-                        // scientific notation number of the form `<coefficient>e{+,-}<exponent>`,
-                        // for example [Literal("10e"), Minus, Literal("3")] => "1e-3".parse().
-                        match (second, third) {
-                            (Some(second), Some(third))
-                                if second == PartialToken::Minus
-                                    || second == PartialToken::Plus =>
-                            {
-                                if let Ok(number) =
-                                    format!("{}{}{}", literal, second, third).parse::<f64>()
-                                {
-                                    cutoff = 3;
-                                    Some(Token::Float(number))
-                                } else {
-                                    Some(Token::Identifier(literal.to_string()))
-                                }
-                            }
-                            _ => Some(Token::Identifier(literal.to_string())),
-                        }
-                    }
-                }
-                PartialToken::Whitespace => {
-                    cutoff = 1;
-                    None
-                }
-                PartialToken::Eq => match second {
-                    Some(PartialToken::Eq) => Some(Token::Eq),
-                    _ => {
-                        cutoff = 1;
-                        Some(Token::Assign)
-                    }
-                },
-                PartialToken::ExclamationMark => match second {
-                    Some(PartialToken::Eq) => Some(Token::Neq),
-                    _ => {
-                        cutoff = 1;
-                        Some(Token::Not)
-                    }
-                },
-                PartialToken::Gt => match second {
-                    Some(PartialToken::Eq) => Some(Token::Geq),
-                    _ => {
-                        cutoff = 1;
-                        Some(Token::Gt)
-                    }
-                },
-                PartialToken::Lt => match second {
-                    Some(PartialToken::Eq) => Some(Token::Leq),
-                    _ => {
-                        cutoff = 1;
-                        Some(Token::Lt)
-                    }
-                },
-                PartialToken::Ampersand => match second {
-                    Some(PartialToken::Ampersand) => match third {
-                        Some(PartialToken::Eq) => {
-                            cutoff = 3;
-                            Some(Token::AndAssign)
-                        }
-                        _ => Some(Token::And),
-                    },
-                    _ => return Err(Error::unmatched_partial_token(first, second)),
-                },
-                PartialToken::VerticalBar => match second {
-                    Some(PartialToken::VerticalBar) => match third {
-                        Some(PartialToken::Eq) => {
-                            cutoff = 3;
-                            Some(Token::OrAssign)
-                        }
-                        _ => Some(Token::Or),
-                    },
-                    _ => return Err(Error::unmatched_partial_token(first, second)),
-                },
-            }
-            .into_iter(),
-        );
-
-        tokens = &tokens[cutoff..];
-    }
-    Ok(result)
-}
-
-impl Display for Token {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        use self::Token::*;
-        match self {
-            Plus => write!(f, "+"),
-            Minus => write!(f, "-"),
-            Star => write!(f, "*"),
-            Slash => write!(f, "/"),
-            Percent => write!(f, "%"),
-            Hat => write!(f, "^"),
-
-            // Logic
-            Eq => write!(f, "=="),
-            Neq => write!(f, "!="),
-            Gt => write!(f, ">"),
-            Lt => write!(f, "<"),
-            Geq => write!(f, ">="),
-            Leq => write!(f, "<="),
-            And => write!(f, "&&"),
-            Or => write!(f, "||"),
-            Not => write!(f, "!"),
-
-            // Precedence
-            LBrace => write!(f, "("),
-            RBrace => write!(f, ")"),
-
-            // Assignment
-            Assign => write!(f, "="),
-            PlusAssign => write!(f, "+="),
-            MinusAssign => write!(f, "-="),
-            StarAssign => write!(f, "*="),
-            SlashAssign => write!(f, "/="),
-            PercentAssign => write!(f, "%="),
-            HatAssign => write!(f, "^="),
-            AndAssign => write!(f, "&&="),
-            OrAssign => write!(f, "||="),
-
-            // Special
-            Comma => write!(f, ","),
-            Semicolon => write!(f, ";"),
-
-            // Values => write!(f, ""), Variables and Functions
-            Identifier(identifier) => identifier.fmt(f),
-            Float(float) => float.fmt(f),
-            Int(int) => int.fmt(f),
-            Boolean(boolean) => boolean.fmt(f),
-            String(string) => fmt::Debug::fmt(string, f),
-            Function(string) => write!(f, "'{string}'"),
-            Yield(_, _) => todo!(),
-        }
-    }
-}
-
-impl Display for PartialToken {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        use self::PartialToken::*;
-        match self {
-            Token(token) => token.fmt(f),
-            Literal(literal) => literal.fmt(f),
-            Whitespace => write!(f, " "),
-            Plus => write!(f, "+"),
-            Minus => write!(f, "-"),
-            Star => write!(f, "*"),
-            Slash => write!(f, "/"),
-            Percent => write!(f, "%"),
-            Hat => write!(f, "^"),
-            Eq => write!(f, "="),
-            ExclamationMark => write!(f, "!"),
-            Gt => write!(f, ">"),
-            Lt => write!(f, "<"),
-            Ampersand => write!(f, "&"),
-            VerticalBar => write!(f, "|"),
-        }
-    }
-}
-
-pub(crate) fn tokenize(string: &str) -> Result<Vec<Token>> {
-    partial_tokens_to_tokens(&str_to_partial_tokens(string)?)
-}
-
-fn parse_dec_or_hex(literal: &str) -> std::result::Result<i64, std::num::ParseIntError> {
-    if let Some(literal) = literal.strip_prefix("0x") {
-        literal.parse()
-    } else {
-        literal.parse()
-    }
-}
-
-#[cfg(test)]
-mod tests {
-    use crate::token::{tokenize, Token};
-
-    #[test]
-    fn assignment_lhs_is_identifier() {
-        let tokens = tokenize("a = 1").unwrap();
-        assert_eq!(
-            tokens.as_slice(),
-            [
-                Token::Identifier("a".to_string()),
-                Token::Assign,
-                Token::Int(1)
-            ]
-        );
-    }
-}

src/tools/collections.rs

@@ -1,416 +0,0 @@
-//! Macros for collection values: strings, lists, maps and tables.
-//!
-//! Tests for this module are written in Dust and can be found at tests/collections.ds.
-
-use crate::{Error, Result, Table, Tool, ToolInfo, Value, ValueType, VariableMap};
-
-pub struct Sort;
-
-impl Tool for Sort {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "sort",
-            description: "Apply default ordering to a list or table.",
-            group: "collections",
-            inputs: vec![ValueType::List, ValueType::Table],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        if let Ok(mut list) = argument.as_list().cloned() {
-            list.sort();
-
-            Ok(Value::List(list))
-        } else if let Ok(mut table) = argument.as_table().cloned() {
-            table.sort();
-
-            Ok(Value::Table(table))
-        } else {
-            Err(crate::Error::ExpectedList {
-                actual: argument.clone(),
-            })
-        }
-    }
-}
-
-pub struct Transform;
-
-impl Tool for Transform {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "transform",
-            description: "Alter a list by calling a function on each value.",
-            group: "collections",
-            inputs: vec![ValueType::ListExact(vec![
-                ValueType::List,
-                ValueType::Function,
-            ])],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let argument = self.check_type(argument)?.as_list()?;
-        let list = argument[0].as_list()?;
-        let function = argument[1].as_function()?;
-        let mut mapped_list = Vec::with_capacity(list.len());
-
-        for value in list {
-            let mut context = VariableMap::new();
-
-            context.set_value("input", value.clone())?;
-
-            let mapped_value = function.run_with_context(&mut context)?;
-
-            mapped_list.push(mapped_value);
-        }
-
-        Ok(Value::List(mapped_list))
-    }
-}
-
-pub struct String;
-
-impl Tool for String {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "string",
-            description: "Stringify a value.",
-            group: "collections",
-            inputs: vec![
-                ValueType::String,
-                ValueType::Function,
-                ValueType::Float,
-                ValueType::Integer,
-                ValueType::Boolean,
-            ],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let argument = self.check_type(argument)?;
-
-        let string = match argument {
-            Value::String(string) => string.clone(),
-            Value::Function(function) => function.to_string(),
-            Value::Float(float) => float.to_string(),
-            Value::Integer(integer) => integer.to_string(),
-            Value::Boolean(boolean) => boolean.to_string(),
-            _ => return self.fail(argument),
-        };
-
-        Ok(Value::String(string))
-    }
-}
-
-pub struct Replace;
-
-impl Tool for Replace {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "replace",
-            description: "Replace all occurences of a substring in a string.",
-            group: "collections",
-            inputs: vec![ValueType::ListExact(vec![
-                ValueType::String,
-                ValueType::String,
-                ValueType::String,
-            ])],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let argument = self.check_type(argument)?.as_list()?;
-        let target = argument[0].as_string()?;
-        let to_remove = argument[1].as_string()?;
-        let replacement = argument[2].as_string()?;
-        let result = target.replace(to_remove, replacement);
-
-        Ok(Value::String(result))
-    }
-}
-
-pub struct Count;
-
-impl Tool for Count {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "count",
-            description: "Return the number of items in a collection.",
-            group: "collections",
-            inputs: vec![
-                ValueType::String,
-                ValueType::List,
-                ValueType::Map,
-                ValueType::Table,
-            ],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let argument = self.check_type(argument)?;
-
-        let len = match argument {
-            Value::String(string) => string.chars().count(),
-            Value::List(list) => list.len(),
-            Value::Map(map) => map.len(),
-            Value::Table(table) => table.len(),
-            Value::Function(_)
-            | Value::Float(_)
-            | Value::Integer(_)
-            | Value::Boolean(_)
-            | Value::Time(_)
-            | Value::Empty => return self.fail(argument),
-        };
-
-        Ok(Value::Integer(len as i64))
-    }
-}
-
-pub struct CreateTable;
-
-impl Tool for CreateTable {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "create_table",
-            description: "Define a new table with a list of column names and list of rows.",
-            group: "collections",
-            inputs: vec![ValueType::ListExact(vec![
-                ValueType::ListOf(Box::new(ValueType::String)),
-                ValueType::ListOf(Box::new(ValueType::List)),
-            ])],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let argument = self.check_type(argument)?.as_list()?;
-
-        let column_name_inputs = argument[0].as_list()?;
-        let mut column_names = Vec::with_capacity(column_name_inputs.len());
-
-        for name in column_name_inputs {
-            column_names.push(name.as_string()?.clone());
-        }
-
-        let column_count = column_names.len();
-        let rows = argument[1].as_list()?;
-        let mut table = Table::new(column_names);
-
-        for row in rows {
-            let row = row.as_fixed_len_list(column_count)?;
-
-            table.insert(row.clone()).unwrap();
-        }
-
-        Ok(Value::Table(table))
-    }
-}
-
-pub struct Rows;
-
-impl Tool for Rows {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "rows",
-            description: "Extract a table's rows as a list.",
-            group: "collections",
-            inputs: vec![ValueType::Table],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let argument = self.check_type(argument)?;
-
-        if let Value::Table(table) = argument {
-            let rows = table
-                .rows()
-                .iter()
-                .map(|row| Value::List(row.clone()))
-                .collect();
-
-            Ok(Value::List(rows))
-        } else {
-            self.fail(argument)
-        }
-    }
-}
-
-pub struct Insert;
-
-impl Tool for Insert {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "insert",
-            description: "Add new rows to a table.",
-            group: "collections",
-            inputs: vec![ValueType::ListExact(vec![
-                ValueType::Table,
-                ValueType::ListOf(Box::new(ValueType::List)),
-            ])],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let argument = argument.as_list()?;
-        let new_rows = argument[1].as_list()?;
-        let mut table = argument[0].as_table()?.clone();
-
-        table.reserve(new_rows.len());
-
-        for row in new_rows {
-            let row = row.as_list()?.clone();
-
-            table.insert(row)?;
-        }
-
-        Ok(Value::Table(table))
-    }
-}
-
-pub struct Select;
-
-impl Tool for Select {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "select",
-            description: "Extract one or more values based on their key.",
-            group: "collections",
-            inputs: vec![
-                ValueType::ListExact(vec![ValueType::Table, ValueType::String]),
-                ValueType::ListExact(vec![
-                    ValueType::Table,
-                    ValueType::ListOf(Box::new(ValueType::String)),
-                ]),
-            ],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let arguments = argument.as_fixed_len_list(2)?;
-        let collection = &arguments[0];
-
-        if let Value::List(list) = collection {
-            let mut selected = Vec::new();
-
-            let index = arguments[1].as_int()?;
-            let value = list.get(index as usize);
-
-            if let Some(value) = value {
-                selected.push(value.clone());
-                return Ok(Value::List(selected));
-            } else {
-                return Ok(Value::List(selected));
-            }
-        }
-
-        let mut column_names = Vec::new();
-
-        if let Value::List(columns) = &arguments[1] {
-            for column in columns {
-                let name = column.as_string()?;
-
-                column_names.push(name.clone());
-            }
-        } else if let Value::String(column) = &arguments[1] {
-            column_names.push(column.clone());
-        } else {
-            todo!()
-        };
-
-        if let Value::Map(map) = collection {
-            let mut selected = VariableMap::new();
-
-            for (key, value) in map.inner() {
-                if column_names.contains(key) {
-                    selected.set_value(key, value.clone())?;
-                }
-            }
-
-            return Ok(Value::Map(selected));
-        }
-
-        if let Value::Table(table) = collection {
-            let selected = table.select(&column_names);
-
-            return Ok(Value::Table(selected));
-        }
-
-        todo!()
-    }
-}
-
-pub struct ForEach;
-
-impl Tool for ForEach {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "for_each",
-            description: "Run an operation on every item in a collection.",
-            group: "collections",
-            inputs: vec![],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let argument = argument.as_list()?;
-
-        Error::expected_minimum_function_argument_amount(
-            self.info().identifier,
-            2,
-            argument.len(),
-        )?;
-
-        let table = argument[0].as_table()?;
-        let columns = argument[1].as_list()?;
-        let mut column_names = Vec::new();
-
-        for column in columns {
-            let name = column.as_string()?;
-
-            column_names.push(name.clone());
-        }
-
-        let selected = table.select(&column_names);
-
-        Ok(Value::Table(selected))
-    }
-}
-
-pub struct Where;
-
-impl Tool for Where {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "where",
-            description: "Keep rows matching a predicate.",
-            group: "collections",
-            inputs: vec![ValueType::ListExact(vec![
-                ValueType::Table,
-                ValueType::Function,
-            ])],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let argument = self.check_type(argument)?.as_list()?;
-        let table = &argument[0].as_table()?;
-        let function = argument[1].as_function()?;
-
-        let mut context = VariableMap::new();
-        let mut new_table = Table::new(table.column_names().clone());
-
-        for row in table.rows() {
-            for (column_index, cell) in row.iter().enumerate() {
-                let column_name = table.column_names().get(column_index).unwrap();
-
-                context.set_value(column_name, cell.clone())?;
-            }
-            let keep_row = function.run_with_context(&mut context)?.as_boolean()?;
-
-            if keep_row {
-                new_table.insert(row.clone())?;
-            }
-        }
-
-        Ok(Value::Table(new_table))
-    }
-}

src/tools/command.rs

@@ -1,119 +0,0 @@
-use std::process::Command;
-
-use crate::{Result, Tool, ToolInfo, Value, ValueType};
-
-pub struct Sh;
-
-impl Tool for Sh {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "sh",
-            description: "Pass input to the Bourne Shell.",
-            group: "command",
-            inputs: vec![ValueType::String],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let argument = argument.as_string()?;
-
-        Command::new("sh").arg("-c").arg(argument).spawn()?.wait()?;
-
-        Ok(Value::Empty)
-    }
-}
-
-pub struct Bash;
-
-impl Tool for Bash {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "bash",
-            description: "Pass input to the Bourne Again Shell.",
-            group: "command",
-            inputs: vec![ValueType::String],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let argument = argument.as_string()?;
-
-        Command::new("bash")
-            .arg("-c")
-            .arg(argument)
-            .spawn()?
-            .wait()?;
-
-        Ok(Value::Empty)
-    }
-}
-pub struct Fish;
-
-impl Tool for Fish {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "fish",
-            description: "Pass input to the fish shell.",
-            group: "command",
-            inputs: vec![ValueType::String],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let argument = argument.as_string()?;
-
-        Command::new("fish")
-            .arg("-c")
-            .arg(argument)
-            .spawn()?
-            .wait()?;
-
-        Ok(Value::Empty)
-    }
-}
-
-pub struct Zsh;
-
-impl Tool for Zsh {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "zsh",
-            description: "Pass input to the Z shell.",
-            group: "command",
-            inputs: vec![ValueType::String],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let argument = argument.as_string()?;
-
-        Command::new("zsh")
-            .arg("-c")
-            .arg(argument)
-            .spawn()?
-            .wait()?;
-
-        Ok(Value::Empty)
-    }
-}
-
-pub struct Raw;
-
-impl Tool for Raw {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "raw",
-            description: "Run input as a command without a shell",
-            group: "command",
-            inputs: vec![ValueType::String],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let argument = argument.as_string()?;
-
-        Command::new(argument).spawn()?.wait()?;
-
-        Ok(Value::Empty)
-    }
-}

src/tools/data_formats.rs

@@ -1,171 +0,0 @@
-//! Convert values to and from data formats like JSON and TOML.
-
-use crate::{Result, Table, Tool, ToolInfo, Value, ValueType};
-
-pub struct FromToml;
-
-impl Tool for FromToml {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "from_toml",
-            description: "Create a value from a TOML string.",
-            group: "data",
-            inputs: vec![ValueType::String],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let argument = argument.as_string()?;
-        let value = toml::from_str(&argument)?;
-
-        Ok(value)
-    }
-}
-
-pub struct FromJson;
-
-impl Tool for FromJson {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "from_json",
-            description: "Get a whale value from a JSON string.",
-            group: "data",
-            inputs: vec![ValueType::String],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let argument = argument.as_string()?;
-        let value = serde_json::from_str(argument)?;
-
-        Ok(value)
-    }
-}
-
-pub struct ToJson;
-
-impl Tool for ToJson {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "to_json",
-            description: "Create a JSON string from a whale value.",
-            group: "data",
-            inputs: vec![ValueType::Any],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let json = serde_json::to_string(argument)?;
-
-        Ok(Value::String(json))
-    }
-}
-
-pub struct FromCsv;
-
-impl Tool for FromCsv {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "from_csv",
-            description: "Create a whale value from a CSV string.",
-            group: "data",
-            inputs: vec![ValueType::String],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let csv = argument.as_string()?;
-        let mut reader = csv::Reader::from_reader(csv.as_bytes());
-
-        let headers = reader
-            .headers()?
-            .iter()
-            .map(|header| header.trim().trim_matches('"').to_string())
-            .collect();
-
-        let mut table = Table::new(headers);
-
-        for result in reader.records() {
-            let row = result?
-                .iter()
-                .map(|column| {
-                    let column = column.trim().trim_matches('"').trim_matches('\'');
-
-                    if let Ok(integer) = column.parse::<i64>() {
-                        Value::Integer(integer)
-                    } else if let Ok(float) = column.parse::<f64>() {
-                        Value::Float(float)
-                    } else {
-                        Value::String(column.to_string())
-                    }
-                })
-                .collect();
-
-            table.insert(row)?;
-        }
-
-        Ok(Value::Table(table))
-    }
-}
-
-pub struct ToCsv;
-
-impl Tool for ToCsv {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "to_csv",
-            description: "Convert a value to a string of comma-separated values.",
-            group: "data",
-            inputs: vec![ValueType::Any],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let mut buffer = Vec::new();
-        let mut writer = csv::Writer::from_writer(&mut buffer);
-
-        match argument {
-            Value::String(string) => {
-                writer.write_record([string])?;
-            }
-            Value::Float(float) => {
-                writer.write_record(&[float.to_string()])?;
-            }
-            Value::Integer(integer) => {
-                writer.write_record(&[integer.to_string()])?;
-            }
-            Value::Boolean(boolean) => {
-                writer.write_record(&[boolean.to_string()])?;
-            }
-            Value::List(list) => {
-                let string_list = list.iter().map(|value| value.to_string());
-
-                writer.write_record(string_list)?;
-            }
-            Value::Empty => {}
-            Value::Map(map) => {
-                writer.write_record(map.inner().keys())?;
-                writer.write_record(map.inner().values().map(|value| value.to_string()))?;
-            }
-            Value::Table(table) => {
-                writer.write_record(table.column_names())?;
-
-                for row in table.rows() {
-                    let row_string = row.iter().map(|value| value.to_string());
-
-                    writer.write_record(row_string)?;
-                }
-            }
-            Value::Function(_) => todo!(),
-            Value::Time(time) => {
-                writer.write_record(&[time.to_string()])?;
-            }
-        }
-
-        writer.flush()?;
-
-        Ok(Value::String(
-            String::from_utf8_lossy(writer.get_ref()).to_string(),
-        ))
-    }
-}

src/tools/disks.rs

@@ -1,123 +0,0 @@
-use std::process::Command;
-
-use sysinfo::{DiskExt, System, SystemExt};
-
-use crate::{Result, Table, Tool, ToolInfo, Value, ValueType};
-
-pub struct ListDisks;
-
-impl Tool for ListDisks {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "list_disks",
-            description: "List all block devices.",
-            group: "disks",
-            inputs: vec![ValueType::Empty],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        argument.as_empty()?;
-
-        let mut sys = System::new_all();
-        sys.refresh_all();
-
-        let mut disk_table = Table::new(vec![
-            "name".to_string(),
-            "kind".to_string(),
-            "file system".to_string(),
-            "mount point".to_string(),
-            "total space".to_string(),
-            "available space".to_string(),
-            "is removable".to_string(),
-        ]);
-
-        for disk in sys.disks() {
-            let name = disk.name().to_string_lossy().to_string();
-            let kind = disk.kind();
-            let file_system = String::from_utf8_lossy(disk.file_system()).to_string();
-            let mount_point = disk.mount_point().to_str().unwrap().to_string();
-            let total_space = disk.total_space() as i64;
-            let available_space = disk.available_space() as i64;
-            let is_removable = disk.is_removable();
-
-            let row = vec![
-                Value::String(name),
-                Value::String(format!("{kind:?}")),
-                Value::String(file_system),
-                Value::String(mount_point),
-                Value::Integer(total_space),
-                Value::Integer(available_space),
-                Value::Boolean(is_removable),
-            ];
-
-            disk_table.insert(row)?;
-        }
-
-        Ok(Value::Table(disk_table))
-    }
-}
-
-pub struct Partition;
-
-impl Tool for Partition {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "partition",
-            description: "Partition a disk, clearing its content.",
-            group: "disks",
-            inputs: vec![ValueType::Map],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let argument = argument.as_map()?;
-        let path = argument
-            .get_value("path")?
-            .unwrap_or(Value::Empty)
-            .as_string()?
-            .clone();
-        let label = argument
-            .get_value("label")?
-            .unwrap_or(Value::Empty)
-            .as_string()?
-            .clone();
-        let name = argument
-            .get_value("name")?
-            .unwrap_or(Value::Empty)
-            .as_string()?
-            .clone();
-        let filesystem = argument
-            .get_value("filesystem")?
-            .unwrap_or(Value::Empty)
-            .as_string()?
-            .clone();
-        let range = argument
-            .get_value("range")?
-            .unwrap_or(Value::Empty)
-            .as_list()?
-            .clone();
-
-        if range.len() != 2 {
-            return Err(crate::Error::ExpectedFixedLenList {
-                expected_len: 2,
-                actual: Value::List(range),
-            });
-        }
-
-        let range_start = range[0].as_string()?;
-        let range_end = range[1].as_string()?;
-
-        let script = format!(
-            "sudo parted {path} mklabel {label} mkpart {name} {filesystem} {range_start} {range_end}"
-        );
-
-        Command::new("fish")
-            .arg("-c")
-            .arg(&script)
-            .spawn()?
-            .wait()?;
-
-        Ok(Value::Empty)
-    }
-}

src/tools/filesystem.rs

@@ -1,487 +0,0 @@
-//! Dust commands for managing files and directories.
-use std::{
-    fs::{self, OpenOptions},
-    io::{Read, Write as IoWrite},
-    path::PathBuf,
-};
-
-use crate::{Error, Result, Table, Time, Tool, ToolInfo, Value, ValueType};
-
-pub struct Append;
-
-impl Tool for Append {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "append",
-            description: "Append data to a file.",
-            group: "filesystem",
-            inputs: vec![ValueType::ListExact(vec![
-                ValueType::String,
-                ValueType::Any,
-            ])],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let arguments = argument.as_fixed_len_list(2)?;
-        let path = arguments[0].as_string()?;
-        let content = arguments[1].as_string()?;
-        let mut file = OpenOptions::new().append(true).open(path)?;
-
-        file.write_all(content.as_bytes())?;
-
-        Ok(Value::Empty)
-    }
-}
-
-pub struct CreateDir;
-
-impl Tool for CreateDir {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "create_dir",
-            description: "Create one or more directories.",
-            group: "filesystem",
-            inputs: vec![
-                ValueType::String,
-                ValueType::ListOf(Box::new(ValueType::String)),
-            ],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let path = argument.as_string()?;
-        fs::create_dir_all(path)?;
-
-        Ok(Value::Empty)
-    }
-}
-
-pub struct FileMetadata;
-
-impl Tool for FileMetadata {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "file_metadata",
-            description: "Get metadata for files.",
-            group: "filesystem",
-            inputs: vec![
-                ValueType::String,
-                ValueType::ListOf(Box::new(ValueType::String)),
-            ],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let path_string = argument.as_string()?;
-        let metadata = PathBuf::from(path_string).metadata()?;
-        let created = metadata.accessed()?.elapsed()?.as_secs() / 60;
-        let accessed = metadata.accessed()?.elapsed()?.as_secs() / 60;
-        let modified = metadata.modified()?.elapsed()?.as_secs() / 60;
-        let read_only = metadata.permissions().readonly();
-        let size = metadata.len();
-
-        let mut file_table = Table::new(vec![
-            "path".to_string(),
-            "size".to_string(),
-            "created".to_string(),
-            "accessed".to_string(),
-            "modified".to_string(),
-            "read only".to_string(),
-        ]);
-
-        file_table.insert(vec![
-            Value::String(path_string.clone()),
-            Value::Integer(size as i64),
-            Value::Integer(created as i64),
-            Value::Integer(accessed as i64),
-            Value::Integer(modified as i64),
-            Value::Boolean(read_only),
-        ])?;
-
-        Ok(Value::Table(file_table))
-    }
-}
-
-pub struct ReadDir;
-
-impl Tool for ReadDir {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "read_dir",
-            description: "Read the content of a directory.",
-            group: "filesystem",
-            inputs: vec![ValueType::String, ValueType::Empty],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let path = if let Ok(path) = argument.as_string() {
-            path
-        } else if argument.is_empty() {
-            "."
-        } else {
-            return Err(Error::TypeError {
-                expected: &[ValueType::Empty, ValueType::String],
-                actual: argument.clone(),
-            });
-        };
-        let dir = fs::read_dir(path)?;
-        let mut file_table = Table::new(vec![
-            "path".to_string(),
-            "size".to_string(),
-            "created".to_string(),
-            "accessed".to_string(),
-            "modified".to_string(),
-            "read only".to_string(),
-        ]);
-
-        for entry in dir {
-            let entry = entry?;
-            let file_type = entry.file_type()?;
-            let file_name = if file_type.is_dir() {
-                let name = entry.file_name().into_string().unwrap_or_default();
-
-                format!("{name}/")
-            } else {
-                entry.file_name().into_string().unwrap_or_default()
-            };
-            let metadata = entry.path().metadata()?;
-
-            let created_timestamp = metadata.accessed()?;
-            let created = Time::from(created_timestamp);
-
-            let accessed_timestamp = metadata.accessed()?;
-            let accessed = Time::from(accessed_timestamp);
-
-            let modified_timestamp = metadata.modified()?;
-            let modified = Time::from(modified_timestamp);
-
-            let read_only = metadata.permissions().readonly();
-            let size = metadata.len();
-
-            file_table.insert(vec![
-                Value::String(file_name),
-                Value::Integer(size as i64),
-                Value::Time(created),
-                Value::Time(accessed),
-                Value::Time(modified),
-                Value::Boolean(read_only),
-            ])?;
-        }
-
-        Ok(Value::Table(file_table))
-    }
-}
-
-pub struct ReadFile;
-
-impl Tool for ReadFile {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "read_file",
-            description: "Read file contents.",
-            group: "filesystem",
-            inputs: vec![ValueType::String],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let path = argument.as_string()?;
-        let mut contents = String::new();
-
-        OpenOptions::new()
-            .read(true)
-            .create(false)
-            .open(path)?
-            .read_to_string(&mut contents)?;
-
-        Ok(Value::String(contents))
-    }
-}
-
-pub struct RemoveDir;
-
-impl Tool for RemoveDir {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "remove_dir",
-            description: "Remove directories.",
-            group: "filesystem",
-            inputs: vec![
-                ValueType::String,
-                ValueType::ListOf(Box::new(ValueType::String)),
-            ],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let path = argument.as_string()?;
-        fs::remove_dir(path)?;
-
-        Ok(Value::Empty)
-    }
-}
-
-pub struct MoveDir;
-
-impl Tool for MoveDir {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "move_dir",
-            description: "Move a directory to a new path.",
-            group: "filesystem",
-            inputs: vec![ValueType::ListExact(vec![
-                ValueType::String,
-                ValueType::String,
-            ])],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let argument = argument.as_list()?;
-
-        Error::expect_function_argument_amount(self.info().identifier, argument.len(), 2)?;
-
-        let current_path = argument[0].as_string()?;
-        let target_path = argument[1].as_string()?;
-        let file_list = ReadDir.run(&Value::String(current_path.clone()))?;
-
-        for path in file_list.as_list()? {
-            let path = PathBuf::from(path.as_string()?);
-            let new_path = PathBuf::from(&target_path).join(&path);
-
-            if path.is_file() {
-                fs::copy(&path, target_path)?;
-            }
-
-            if path.is_symlink() && path.symlink_metadata()?.is_file() {
-                fs::copy(&path, new_path)?;
-            }
-        }
-
-        Ok(Value::Empty)
-    }
-}
-
-pub struct Trash;
-
-impl Tool for Trash {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "trash",
-            description: "Move a file or directory to the trash.",
-            group: "filesystem",
-            inputs: vec![
-                ValueType::String,
-                ValueType::ListOf(Box::new(ValueType::String)),
-            ],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let path = argument.as_string()?;
-
-        trash::delete(path)?;
-
-        Ok(Value::Empty)
-    }
-}
-
-pub struct Write;
-
-impl Tool for Write {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "write",
-            description: "Write data to a file.",
-            group: "filesystem",
-            inputs: vec![ValueType::ListExact(vec![
-                ValueType::String,
-                ValueType::Any,
-            ])],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let strings = argument.as_list()?;
-
-        Error::expect_function_argument_amount(self.info().identifier, strings.len(), 2)?;
-
-        let path = strings.first().unwrap().as_string()?;
-        let mut file = OpenOptions::new()
-            .write(true)
-            .create(true)
-            .truncate(true)
-            .open(path)?;
-
-        for content in &strings[1..] {
-            let content = content.to_string();
-
-            file.write_all(content.as_bytes())?;
-        }
-
-        Ok(Value::Empty)
-    }
-}
-
-pub struct RemoveFile;
-
-impl Tool for RemoveFile {
-    fn info(&self) -> ToolInfo<'static> {
-        ToolInfo {
-            identifier: "remove_file",
-            description: "Permanently delete a file.",
-            group: "filesystem",
-            inputs: vec![
-                ValueType::String,
-                ValueType::ListOf(Box::new(ValueType::String)),
-            ],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        if let Ok(path) = argument.as_string() {
-            fs::remove_file(path)?;
-
-            return Ok(Value::Empty);
-        }
-
-        if let Ok(path_list) = argument.as_list() {
-            for path in path_list {
-                let path = path.as_string()?;
-
-                fs::remove_file(path)?;
-            }
-
-            return Ok(Value::Empty);
-        }
-
-        Err(Error::expected_string(argument.clone()))
-    }
-}
-
-pub struct Watch;
-
-impl Tool for Watch {
-    fn info(&self) -> crate::ToolInfo<'static> {
-        crate::ToolInfo {
-            identifier: "watch",
-            description: "Wait until a file changes.",
-            group: "filesystem",
-            inputs: vec![ValueType::String],
-        }
-    }
-
-    fn run(&self, argument: &Value) -> Result<Value> {
-        let argument = argument.as_string()?;
-        let path = PathBuf::from(argument);
-        let modified_old = path.metadata()?.modified()?;
-        let wait_time = loop {
-            let modified_new = path.metadata()?.modified()?;
-
-            if modified_old != modified_new {
-                break modified_new
-                    .duration_since(modified_old)
-                    .unwrap_or_default()
-                    .as_millis() as i64;
-            }
-        };
-
-        Ok(Value::Integer(wait_time))
-    }
-}
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-
-    #[test]
-    fn create_dir() {
-        let path = PathBuf::from("./target/create_dir/");
-        let path_value = Value::String(path.to_string_lossy().to_string());
-        let _ = std::fs::remove_file(&path);
-
-        CreateDir.run(&path_value).unwrap();
-
-        assert!(path.is_dir());
-    }
-
-    #[test]
-    fn create_dir_nested() {
-        let path = PathBuf::from("./target/create_dir/nested");
-        let path_value = Value::String(path.to_string_lossy().to_string());
-        let _ = std::fs::remove_file(&path);
-
-        CreateDir.run(&path_value).unwrap();
-
-        assert!(path.is_dir());
-    }
-
-    #[test]
-    fn write() {
-        let path = PathBuf::from("./target/write.txt");
-        let path_value = Value::String(path.to_string_lossy().to_string());
-        let message = "hiya".to_string();
-        let message_value = Value::String(message.clone());
-        let _ = std::fs::remove_file(&path);
-
-        Write
-            .run(&Value::List(vec![path_value, message_value]))
-            .unwrap();
-
-        assert!(path.is_file());
-    }
-
-    #[test]
-    fn append() {
-        let path = PathBuf::from("./target/append.txt");
-        let path_value = Value::String(path.to_string_lossy().to_string());
-        let message = "hiya".to_string();
-        let message_value = Value::String(message.clone());
-        let _ = std::fs::remove_file(&path);
-
-        Write
-            .run(&Value::List(vec![
-                path_value.clone(),
-                message_value.clone(),
-            ]))
-            .unwrap();
-        Append
-            .run(&Value::List(vec![path_value, message_value]))
-            .unwrap();
-
-        let read = fs::read_to_string(&path).unwrap();
-
-        assert_eq!("hiyahiya", read);
-    }
-
-    #[test]
-    fn read_file() {
-        let path = PathBuf::from("./target/read_file.txt");
-        let path_value = Value::String(path.to_string_lossy().to_string());
-        let message = "hiya".to_string();
-        let message_value = Value::String(message.clone());
-        let _ = std::fs::remove_file(&path);
-
-        Write
-            .run(&Value::List(vec![path_value.clone(), message_value]))
-            .unwrap();
-
-        let test = ReadFile.run(&path_value).unwrap();
-        let read = fs::read_to_string(&path).unwrap();
-
-        assert_eq!(test, Value::String(read));
-    }
-
-    #[test]
-    fn remove_file() {
-        let path = PathBuf::from("./target/remove_file.txt");
-        let path_value = Value::String(path.to_string_lossy().to_string());
-        let _ = std::fs::File::create(&path);
-
-        RemoveFile.run(&path_value).unwrap();
-
-        assert!(!path.exists());
-    }
-}