Clean up examples

This reverts commit 5e685d6641.

Cargo.toml

@@ -1,10 +1,12 @@
 [package]
 name = "dust-lang"
-description = "Data-Oriented Programming Language"
+description = "General purpose programming language"
-version = "0.3.7"
+version = "0.4.2"
 repository = "https://git.jeffa.io/jeff/dust.git"
 edition = "2021"
 license = "MIT"
+authors = ["Jeff Anderson"]
+default-run = "dust"
 
 [[bin]]
 name = "dust"
@@ -16,20 +18,33 @@ opt-level = 1
 opt-level = 3
 
 [dependencies]
-ansi_term = "0.12.1"
-async-std = { version = "1.12.0", features = ["attributes"] }
 clap = { version = "4.4.4", features = ["derive"] }
-comfy-table = "7.0.1"
 csv = "1.2.2"
-git2 = "0.18.1"
+libc = "0.2.148"
+log = "0.4.20"
 rand = "0.8.5"
 rayon = "1.8.0"
 reqwest = { version = "0.11.20", features = ["blocking", "json"] }
-rustyline = { version = "12.0.0", features = ["derive", "with-file-history"] }
 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

@@ -1,292 +1,100 @@
 # Dust
 
-Dust is a general purpose programming language that emphasises concurrency and correctness.
+High-level programming language with effortless concurrency, automatic memory management, type safety and strict error handling.
 
-A basic dust program:
+![Dust version of an example from The Rust Programming Language.](https://git.jeffa.io/jeff/dust/raw/branch/main/docs/assets/example_0.png)
-
-```dust
-(output  "Hello world!")
-```
-
-Dust can do two (or more) things at the same time with effortless concurrency:
-
-```dust
-async {
-    (output 'will this one finish first?')
-    (output 'or will this one?')
-}
-```
-
-You can make *any* block, i.e. `{}`, run its statements in parallel by changing it to `async {}`.
-
-```dust
-if (random_boolean) {
-    (output "Do something...")
-} else async {
-    (output "Do something else instead...")
-    (output "And another thing at the same time...")
-}
-```
-
-Dust is an interpreted, strictly typed language with first class functions. It emphasises concurrency by allowing any group of statements to be executed in parallel. Dust includes built-in tooling to import and export data in a variety of formats, including JSON, TOML, YAML and CSV.
 
 <!--toc:start-->
 - [Dust](#dust)
   - [Features](#features)
-  - [Usage](#usage)
+    - [Easy to Read and Write](#easy-to-read-and-write)
-  - [Installation](#installation)
+    - [Effortless Concurrency](#effortless-concurrency)
-  - [Benchmarks](#benchmarks)
+    - [Helpful Errors](#helpful-errors)
-  - [Implementation](#implementation)
+    - [Static analysis](#static-analysis)
-  - [The Dust Programming Language](#the-dust-programming-language)
+    - [Debugging](#debugging)
-    - [Declaring Variables](#declaring-variables)
+    - [Automatic Memory Management](#automatic-memory-management)
-    - [Lists](#lists)
+    - [Error Handling](#error-handling)
-    - [Maps](#maps)
+  - [Installation and Usage](#installation-and-usage)
-    - [Loops](#loops)
-    - [Tables](#tables)
-    - [Functions](#functions)
-    - [Concurrency](#concurrency)
 <!--toc:end-->
 
 ## Features
 
-- Simplicity: Dust is designed to be easy to learn.
+### Easy to Read and Write
-- Speed: Dust is built on [Tree Sitter] and [Rust] to prioritize performance and correctness. See [Benchmarks] below.
-- Concurrency: Easily and safely write code that runs in parallel.
-- Safety: Written in safe, stable Rust.
-- Correctness: Type checking makes it easy to write good code that works.
 
-## 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.
+```js
-
+output('Hello world!')
-To get help with the shell you can use the "help" tool.
-
-```dust
-(help) # Returns a table with tool info.
 ```
 
-## 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 build the parser. To do so, enter the `tree-sitter-dust` directory and run `tree-sitter-generate`. In the project root, run `cargo run` to start the shell. To see other command line options, use `cargo run -- --help`.
+```js
+async {
+    output('Will this one print first?')
+    output('Or will this one?')
+    output('Who knows! Each "output" will run in its own thread!')
+}
+```
+
+### Helpful Errors
+
+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
+match io:stdin() {
+    Result::Ok(input) -> output("We read this input: " + input)
+    Result::Error(message) -> output("We got this error: " + message)
+}
+```
+
+## 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
+```
+
+To install with cargo:
+
+```fish
+cargo install dust-lang
+dust
+```
 
 ## Benchmarks
 
-Dust is at a very early development stage but performs strongly in preliminary benchmarks. The examples given were tested using [Hyperfine] on a single-core cloud instance with 1024 MB RAM. Each test was run 1000 times. The test script is shown below. Each test asks the program to read a JSON file and count the objects. Dust is a command line shell, programming language and data manipulation tool so three appropriate targets were chosen for comparison: nushell, NodeJS and jq. The programs produced identical output with the exception that NodeJS printed in color.
+## Development Status
 
-For the first test, a file with four entries was used.
+Currently, Dust is being prepared for version 1.0. Until then, there may be breaking changes to the language and CLI.
-
-| Command | Mean [ms] | Min [ms] | Max [ms] 
-|:---|---:|---:|---:|
-| Dust | 3.1 ± 0.5 | 2.4 | 8.4 |
-| jq | 33.7 ± 2.2 | 30.0 | 61.8 |
-| NodeJS | 226.4 ± 13.1 | 197.6 | 346.2 |
-| Nushell | 51.6 ± 3.7 | 45.4 | 104.3 |
-
-The second set of data is from the GitHub API, it consists of 100 commits from the jq GitHub repo.
-
-| Command | Mean [ms] | Min [ms] | Max [ms] |
-|:---|---:|---:|---:|
-| Dust | 6.8 ± 0.6 | 5.7 | 12.0 | 2.20 ± 0.40 |
-| jq | 43.3 ± 3.6 | 37.6 | 81.6 | 13.95 ± 2.49 |
-| NodeJS | 224.9 ± 12.3 | 194.8 | 298.5 |
-| Nushell | 59.2 ± 5.7 | 49.7 | 125.0 | 19.11 ± 3.55 |
-
-This data came from CERN, it is a massive file of 100,000 entries.
-
-| Command | Mean [ms] | Min [ms] | Max [ms] |
-|:---|---:|---:|---:|
-| Dust | 1080.8 ± 38.7 | 975.3 | 1326.6 |
-| jq | 1305.3 ± 64.3 | 1159.7 | 1925.1 |
-| NodeJS | 1850.5 ± 72.5 | 1641.9 | 2395.1 |
-| Nushell | 1850.5 ± 86.2 | 1625.5 | 2400.7 |
-
-The tests were run after 5 warmup runs and the cache was cleared before each run.
-
-```sh
-hyperfine \
-	--shell none \
-	--warmup 5 \
-	--prepare "rm -rf /root/.cache" \
-	--runs 1000 \
-	--parameter-list data_path seaCreatures.json,jq_data.json,dielectron.json \
-	--export-markdown test_output.md \
-	"dust -c '(length (from_json input))' -p {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'"
-```
-
-## Implementation
-
-Dust is formally defined as a Tree Sitter grammar in the tree-sitter-dust directory. Tree sitter generates a parser, written in C, from a set of rules defined in JavaScript. Dust itself is a rust binary that calls the C parser using FFI.
-
-Tests are written in three places: in the Rust library, in Dust as examples and in the Tree Sitter test format. Generally, features are added by implementing and testing the syntax in the tree-sitter-dust repository, then writing library tests to evaluate the new syntax. Implementation tests run the Dust files in the "examples" directory and should be used to demonstrate and verify that features work together.
-
-Tree Sitter generates a concrete syntax tree, which Dust traverses to create an abstract syntax tree that can run the Dust code. The CST generation is an extra step but it allows easy testing of the parser, defining the language in one file and makes the syntax easy to modify and expand. Because it uses Tree Sitter, developer-friendly features like syntax highlighting and code navigation are already available in any text editor that supports Tree Sitter.
-
-## The Dust Programming Language
-
-Dust is easy to learn. Aside from this guide, the best way to learn Dust is to read the examples and tests to get a better idea of what it can do.
-
-### Declaring Variables
-
-Variables have two parts: a key and a value. The key is always a string. The value can be any of the following data types:
-
-- string
-- integer
-- floating point value
-- boolean
-- list
-- map
-- table
-- function
-
-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] # Commas are optional when writing lists.
-map = {
-    key = 'value'
-}
-```
-
-Note that strings can be wrapped with any kind of quote: single, double or backticks. Numbers are always integers by default. Floats are declared by adding a decimal. If you divide integers or do any kind of math with a float, you will create a float value.
-
-### Lists
-
-Lists are sequential collections. They can be built by grouping values with square brackets. Commas are optional. Values can be indexed by their position using a colon `:` followed by an integer. Dust lists are zero-indexed.
-
-```dust
-list = [true 41 "Ok"]
-
-(assert_equal list:0 true)
-
-the_answer = list:1 + 1
-
-(assert_equal the_answer, 42) # You can also use commas when passing values to
-                              # a function. 
-```
-
-### Maps
-
-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 `:`.
-
-```dust
-reminder = {
-    message = "Buy milk"
-    tags = ["groceries", "home"]
-}
-
-(output reminder:message)
-```
-
-### Loops
-
-A **while** loop continues until a predicate is false.
-
-```dust
-i = 0
-while i < 10 {
-    (output i)
-    i += 1
-}
-```
-
-A **for** loop operates on a list without mutating it or the items inside. It does not return a value.
-
-```dust
-list = [ 1, 2, 3 ]
-
-for number in list {
-    (output number + 1)
-}
-```
-
-### Tables
-
-Tables are strict collections, each row must have a value for each column. If a value is "missing" it should be set to an appropriate value for that type. For example, a string can be empty and a number can be set to zero. Dust table declarations consist of a list of column names, which are identifiers enclosed in pointed braces, followed by a list of rows.
-
-```dust
-animals = table <name species age> [
-    ["rover" "cat" 14]
-    ["spot" "snake" 9]
-    ["bob" "giraffe" 2]
-]
-```
-
-Querying a table is similar to SQL.
-
-```dust
-names = select name from animals
-youngins = select species from animals {
-    age <= 10
-}
-```
-
-The keywords `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 into animals [
-    ["eliza" "ostrich" 4]
-    ["pat" "white rhino" 7]
-    ["jim" "walrus" 9]
-]
-
-(assert_equal 6 (length animals))
-```
-
-### Functions
-
-Functions are first-class values in dust, so they are assigned to variables like any other value.
-
-```dust
-# This simple function has no arguments.
-say_hi = || => {
-    (output "hi")
-}
-
-# This function has one argument and will return a value.
-add_one = |number| => {
-    number + 1
-}
-
-(say_hi)
-(assert_equal (add_one 3), 4)
-```
-
-You don't need commas when listing arguments and you don't need to add whitespace inside the function body but doing so may make your code easier to read.
-
-### Concurrency
-
-Dust features effortless concurrency anywhere in your code. Any block of code can be made to run its contents asynchronously. Dust's concurrency is written in safe Rust and uses a thread pool whose size depends on the number of cores available.
-
-```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")
-}
-```
-
-### Acknowledgements
-
-Dust began as a fork of [evalexpr]. Some of the original code is still in place but the project has dramatically changed and no longer uses any of its parsing or interpreting.
-
-[Tree Sitter]: https://tree-sitter.github.io/tree-sitter/
-[Rust]: https://rust-lang.org
-[evalexpr]: https://github.com/ISibboI/evalexpr
-[rustup]: https://rustup.rs
-[Hyperfine]: https://github.com/sharkdp/hyperfine

build.rs

@@ -1,15 +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);
 
+    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/read-data.js

@@ -1,6 +0,0 @@
-const fs = require('node:fs');
-const data = fs.readFileSync("examples/assets/jq_data.json");
-const items = JSON.parse(data);
-const output = items.map((item) => item.commit.committer.name);
-
-console.log(output)

examples/async.ds

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

@@ -1,9 +1,20 @@
-cast = (download "https://api.sampleapis.com/futurama/cast")
+cast_len = 0
-characters = (download "https://api.sampleapis.com/futurama/characters")	
+characters_len = 0
-episodes = (download "https://api.sampleapis.com/futurama/episodes")
+episodes_len = 0
 
-cast_len = (length (from_json cast))
+async {	
-characters_len = (length (from_json characters))
+	{
-episodes_len = (length (from_json episodes))
+		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])
+output ([cast_len, characters_len, episodes_len])

examples/clue_solver.ds

@@ -1,51 +1,53 @@
-all_cards = {
+cards = {
 	rooms = ['Library' 'Kitchen' 'Conservatory']
 	suspects = ['White' 'Green' 'Scarlett']
 	weapons = ['Rope' 'Lead_Pipe' 'Knife']
 }
 
-is_ready_to_solve = |cards <map>| <bool> {
+is_ready_to_solve = (cards <map>) <bool> {
-	((length cards:suspects) == 1)
+	(length(cards:suspects) == 1)
-	&& ((length cards:rooms) == 1)
+	&& (length(cards:rooms) == 1)
-	&& ((length cards:weapons) == 1)
+	&& (length(cards:weapons) == 1)
 }
 
-take_turn = |opponent_card <str>, current_room <str>, cards <map>| <map> {
+remove_card = (cards <map>, opponent_card <str>) <none> {
-	(remove_card opponent_card cards)
-	(make_guess current_room cards)
-	cards
-}
-
-remove_card = |opponent_card <str>, cards <map>| {
 	cards:rooms -= opponent_card
 	cards:suspects -= opponent_card
 	cards:weapons -= opponent_card
 }
 
-make_guess = |current_room <str>, cards <map>| {
+make_guess = (cards <map>, current_room <str>) <none> {
-	if (is_ready_to_solve cards) {
+	if is_ready_to_solve(cards) {
-		(output 'It was '
+		output(
+			'I accuse '
 			+ cards:suspects:0
 			+ ' in the ' 
 			+ cards:rooms:0  
 			+ ' with the ' 
 			+ cards:weapons:0 
-			+ '!')
+			+ '!'
+		)
 	} else {
-		(output 'I accuse '
+		output(
-			+ (random cards:suspects)
+			'I question '
+			+ random:from(cards:suspects)
 			+ ' in the '
 			+ current_room
 			+ ' with the '
-			+ (random cards:weapons)
+			+ random:from(cards:weapons)
-			+ '.')
+			+ '.'
+		)
 	}
 }
 
-(take_turn 'Rope' 'Kitchen'
+take_turn = (cards <map>, opponent_card <str>, current_room <str>) <none> {
-	(take_turn 'Library' 'Kitchen'
+	remove_card(cards opponent_card)
-		(take_turn 'Conservatory' 'Kitchen'
+	make_guess(cards current_room)
-			(take_turn 'White' 'Kitchen'
+}
-				(take_turn 'Green' 'Kitchen'
-					(take_turn 'Knife' 'Kitchen' all_cards))))))
 
+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/download.ds

@@ -1,9 +1,10 @@
-cast = (download "https://api.sampleapis.com/futurama/cast")
+raw_data = download("https://api.sampleapis.com/futurama/cast")
-characters = (download "https://api.sampleapis.com/futurama/characters")	
+cast_data = from_json(raw_data)
-episodes = (download "https://api.sampleapis.com/futurama/episodes")
 
-cast_len = (length (from_json cast))
+names = []
-characters_len = (length (from_json characters))
-episodes_len = (length (from_json episodes))
 
-(output [cast_len, characters_len, episodes_len])
+for cast_member in cast_data {
+	names += cast_member:name
+}
+
+assert_equal("Billy West", names:0)

examples/fetch.ds

@@ -1,10 +0,0 @@
-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/fibonacci.ds

@@ -1,9 +1,9 @@
-fib = |i <int>| <int> {
+fib = (i <int>) <int> {
 	if i <= 1 {
 		1
 	} else {
-		(fib i - 1) + (fib i - 2)
+		fib(i - 1) + fib(i - 2)
 	}
 }
 
-(fib 5)
+fib(8)

examples/fizzbuzz.ds

@@ -5,13 +5,13 @@ while count <= 15 {
 	divides_by_5 = count % 5 == 0
 
 	if divides_by_3 && divides_by_5 { 
-		(output 'fizzbuzz')
+		output('fizzbuzz')
 	} else if divides_by_3 { 
-		(output 'fizz')
+		output('fizz')
 	} else if divides_by_5 {
-		(output 'buzz')
+		output('buzz')
 	} else { 
-		(output count)
+		output(count)
 	}
 	
 	count += 1

examples/for_loop.ds

@@ -1,6 +0,0 @@
-list = [1 2 3]
-
-for i in list {
-	i += 1
-	(output i)
-}

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/hello_world.ds

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

examples/jq_data.ds

@@ -1,11 +1,11 @@
-data = (from_json (read 'examples/assets/jq_data.json'))
+data = json:parse(fs:read_file('examples/assets/jq_data.json'))
 
-new_data = [];
+new_data = []
 
-for commit_data in data {	
+for commit_data in data as collection {	
 	new_data += {
-		message = commit_data.commit.message
+		message = commit_data:commit:message
-		name = commit_data.commit.committer.name
+		name = commit_data:commit:committer:name
 	}
 }
 

examples/list.ds

@@ -1,7 +0,0 @@
-numbers = [1, 2, 3]
-
-x = numbers:0
-y = numbers:1
-z = numbers:2
-
-(assert_equal x + y, z)

examples/map.ds

@@ -1,11 +0,0 @@
-dictionary = {
-	dust = "awesome"
-	answer = 42
-}
-
-(output 
-	'Dust is '
-	+ dictionary:dust
-	+ '! The answer is ' 
-	+ dictionary:answer
-)

examples/random.ds

@@ -1,16 +1,13 @@
 stuff = [
-	(random_integer)
+	random:integer()
-	(random_integer)
+	random:integer()
-	(random_integer)
+	random:integer()
-	(random_float)
+	random:float()
-	(random_float)
+	random:float()
-	(random_float)
+	random:float()
-	(random_boolean)
+	random:boolean()
-	(random_boolean)
+	random:boolean()
-	(random_boolean)
+	random:boolean()
-	"foobar_1"
-	"foobar_2"
-	"foobar_3"
 ]
 
-(random stuff)
+random:from(stuff)

examples/sea_creatures.ds

@@ -1,5 +1,5 @@
-raw_data = (read 'examples/assets/seaCreatures.json')
+raw_data = fs:read_file('examples/assets/seaCreatures.json')
-sea_creatures = (from_json raw_data)
+sea_creatures = json:parse(raw_data)
 
 data = {
 	creatures = []

examples/select.ds

@@ -1,15 +0,0 @@
-my_table = table |text number bool| [
-	["a", 1, true]
-	["b", 2, true]
-	["a", 3, true]
-]
-
-test_table = table |text bool| [
-	["a", true]
-	["b", true]
-	["a", true]
-]
-
-test_select = select |text bool| from my_table;
-
-(assert_equal test_select, test_table)

examples/table.ds

@@ -1,43 +0,0 @@
-my_table = table |text number bool| [
-	["a", 1, true]
-	["b", 2, true]
-	["a", 3, true]
-]
-
-test_table = table |text bool| [
-	["a", true]
-	["b", true]
-	["a", true]
-]
-
-test_select = select |text bool| from my_table;
-
-(assert_equal test_select, test_table)
-
-test_table = table |number bool| [
-	[1, true]
-	[3, true]
-]
-
-test_select_where = select |number, bool| from my_table {
-	text == "a"
-}
-
-(assert_equal test_select_where, test_table)
-
-test_table = table |text number bool| [
-	["a", 1, true]
-	["b", 2, true]
-	["a", 3, true]
-	["c", 4, true]
-	["d", 5, true]
-	["e", 6, true]
-]
-
-insert into my_table [
-	["c", 4, true]
-	["d", 5, true]
-	["e", 6, true]
-]
-
-(assert_equal test_table, my_table)

examples/variables.ds

@@ -1,10 +0,0 @@
-x = 1
-y = "hello dust!"
-z = 42.0
-list = [3, 2, x]
-big_list = [x, y, z, list]
-foo = {
-	x = "bar" 
-	y = 42
-	z = 0
-}

examples/while_loop.ds

@@ -1,6 +0,0 @@
-i = 0
-
-while i < 10 {
-	(output i)
-	i += 1
-}

examples/yield.ds

@@ -1,15 +0,0 @@
-1 -> (output)
-
-add_one = |numbers <list>| <list> {
-	new_numbers = []
-	
-	for number in numbers {
-		new_numbers += number + 1
-	}
-
-	new_numbers
-}
-
-foo = [1, 2, 3] -> (add_one)
-
-(assert_equal [2 3 4] foo)

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

@@ -1,97 +1,186 @@
 use serde::{Deserialize, Serialize};
-use tree_sitter::Node;
 
-use crate::{AbstractTree, Error, Identifier, Map, Result, Statement, Type, Value};
+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,
-    r#type: Option<Type>,
+    type_specification: Option<TypeSpecification>,
     operator: AssignmentOperator,
     statement: Statement,
-}
+    syntax_position: SourcePosition,
-
-#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
-pub enum AssignmentOperator {
-    Equal,
-    PlusEqual,
-    MinusEqual,
 }
 
 impl AbstractTree for Assignment {
-    fn from_syntax_node(source: &str, node: Node) -> Result<Self> {
+    fn from_syntax(
-        Error::expect_syntax_node(source, "assignment", node)?;
+        syntax_node: SyntaxNode,
+        source: &str,
+        context: &Context,
+    ) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("assignment", syntax_node)?;
 
-        let identifier_node = node.child_by_field_name("identifier").unwrap();
+        let child_count = syntax_node.child_count();
-        let identifier = Identifier::from_syntax_node(source, identifier_node)?;
 
-        let type_node = node.child_by_field_name("type");
+        let identifier_node = syntax_node.child(0).unwrap();
-        let r#type = if let Some(type_node) = type_node {
+        let identifier = Identifier::from_syntax(identifier_node, source, context)?;
-            Some(Type::from_syntax_node(source, type_node)?)
+
+        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 = node
+        let operator_node = syntax_node.child(child_count - 2).unwrap();
-            .child_by_field_name("assignment_operator")
+        let operator = AssignmentOperator::from_syntax(operator_node, source, context)?;
-            .unwrap()
-            .child(0)
-            .unwrap();
-        let operator = match operator_node.kind() {
-            "=" => AssignmentOperator::Equal,
-            "+=" => AssignmentOperator::PlusEqual,
-            "-=" => AssignmentOperator::MinusEqual,
-            _ => {
-                return Err(Error::UnexpectedSyntaxNode {
-                    expected: "=, += or -=",
-                    actual: operator_node.kind(),
-                    location: operator_node.start_position(),
-                    relevant_source: source[operator_node.byte_range()].to_string(),
-                })
-            }
-        };
 
-        let statement_node = node.child_by_field_name("statement").unwrap();
+        let statement_node = syntax_node.child(child_count - 1).unwrap();
-        let statement = Statement::from_syntax_node(source, statement_node)?;
+        let statement = Statement::from_syntax(statement_node, source, context)?;
 
         Ok(Assignment {
             identifier,
-            r#type,
+            type_specification,
             operator,
             statement,
+            syntax_position: syntax_node.range().into(),
         })
     }
 
-    fn run(&self, source: &str, context: &mut Map) -> Result<Value> {
+    fn validate(&self, source: &str, context: &Context) -> Result<(), ValidationError> {
-        let key = self.identifier.inner();
+        if let AssignmentOperator::Equal = self.operator {
-        let value = self.statement.run(source, context)?;
+            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 => {
-                if let Some(mut previous_value) = context.variables()?.get(key).cloned() {
+                let left = self.identifier.run(source, context)?;
-                    previous_value += value;
+
-                    previous_value
+                left.add(right, self.syntax_position)?
-                } else {
-                    return Err(Error::VariableIdentifierNotFound(key.clone()));
-                }
             }
             AssignmentOperator::MinusEqual => {
-                if let Some(mut previous_value) = context.variables()?.get(key).cloned() {
+                if let Some(left) = context.get_value(&self.identifier)? {
-                    previous_value -= value;
+                    left.subtract(right, self.syntax_position)?
-                    previous_value
                 } else {
-                    return Err(Error::VariableIdentifierNotFound(key.clone()));
+                    return Err(RuntimeError::ValidationFailure(
+                        ValidationError::VariableIdentifierNotFound(self.identifier.clone()),
+                    ));
                 }
             }
-            AssignmentOperator::Equal => value,
+            AssignmentOperator::Equal => right,
         };
 
-        if let Some(r#type) = &self.r#type {
+        if let Value::Function(Function::ContextDefined(function_node)) = &new_value {
-            r#type.check(&new_value)?;
+            function_node
+                .context()
+                .set_value(self.identifier.clone(), new_value.clone())?;
         }
 
-        context.variables_mut()?.insert(key.clone(), new_value);
+        log::info!("RUN assignment: {} = {}", self.identifier, new_value);
 
-        Ok(Value::Empty)
+        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

@@ -1,23 +1,44 @@
-use std::sync::RwLock;
+use std::{
+    fmt::{self, Formatter},
+    sync::RwLock,
+};
 
 use rayon::prelude::*;
 use serde::{Deserialize, Serialize};
-use tree_sitter::Node;
 
-use crate::{AbstractTree, Error, Map, Result, Statement, Value};
+use crate::{
+    error::{rw_lock_error::RwLockError, RuntimeError, SyntaxError, ValidationError},
+    AbstractTree, Context, Format, Statement, SyntaxNode, Type, Value,
+};
 
-#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+/// 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(source: &str, node: Node) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
-        Error::expect_syntax_node(source, "block", node)?;
+        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
@@ -25,60 +46,125 @@ impl AbstractTree for Block {
             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.is_named() {
+            if child_node.kind() == "statement" {
-                let statement = Statement::from_syntax_node(source, child_node)?;
+                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 run(&self, source: &str, context: &mut Map) -> Result<Value> {
+    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::Empty));
+            let final_result = RwLock::new(Ok(Value::none()));
 
             statements
                 .into_par_iter()
                 .enumerate()
                 .find_map_first(|(index, statement)| {
-                    if let Statement::Return(expression) = statement {
+                    let result = statement.run(_source, _context);
-                        return Some(expression.run(source, &mut context.clone()));
+                    let should_return = if self.contains_return {
-                    }
+                        statement.is_return()
+                    } else {
+                        index == statements.len() - 1
+                    };
 
-                    let result = statement.run(source, &mut context.clone());
+                    if should_return {
-
+                        let get_write_lock = final_result.write();
-                    if result.is_err() {
-                        Some(result)
-                    } else if index == statements.len() - 1 {
-                        let _ = final_result.write().unwrap().as_mut().map(|_| result);
 
+                        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().unwrap())
+                .unwrap_or(final_result.into_inner().map_err(|_| RwLockError)?)
         } else {
-            let mut prev_result = None;
+            for (index, statement) in self.statements.iter().enumerate() {
-
+                if statement.is_return() {
-            for statement in &self.statements {
+                    return statement.run(_source, _context);
-                if let Statement::Return(expression) = statement {
-                    return expression.run(source, context);
                 }
 
-                prev_result = Some(statement.run(source, context));
+                if index == self.statements.len() - 1 {
+                    return statement.run(_source, _context);
+                }
             }
 
-            prev_result.unwrap_or(Ok(Value::Empty))
+            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/built_in_function.rs

@@ -1,650 +0,0 @@
-use std::{
-    env::current_dir,
-    fs::{copy, metadata, read_dir, read_to_string, remove_file, write, File},
-    io::Write,
-    path::PathBuf,
-    process::Command,
-};
-
-use rand::{random, thread_rng, Rng};
-use reqwest::blocking::get;
-use serde::{Deserialize, Serialize};
-use tree_sitter::Node;
-
-use crate::{AbstractTree, Error, Expression, List, Map, Result, Table, Type, Value};
-
-#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
-pub enum BuiltInFunction {
-    // General
-    Assert(Vec<Expression>),
-    AssertEqual(Vec<Expression>),
-    Download(Expression),
-    Context,
-    Help(Option<Expression>),
-    Length(Expression),
-    Output(Vec<Expression>),
-    OutputError(Vec<Expression>),
-    Type(Expression),
-    Workdir,
-
-    // Filesystem
-    Append(Vec<Expression>),
-    Metadata(Expression),
-    Move(Vec<Expression>),
-    Read(Option<Expression>),
-    Remove(Expression),
-    Write(Vec<Expression>),
-
-    // Format conversion
-    FromJson(Expression),
-    ToJson(Expression),
-    ToString(Expression),
-    ToFloat(Expression),
-
-    // Command
-    Bash(Vec<Expression>),
-    Fish(Vec<Expression>),
-    Raw(Vec<Expression>),
-    Sh(Vec<Expression>),
-    Zsh(Vec<Expression>),
-
-    // Random
-    Random(Vec<Expression>),
-    RandomBoolean,
-    RandomInteger,
-    RandomFloat,
-
-    // Table
-    Columns(Expression),
-    Rows(Expression),
-
-    // List
-    Reverse(Expression, Option<(Expression, Expression)>),
-}
-
-impl AbstractTree for BuiltInFunction {
-    fn from_syntax_node(source: &str, node: Node) -> Result<Self> {
-        debug_assert_eq!("built_in_function", node.kind());
-
-        fn parse_expressions(source: &str, node: Node) -> Result<Vec<Expression>> {
-            let mut expressions = Vec::new();
-
-            for index in 1..node.child_count() {
-                let child_node = node.child(index).unwrap();
-
-                if child_node.kind() == "expression" {
-                    let expression = Expression::from_syntax_node(source, child_node)?;
-
-                    expressions.push(expression);
-                }
-            }
-
-            Ok(expressions)
-        }
-
-        let tool_node = node.child(0).unwrap();
-        let tool = match tool_node.kind() {
-            "assert" => {
-                let expressions = parse_expressions(source, node)?;
-
-                BuiltInFunction::Assert(expressions)
-            }
-            "assert_equal" => {
-                let expressions = parse_expressions(source, node)?;
-
-                BuiltInFunction::AssertEqual(expressions)
-            }
-            "context" => BuiltInFunction::Context,
-            "download" => {
-                let expression_node = node.child(1).unwrap();
-                let expression = Expression::from_syntax_node(source, expression_node)?;
-
-                BuiltInFunction::Download(expression)
-            }
-            "help" => {
-                let child_node = node.child(1).unwrap();
-                let expression = if child_node.is_named() {
-                    Some(Expression::from_syntax_node(source, child_node)?)
-                } else {
-                    None
-                };
-
-                BuiltInFunction::Help(expression)
-            }
-            "length" => {
-                let expression_node = node.child(1).unwrap();
-                let expression = Expression::from_syntax_node(source, expression_node)?;
-
-                BuiltInFunction::Length(expression)
-            }
-            "output" => {
-                let expressions = parse_expressions(source, node)?;
-
-                BuiltInFunction::Output(expressions)
-            }
-            "output_error" => {
-                let expressions = parse_expressions(source, node)?;
-
-                BuiltInFunction::OutputError(expressions)
-            }
-            "type" => {
-                let expression_node = node.child(1).unwrap();
-                let expression = Expression::from_syntax_node(source, expression_node)?;
-
-                BuiltInFunction::Type(expression)
-            }
-            "workdir" => BuiltInFunction::Workdir,
-            "append" => {
-                let expressions = parse_expressions(source, node)?;
-
-                Error::expect_tool_argument_amount("append", 2, expressions.len())?;
-
-                BuiltInFunction::Append(expressions)
-            }
-            "metadata" => {
-                let expression_node = node.child(1).unwrap();
-                let expression = Expression::from_syntax_node(source, expression_node)?;
-
-                BuiltInFunction::Metadata(expression)
-            }
-            "move" => {
-                let expressions = parse_expressions(source, node)?;
-
-                Error::expect_tool_argument_amount("move", 2, expressions.len())?;
-
-                BuiltInFunction::Move(expressions)
-            }
-            "read" => {
-                let expression = if let Some(node) = node.child(1) {
-                    Some(Expression::from_syntax_node(source, node)?)
-                } else {
-                    None
-                };
-
-                BuiltInFunction::Read(expression)
-            }
-            "remove" => {
-                let expression_node = node.child(1).unwrap();
-                let expression = Expression::from_syntax_node(source, expression_node)?;
-
-                BuiltInFunction::Remove(expression)
-            }
-            "write" => {
-                let expressions = parse_expressions(source, node)?;
-
-                Error::expect_tool_argument_amount("write", 2, expressions.len())?;
-
-                BuiltInFunction::Write(expressions)
-            }
-            "from_json" => {
-                let expression_node = node.child(1).unwrap();
-                let expression = Expression::from_syntax_node(source, expression_node)?;
-
-                BuiltInFunction::FromJson(expression)
-            }
-            "to_json" => {
-                let expression_node = node.child(1).unwrap();
-                let expression = Expression::from_syntax_node(source, expression_node)?;
-
-                BuiltInFunction::ToJson(expression)
-            }
-            "to_string" => {
-                let expression_node = node.child(1).unwrap();
-                let expression = Expression::from_syntax_node(source, expression_node)?;
-
-                BuiltInFunction::ToString(expression)
-            }
-            "to_float" => {
-                let expression_node = node.child(1).unwrap();
-                let expression = Expression::from_syntax_node(source, expression_node)?;
-
-                BuiltInFunction::ToFloat(expression)
-            }
-            "bash" => {
-                let expressions = parse_expressions(source, node)?;
-
-                BuiltInFunction::Bash(expressions)
-            }
-            "fish" => {
-                let expressions = parse_expressions(source, node)?;
-
-                BuiltInFunction::Fish(expressions)
-            }
-            "raw" => {
-                let expressions = parse_expressions(source, node)?;
-
-                BuiltInFunction::Raw(expressions)
-            }
-            "sh" => {
-                let expressions = parse_expressions(source, node)?;
-
-                BuiltInFunction::Sh(expressions)
-            }
-            "zsh" => {
-                let expressions = parse_expressions(source, node)?;
-
-                BuiltInFunction::Zsh(expressions)
-            }
-            "random" => {
-                let expressions = parse_expressions(source, node)?;
-
-                BuiltInFunction::Random(expressions)
-            }
-            "random_boolean" => BuiltInFunction::RandomBoolean,
-            "random_float" => BuiltInFunction::RandomFloat,
-            "random_integer" => BuiltInFunction::RandomInteger,
-            "columns" => {
-                let expression_node = node.child(2).unwrap();
-                let expression = Expression::from_syntax_node(source, expression_node)?;
-
-                BuiltInFunction::Columns(expression)
-            }
-            "rows" => {
-                let expression_node = node.child(2).unwrap();
-                let expression = Expression::from_syntax_node(source, expression_node)?;
-
-                BuiltInFunction::Rows(expression)
-            }
-            "reverse" => {
-                let list_node = node.child(2).unwrap();
-                let list_expression = Expression::from_syntax_node(source, list_node)?;
-
-                let slice_range_nodes =
-                    if let (Some(start_node), Some(end_node)) = (node.child(3), node.child(4)) {
-                        let start = Expression::from_syntax_node(source, start_node)?;
-                        let end = Expression::from_syntax_node(source, end_node)?;
-
-                        Some((start, end))
-                    } else {
-                        None
-                    };
-
-                BuiltInFunction::Reverse(list_expression, slice_range_nodes)
-            }
-            _ => {
-                return Err(Error::UnexpectedSyntaxNode {
-                    expected: "built-in function",
-                    actual: tool_node.kind(),
-                    location: tool_node.start_position(),
-                    relevant_source: source[tool_node.byte_range()].to_string(),
-                })
-            }
-        };
-
-        Ok(tool)
-    }
-
-    fn run(&self, source: &str, context: &mut Map) -> Result<Value> {
-        match self {
-            BuiltInFunction::Assert(expressions) => {
-                for expression in expressions {
-                    let value = expression.run(source, context)?;
-
-                    if value.as_boolean()? {
-                        continue;
-                    } else {
-                        return Err(Error::AssertFailed);
-                    }
-                }
-
-                Ok(Value::Empty)
-            }
-            BuiltInFunction::AssertEqual(expressions) => {
-                let mut prev_value = None;
-
-                for expression in expressions {
-                    let value = expression.run(source, context)?;
-
-                    if let Some(prev_value) = &prev_value {
-                        if &value == prev_value {
-                            continue;
-                        } else {
-                            return Err(Error::AssertEqualFailed {
-                                expected: prev_value.clone(),
-                                actual: value,
-                            });
-                        }
-                    }
-
-                    prev_value = Some(value);
-                }
-
-                Ok(Value::Empty)
-            }
-            BuiltInFunction::Context => Ok(Value::Map(context.clone())),
-            BuiltInFunction::Download(expression) => {
-                let value = expression.run(source, context)?;
-                let url = value.as_string()?;
-                let data = get(url)?.text()?;
-
-                Ok(Value::String(data))
-            }
-            BuiltInFunction::Length(expression) => {
-                let value = expression.run(source, context)?;
-                let length = match value {
-                    Value::List(list) => list.items().len(),
-                    Value::Map(map) => map.variables()?.len(),
-                    Value::Table(table) => table.len(),
-                    Value::String(string) => string.chars().count(),
-                    Value::Function(_) => todo!(),
-                    Value::Float(_) => todo!(),
-                    Value::Integer(_) => todo!(),
-                    Value::Boolean(_) => todo!(),
-                    Value::Empty => todo!(),
-                };
-
-                Ok(Value::Integer(length as i64))
-            }
-            BuiltInFunction::Help(_expression) => {
-                let mut help_table =
-                    Table::new(vec!["tool".to_string(), "description".to_string()]);
-
-                help_table.insert(vec![
-                    Value::String("help".to_string()),
-                    Value::String("Get info on tools.".to_string()),
-                ])?;
-
-                Ok(Value::Table(help_table))
-            }
-            BuiltInFunction::Output(expressions) => {
-                for expression in expressions {
-                    let value = expression.run(source, context)?;
-
-                    println!("{value}");
-                }
-
-                Ok(Value::Empty)
-            }
-            BuiltInFunction::OutputError(expressions) => {
-                for expression in expressions {
-                    let value = expression.run(source, context)?;
-
-                    eprintln!("{value}");
-                }
-
-                Ok(Value::Empty)
-            }
-            BuiltInFunction::Type(expression) => {
-                let run_expression = expression.run(source, context);
-                let value_type = if let Ok(value) = run_expression {
-                    value.r#type()
-                } else if let Err(Error::VariableIdentifierNotFound(_)) = run_expression {
-                    Type::Any
-                } else {
-                    return run_expression;
-                };
-
-                Ok(Value::String(value_type.to_string()))
-            }
-            BuiltInFunction::Workdir => {
-                let workdir = current_dir()?.to_string_lossy().to_string();
-
-                Ok(Value::String(workdir))
-            }
-            BuiltInFunction::Append(expressions) => {
-                let path_value = expressions[0].run(source, context)?;
-                let path = path_value.as_string()?;
-                let data = expressions[1].run(source, context)?.to_string();
-                let mut file = File::options().append(true).open(path)?;
-
-                file.write(data.as_bytes())?;
-
-                Ok(Value::Empty)
-            }
-            BuiltInFunction::Metadata(expression) => {
-                let path_value = expression.run(source, context)?;
-                let path = path_value.as_string()?;
-                let metadata = metadata(path)?;
-                let file_type = if metadata.is_dir() {
-                    "dir".to_string()
-                } else if metadata.is_file() {
-                    "file".to_string()
-                } else if metadata.is_symlink() {
-                    "link".to_string()
-                } else {
-                    "unknown".to_string()
-                };
-                let size = metadata.len() as i64;
-                let created = metadata.created()?.elapsed()?.as_secs() as i64;
-                let modified = metadata.modified()?.elapsed()?.as_secs() as i64;
-                let accessed = metadata.accessed()?.elapsed()?.as_secs() as i64;
-                let metadata_output = Map::new();
-
-                {
-                    let mut metadata_variables = metadata_output.variables_mut()?;
-
-                    metadata_variables.insert("type".to_string(), Value::String(file_type));
-                    metadata_variables.insert("size".to_string(), Value::Integer(size));
-                    metadata_variables.insert("created".to_string(), Value::Integer(created));
-                    metadata_variables.insert("modified".to_string(), Value::Integer(modified));
-                    metadata_variables.insert("accessed".to_string(), Value::Integer(accessed));
-                }
-
-                Ok(Value::Map(metadata_output))
-            }
-            BuiltInFunction::Move(expressions) => {
-                let from_value = expressions[0].run(source, context)?;
-                let from = from_value.as_string()?;
-                let to_value = expressions[1].run(source, context)?;
-                let to = to_value.as_string()?;
-
-                copy(from, to)?;
-                remove_file(from)?;
-
-                Ok(Value::Empty)
-            }
-            BuiltInFunction::Read(expression) => {
-                let path = if let Some(expression) = expression {
-                    let path_value = expression.run(source, context)?;
-
-                    PathBuf::from(path_value.as_string()?)
-                } else {
-                    PathBuf::from(".")
-                };
-                let content = if path.is_dir() {
-                    let dir = read_dir(&path)?;
-                    let mut contents = Vec::new();
-
-                    for file in dir {
-                        let file = file?;
-                        let file_path = file.path().to_string_lossy().to_string();
-
-                        contents.push(Value::String(file_path));
-                    }
-
-                    Value::List(List::with_items(contents))
-                } else {
-                    Value::String(read_to_string(path)?)
-                };
-
-                Ok(content)
-            }
-            BuiltInFunction::Remove(expression) => {
-                let path_value = expression.run(source, context)?;
-                let path = PathBuf::from(path_value.as_string()?);
-
-                remove_file(path)?;
-
-                Ok(Value::Empty)
-            }
-            BuiltInFunction::Write(expressions) => {
-                let path_value = expressions[0].run(source, context)?;
-                let path = path_value.as_string()?;
-                let data_value = expressions[1].run(source, context)?;
-                let data = data_value.as_string()?;
-
-                write(path, data)?;
-
-                Ok(Value::Empty)
-            }
-            BuiltInFunction::FromJson(expression) => {
-                let json_value = expression.run(source, context)?;
-                let json = json_value.as_string()?;
-                let value = serde_json::from_str(json)?;
-
-                Ok(value)
-            }
-            BuiltInFunction::ToJson(expression) => {
-                let value = expression.run(source, context)?;
-                let json = serde_json::to_string(&value)?;
-
-                Ok(Value::String(json))
-            }
-            BuiltInFunction::ToString(expression) => {
-                let value = expression.run(source, context)?;
-                let string = value.to_string();
-
-                Ok(Value::String(string))
-            }
-            BuiltInFunction::ToFloat(expression) => {
-                let value = expression.run(source, context)?;
-                let float = match value {
-                    Value::String(string) => string.parse()?,
-                    Value::Float(float) => float,
-                    Value::Integer(integer) => integer as f64,
-                    _ => return Err(Error::ExpectedNumberOrString { actual: value }),
-                };
-
-                Ok(Value::Float(float))
-            }
-            BuiltInFunction::Bash(expressions) => {
-                let mut command = Command::new("bash");
-
-                for expression in expressions {
-                    let value = expression.run(source, context)?;
-                    let command_input = value.as_string()?;
-
-                    command.arg(command_input);
-                }
-
-                let output = command.spawn()?.wait_with_output()?.stdout;
-
-                Ok(Value::String(String::from_utf8(output)?))
-            }
-            BuiltInFunction::Fish(expressions) => {
-                let mut command = Command::new("fish");
-
-                for expression in expressions {
-                    let value = expression.run(source, context)?;
-                    let command_input = value.as_string()?;
-
-                    command.arg(command_input);
-                }
-
-                let output = command.spawn()?.wait_with_output()?.stdout;
-
-                Ok(Value::String(String::from_utf8(output)?))
-            }
-            BuiltInFunction::Raw(expressions) => {
-                let raw_command = expressions[0].run(source, context)?;
-                let mut command = Command::new(raw_command.as_string()?);
-
-                for expression in &expressions[1..] {
-                    let value = expression.run(source, context)?;
-                    let command_input = value.as_string()?;
-
-                    command.arg(command_input);
-                }
-
-                let output = command.spawn()?.wait_with_output()?.stdout;
-
-                Ok(Value::String(String::from_utf8(output)?))
-            }
-            BuiltInFunction::Sh(expressions) => {
-                let mut command = Command::new("sh");
-
-                for expression in expressions {
-                    let value = expression.run(source, context)?;
-                    let command_input = value.as_string()?;
-
-                    command.arg(command_input);
-                }
-
-                let output = command.spawn()?.wait_with_output()?.stdout;
-
-                Ok(Value::String(String::from_utf8(output)?))
-            }
-            BuiltInFunction::Zsh(expressions) => {
-                let mut command = Command::new("zsh");
-
-                for expression in expressions {
-                    let value = expression.run(source, context)?;
-                    let command_input = value.as_string()?;
-
-                    command.arg(command_input);
-                }
-
-                let output = command.spawn()?.wait_with_output()?.stdout;
-
-                Ok(Value::String(String::from_utf8(output)?))
-            }
-            BuiltInFunction::Random(expressions) => {
-                if expressions.len() == 1 {
-                    let value = expressions[0].run(source, context)?;
-                    let list = value.as_list()?.items();
-
-                    if list.len() == 1 {
-                        return Ok(list.first().cloned().unwrap());
-                    }
-
-                    let range = 0..list.len();
-                    let random_index = thread_rng().gen_range(range);
-                    let random_value = list.get(random_index).ok_or(Error::ExpectedList {
-                        actual: value.clone(),
-                    })?;
-
-                    return Ok(random_value.clone());
-                }
-
-                let range = 0..expressions.len();
-                let random_index = thread_rng().gen_range(range);
-                let random_expression = expressions.get(random_index).unwrap();
-                let value = random_expression.run(source, context)?;
-
-                Ok(value)
-            }
-            BuiltInFunction::RandomBoolean => Ok(Value::Boolean(random())),
-            BuiltInFunction::RandomFloat => Ok(Value::Float(random())),
-            BuiltInFunction::RandomInteger => Ok(Value::Integer(random())),
-            BuiltInFunction::Columns(expression) => {
-                let column_names = expression
-                    .run(source, context)?
-                    .as_table()?
-                    .headers()
-                    .iter()
-                    .cloned()
-                    .map(Value::String)
-                    .collect();
-
-                Ok(Value::List(List::with_items(column_names)))
-            }
-            BuiltInFunction::Rows(expression) => {
-                let rows = expression
-                    .run(source, context)?
-                    .as_table()?
-                    .rows()
-                    .iter()
-                    .cloned()
-                    .map(|row| Value::List(List::with_items(row)))
-                    .collect();
-
-                Ok(Value::List(List::with_items(rows)))
-            }
-            BuiltInFunction::Reverse(list_expression, slice_range) => {
-                let expression_run = list_expression.run(source, context)?;
-                let list = expression_run.as_list()?;
-
-                if let Some((start, end)) = slice_range {
-                    let start = start.run(source, context)?.as_integer()? as usize;
-                    let end = end.run(source, context)?.as_integer()? as usize;
-
-                    list.items_mut()[start..end].reverse()
-                } else {
-                    list.items_mut().reverse()
-                };
-
-                Ok(Value::List(list.clone()))
-            }
-        }
-    }
-}

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

@@ -1,13 +1,17 @@
 use serde::{Deserialize, Serialize};
-use tree_sitter::Node;
 
 use crate::{
-    value_node::ValueNode, AbstractTree, BuiltInFunction, Error, Identifier, Index, Map, Result,
+    error::{RuntimeError, SyntaxError, ValidationError},
-    Value, Yield,
+    value_node::ValueNode,
+    AbstractTree, As, Command, Context, Format, FunctionCall, Identifier, Index, Logic, Math,
+    SyntaxNode, Type, Value,
 };
 
-use super::{function_call::FunctionCall, logic::Logic, math::Math};
+/// 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),
@@ -16,13 +20,17 @@ pub enum Expression {
     Math(Box<Math>),
     Logic(Box<Logic>),
     FunctionCall(Box<FunctionCall>),
-    Tool(Box<BuiltInFunction>),
+    Command(Command),
-    Yield(Box<Yield>),
+    As(Box<As>),
 }
 
 impl AbstractTree for Expression {
-    fn from_syntax_node(source: &str, node: Node) -> Result<Self> {
+    fn from_syntax(
-        Error::expect_syntax_node(source, "expression", node)?;
+        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()
@@ -31,22 +39,24 @@ impl AbstractTree for Expression {
         };
 
         let expression = match child.kind() {
-            "value" => Expression::Value(ValueNode::from_syntax_node(source, child)?),
+            "as" => Expression::As(Box::new(As::from_syntax(child, source, _context)?)),
-            "identifier" => Expression::Identifier(Identifier::from_syntax_node(source, child)?),
+            "value" => Expression::Value(ValueNode::from_syntax(child, source, _context)?),
-            "index" => Expression::Index(Box::new(Index::from_syntax_node(source, child)?)),
+            "identifier" => {
-            "math" => Expression::Math(Box::new(Math::from_syntax_node(source, child)?)),
+                Expression::Identifier(Identifier::from_syntax(child, source, _context)?)
-            "logic" => Expression::Logic(Box::new(Logic::from_syntax_node(source, child)?)),
-            "function_call" => {
-                Expression::FunctionCall(Box::new(FunctionCall::from_syntax_node(source, child)?))
             }
-            "tool" => Expression::Tool(Box::new(BuiltInFunction::from_syntax_node(source, child)?)),
+            "index" => Expression::Index(Box::new(Index::from_syntax(child, source, _context)?)),
-            "yield" => Expression::Yield(Box::new(Yield::from_syntax_node(source, child)?)),
+            "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(Error::UnexpectedSyntaxNode {
+                return Err(SyntaxError::UnexpectedSyntaxNode {
-                    expected: "value, identifier, index, math, logic, function_call or yield",
+                    expected: "value, identifier, index, math, logic, function call, as or command"
-                    actual: child.kind(),
+                        .to_string(),
-                    location: child.start_position(),
+                    actual: child.kind().to_string(),
-                    relevant_source: source[child.byte_range()].to_string(),
+                    position: node.range().into(),
                 })
             }
         };
@@ -54,16 +64,57 @@ impl AbstractTree for Expression {
         Ok(expression)
     }
 
-    fn run(&self, source: &str, context: &mut Map) -> Result<Value> {
+    fn expected_type(&self, _context: &Context) -> Result<Type, ValidationError> {
         match self {
-            Expression::Value(value_node) => value_node.run(source, context),
+            Expression::Value(value_node) => value_node.expected_type(_context),
-            Expression::Identifier(identifier) => identifier.run(source, context),
+            Expression::Identifier(identifier) => identifier.expected_type(_context),
-            Expression::Math(math) => math.run(source, context),
+            Expression::Math(math) => math.expected_type(_context),
-            Expression::Logic(logic) => logic.run(source, context),
+            Expression::Logic(logic) => logic.expected_type(_context),
-            Expression::FunctionCall(function_call) => function_call.run(source, context),
+            Expression::FunctionCall(function_call) => function_call.expected_type(_context),
-            Expression::Tool(tool) => tool.run(source, context),
+            Expression::Index(index) => index.expected_type(_context),
-            Expression::Index(index) => index.run(source, context),
+            Expression::Command(command) => command.expected_type(_context),
-            Expression::Yield(r#yield) => r#yield.run(source, 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/filter.rs

@@ -1,77 +0,0 @@
-use rayon::prelude::*;
-use serde::{Deserialize, Serialize};
-use tree_sitter::Node;
-
-use crate::{AbstractTree, Block, Error, Expression, Identifier, List, Map, Result, Value};
-
-#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
-pub struct Filter {
-    count: Option<Expression>,
-    item_id: Identifier,
-    collection: Expression,
-    predicate: Block,
-}
-
-impl AbstractTree for Filter {
-    fn from_syntax_node(source: &str, node: Node) -> Result<Self> {
-        let count = match node.child_by_field_name("count") {
-            Some(node) => Some(Expression::from_syntax_node(source, node)?),
-            None => None,
-        };
-
-        let item_id_node = node.child_by_field_name("item_id").unwrap();
-        let item_id = Identifier::from_syntax_node(source, item_id_node)?;
-
-        let collection_node = node.child_by_field_name("collection").unwrap();
-        let collection = Expression::from_syntax_node(source, collection_node)?;
-
-        let predicate_node = node.child_by_field_name("predicate").unwrap();
-        let predicate = Block::from_syntax_node(source, predicate_node)?;
-
-        Ok(Filter {
-            count,
-            item_id,
-            collection,
-            predicate,
-        })
-    }
-
-    fn run(&self, source: &str, context: &mut Map) -> Result<Value> {
-        let value = self.collection.run(source, context)?;
-        let values = value.as_list()?.items();
-        let key = self.item_id.inner();
-        let new_values = List::new();
-        let count = match &self.count {
-            Some(expression) => Some(expression.run(source, context)?.as_integer()? as usize),
-            None => None,
-        };
-        let loop_context = Map::clone_from(context)?;
-
-        values.par_iter().try_for_each(|value| {
-            if let Some(max) = count {
-                if new_values.items().len() == max {
-                    return Ok(());
-                }
-            }
-
-            let mut iter_context = loop_context.clone();
-
-            iter_context
-                .variables_mut()?
-                .insert(key.clone(), value.clone());
-
-            let should_include = self
-                .predicate
-                .run(source, &mut iter_context)?
-                .as_boolean()?;
-
-            if should_include {
-                new_values.items_mut().push(value.clone());
-            }
-
-            Ok::<(), Error>(())
-        })?;
-
-        Ok(Value::List(new_values))
-    }
-}

src/abstract_tree/find.rs

@@ -1,70 +0,0 @@
-use rayon::prelude::*;
-use serde::{Deserialize, Serialize};
-use tree_sitter::Node;
-
-use crate::{AbstractTree, Block, Error, Expression, Identifier, Map, Result, Value};
-
-#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
-pub struct Find {
-    identifier: Identifier,
-    expression: Expression,
-    item: Block,
-}
-
-impl AbstractTree for Find {
-    fn from_syntax_node(source: &str, node: Node) -> Result<Self> {
-        let identifier_node = node.child(1).unwrap();
-        let identifier = Identifier::from_syntax_node(source, identifier_node)?;
-
-        let expression_node = node.child(3).unwrap();
-        let expression = Expression::from_syntax_node(source, expression_node)?;
-
-        let item_node = node.child(4).unwrap();
-        let item = Block::from_syntax_node(source, item_node)?;
-
-        Ok(Find {
-            identifier,
-            expression,
-            item,
-        })
-    }
-
-    fn run(&self, source: &str, context: &mut Map) -> Result<Value> {
-        let expression_run = self.expression.run(source, context)?;
-        let list = expression_run.as_list()?.items();
-        let key = self.identifier.inner();
-
-        let find_result = list.par_iter().find_map_first(|value| {
-            let loop_context = Map::clone_from(context).unwrap();
-
-            loop_context
-                .variables_mut()
-                .unwrap()
-                .insert(key.clone(), (*value).clone());
-
-            let run_result = self.item.run(source, &mut loop_context.clone());
-
-            if let Ok(run_result_value) = run_result {
-                if let Ok(should_return) = run_result_value.as_boolean() {
-                    if should_return {
-                        Some(Ok(value.clone()))
-                    } else {
-                        None
-                    }
-                } else {
-                    Some(Err(Error::ExpectedBoolean {
-                        actual: value.clone(),
-                    }))
-                }
-            } else {
-                Some(run_result)
-            }
-        });
-
-        if let Some(result) = find_result {
-            result
-        } else {
-            Ok(Value::Empty)
-        }
-    }
-}

src/abstract_tree/for.rs

@@ -1,79 +1,153 @@
 use rayon::prelude::*;
 use serde::{Deserialize, Serialize};
-use tree_sitter::Node;
 
-use crate::{AbstractTree, Block, Error, Expression, Identifier, Map, Result, Value};
+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(source: &str, node: Node) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
-        Error::expect_syntax_node(source, "for", node)?;
+        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(Error::UnexpectedSyntaxNode {
+                return Err(SyntaxError::UnexpectedSyntaxNode {
-                    expected: "for or async for",
+                    expected: "for or async for".to_string(),
-                    actual: for_node.kind(),
+                    actual: for_node.kind().to_string(),
-                    location: for_node.start_position(),
+                    position: node.range().into(),
-                    relevant_source: source[for_node.byte_range()].to_string(),
                 })
             }
         };
 
         let identifier_node = node.child(1).unwrap();
-        let identifier = Identifier::from_syntax_node(source, identifier_node)?;
+        let identifier = Identifier::from_syntax(identifier_node, source, context)?;
 
         let expression_node = node.child(3).unwrap();
-        let expression = Expression::from_syntax_node(source, expression_node)?;
+        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_node(source, item_node)?;
+        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 run(&self, source: &str, context: &mut Map) -> Result<Value> {
+    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 values = expression_run.as_list()?.items();
+        let key = &self.item_id;
-        let key = self.item_id.inner();
 
+        if let Value::Range(range) = expression_run {
             if self.is_async {
-            values.par_iter().try_for_each(|value| {
+                range.into_par_iter().try_for_each(|integer| {
-                let mut iter_context = Map::clone_from(context)?;
+                    self.context.add_allowance(key)?;
-
+                    self.context
-                iter_context
+                        .set_value(key.clone(), Value::Integer(integer))?;
-                    .variables_mut()?
+                    self.block.run(source, &self.context).map(|_value| ())
-                    .insert(key.clone(), value.clone());
-
-                self.block.run(source, &mut iter_context).map(|_value| ())
                 })?;
             } else {
-            let loop_context = Map::clone_from(context)?;
+                for i in range {
-
+                    self.context.add_allowance(key)?;
-            for value in values.iter() {
+                    self.context.set_value(key.clone(), Value::Integer(i))?;
-                loop_context
+                    self.block.run(source, &self.context)?;
-                    .variables_mut()?
-                    .insert(key.clone(), value.clone());
-
-                self.block.run(source, &mut loop_context.clone())?;
                 }
             }
 
-        Ok(Value::Empty)
+            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

@@ -1,79 +1,202 @@
 use serde::{Deserialize, Serialize};
-use tree_sitter::Node;
 
-use crate::{AbstractTree, BuiltInFunction, Error, Map, Result, Value};
+use crate::{
-
+    built_in_functions::Callable,
-use super::expression::Expression;
+    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 enum FunctionCall {
+pub struct FunctionCall {
-    BuiltIn(Box<BuiltInFunction>),
+    function_expression: FunctionExpression,
-    ContextDefined {
-        name: Expression,
     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(source: &str, node: Node) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
-        debug_assert_eq!("function_call", node.kind());
+        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 function_node = node.child(1).unwrap();
         let mut arguments = Vec::new();
 
         for index in 2..node.child_count() - 1 {
-            let node = node.child(index).unwrap();
+            let child = node.child(index).unwrap();
 
-            if node.is_named() {
+            if child.is_named() {
-                let expression = Expression::from_syntax_node(source, node)?;
+                let expression = Expression::from_syntax(child, source, context)?;
 
                 arguments.push(expression);
             }
         }
 
-        let function_call = if function_node.kind() == "built_in_function" {
+        Ok(FunctionCall {
-            let function = BuiltInFunction::from_syntax_node(source, function_node)?;
+            function_expression,
-
+            arguments,
-            FunctionCall::BuiltIn(Box::new(function))
+            syntax_position: node.range().into(),
-        } else {
+        })
-            let name = Expression::from_syntax_node(source, function_node)?;
-
-            FunctionCall::ContextDefined { name, arguments }
-        };
-
-        Ok(function_call)
     }
 
-    fn run(&self, source: &str, context: &mut Map) -> Result<Value> {
+    fn expected_type(&self, context: &Context) -> Result<Type, ValidationError> {
-        let (name, arguments) = match self {
+        match &self.function_expression {
-            FunctionCall::BuiltIn(function) => return function.run(source, context),
+            FunctionExpression::Identifier(identifier) => {
-            FunctionCall::ContextDefined { name, arguments } => (name, arguments),
+                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,
+            });
         };
 
-        let function = if let Expression::Identifier(identifier) = name {
+        if self.arguments.len() != parameter_types.len() {
-            if let Some(value) = context.variables()?.get(identifier.inner()) {
+            return Err(ValidationError::ExpectedFunctionArgumentAmount {
-                value.as_function().cloned()
+                expected: parameter_types.len(),
-            } else {
+                actual: self.arguments.len(),
-                return Err(Error::FunctionIdentifierNotFound(identifier.clone()));
+                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 {
-            let name_run = name.run(source, context)?;
+                    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()?;
 
-            name_run.as_function().cloned()
+        match function {
-        }?;
+            Function::BuiltIn(built_in_function) => {
+                let mut arguments = Vec::with_capacity(self.arguments.len());
 
-        let mut function_context = Map::clone_from(context)?;
+                for expression in &self.arguments {
-        let parameter_expression_pairs = function.parameters().iter().zip(arguments.iter());
-
-        for ((identifier, r#type), expression) in parameter_expression_pairs {
-            let key = identifier.clone().take_inner();
                     let value = expression.run(source, context)?;
 
-            r#type.check(&value)?;
+                    arguments.push(value);
-
-            function_context.variables_mut()?.insert(key, value);
                 }
 
-        function.run(source, &mut function_context)
+                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

@@ -1,39 +1,167 @@
-use serde::{Deserialize, Serialize};
+use std::{
-use tree_sitter::Node;
+    fmt::{self, Display, Formatter},
+    sync::Arc,
+};
 
-use crate::{AbstractTree, Error, Map, Result, Value};
+use serde::{de::Visitor, Deserialize, Serialize};
 
-#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+use crate::{
-pub struct Identifier(String);
+    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(inner: String) -> Self {
+    pub fn new(key: &str) -> Self {
-        Identifier(inner)
+        for built_in_identifier in all_built_in_identifiers() {
+            let identifier = built_in_identifier.get();
+
+            if &key == identifier.inner().as_ref() {
+                return identifier.clone();
+            }
         }
 
-    pub fn take_inner(self) -> String {
+        Identifier(Arc::new(key.to_string()))
-        self.0
     }
 
-    pub fn inner(&self) -> &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(source: &str, node: Node) -> Result<Self> {
+    fn from_syntax(
-        Error::expect_syntax_node(source, "identifier", node)?;
+        node: SyntaxNode,
+        source: &str,
+        _context: &Context,
+    ) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("identifier", node)?;
 
-        let identifier = &source[node.byte_range()];
+        let text = &source[node.byte_range()];
 
-        Ok(Identifier(identifier.to_string()))
+        debug_assert!(!text.is_empty());
+
+        Ok(Identifier::new(text))
     }
 
-    fn run(&self, _source: &str, context: &mut Map) -> Result<Value> {
+    fn validate(&self, _source: &str, context: &Context) -> Result<(), ValidationError> {
-        if let Some(value) = context.variables()?.get(&self.0) {
+        let variable_exists = context.add_allowance(self)?;
-            Ok(value.clone())
+
+        if variable_exists {
+            Ok(())
         } else {
-            Err(Error::VariableIdentifierNotFound(self.inner().clone()))
+            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/identifier_assignment.rs

@@ -1,79 +0,0 @@
-use serde::{Deserialize, Serialize};
-use tree_sitter::Node;
-
-use crate::{AbstractTree, Error, Identifier, Map, Result, Statement, Value};
-
-#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
-pub struct IdentifierAssignment {
-    identifier: Identifier,
-    operator: AssignmentOperator,
-    statement: Statement,
-}
-
-#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
-pub enum AssignmentOperator {
-    Equal,
-    PlusEqual,
-    MinusEqual,
-}
-
-impl AbstractTree for IdentifierAssignment {
-    fn from_syntax_node(source: &str, node: Node) -> Result<Self> {
-        let identifier_node = node.child(0).unwrap();
-        let identifier = Identifier::from_syntax_node(source, identifier_node)?;
-
-        let operator_node = node.child(1).unwrap().child(0).unwrap();
-        let operator = match operator_node.kind() {
-            "=" => AssignmentOperator::Equal,
-            "+=" => AssignmentOperator::PlusEqual,
-            "-=" => AssignmentOperator::MinusEqual,
-            _ => {
-                return Err(Error::UnexpectedSyntaxNode {
-                    expected: "=, += or -=",
-                    actual: operator_node.kind(),
-                    location: operator_node.start_position(),
-                    relevant_source: source[operator_node.byte_range()].to_string(),
-                })
-            }
-        };
-
-        let statement_node = node.child(2).unwrap();
-        let statement = Statement::from_syntax_node(source, statement_node)?;
-
-        Ok(IdentifierAssignment {
-            identifier,
-            operator,
-            statement,
-        })
-    }
-
-    fn run(&self, source: &str, context: &mut Map) -> Result<Value> {
-        let key = self.identifier.inner().clone();
-        let value = self.statement.run(source, context)?;
-        let mut context = context.variables_mut();
-
-        let new_value = match self.operator {
-            AssignmentOperator::PlusEqual => {
-                if let Some(mut previous_value) = context.get(&key).cloned() {
-                    previous_value += value;
-                    previous_value
-                } else {
-                    Value::Empty
-                }
-            }
-            AssignmentOperator::MinusEqual => {
-                if let Some(mut previous_value) = context.get(&key).cloned() {
-                    previous_value -= value;
-                    previous_value
-                } else {
-                    Value::Empty
-                }
-            }
-            AssignmentOperator::Equal => value,
-        };
-
-        context.insert(key, new_value);
-
-        Ok(Value::Empty)
-    }
-}

src/abstract_tree/if_else.rs

@@ -1,7 +1,9 @@
 use serde::{Deserialize, Serialize};
-use tree_sitter::Node;
 
-use crate::{AbstractTree, Block, Expression, Map, Result, Value};
+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 {
@@ -10,15 +12,16 @@ pub struct IfElse {
     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(source: &str, node: Node) -> Result<Self> {
+    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_node(source, if_expression_node)?;
+        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_node(source, if_block_node)?;
+        let if_block = Block::from_syntax(if_block_node, source, context)?;
 
         let child_count = node.child_count();
         let mut else_if_expressions = Vec::new();
@@ -30,19 +33,19 @@ impl AbstractTree for IfElse {
 
             if child.kind() == "else_if" {
                 let expression_node = child.child(1).unwrap();
-                let expression = Expression::from_syntax_node(source, expression_node)?;
+                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_node(source, block_node)?;
+                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_node(source, else_node)?);
+                else_block = Some(Block::from_syntax(else_node, source, context)?);
             }
         }
 
@@ -52,23 +55,71 @@ impl AbstractTree for IfElse {
             else_if_expressions,
             else_if_blocks,
             else_block,
+            source_position: SourcePosition::from(node.range()),
         })
     }
 
-    fn run(&self, source: &str, context: &mut Map) -> Result<Value> {
+    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 expressions = &self.else_if_expressions;
+            let else_ifs = self
+                .else_if_expressions
+                .iter()
+                .zip(self.else_if_blocks.iter());
 
-            for (index, expression) in expressions.iter().enumerate() {
+            for (expression, block) in else_ifs {
                 let if_boolean = expression.run(source, context)?.as_boolean()?;
 
                 if if_boolean {
-                    let block = self.else_if_blocks.get(index).unwrap();
-
                     return block.run(source, context);
                 }
             }
@@ -76,8 +127,34 @@ impl AbstractTree for IfElse {
             if let Some(block) = &self.else_block {
                 block.run(source, context)
             } else {
-                Ok(Value::Empty)
+                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

@@ -1,103 +1,134 @@
 use serde::{Deserialize, Serialize};
-use tree_sitter::Node;
 
-use crate::{AbstractTree, Error, Expression, List, Map, Result, Value};
+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: Expression,
+    pub collection: IndexExpression,
-    pub index: Expression,
+    pub index: IndexExpression,
-    pub index_end: Option<Expression>,
+    source_position: SourcePosition,
 }
 
 impl AbstractTree for Index {
-    fn from_syntax_node(source: &str, node: Node) -> Result<Self> {
+    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 = Expression::from_syntax_node(source, collection_node)?;
+        let collection = IndexExpression::from_syntax(collection_node, source, context)?;
 
         let index_node = node.child(2).unwrap();
-        let index = Expression::from_syntax_node(source, index_node)?;
+        let index = IndexExpression::from_syntax(index_node, source, context)?;
-
-        let index_end_node = node.child(4);
-        let index_end = if let Some(index_end_node) = index_end_node {
-            Some(Expression::from_syntax_node(source, index_end_node)?)
-        } else {
-            None
-        };
 
         Ok(Index {
             collection,
             index,
-            index_end,
+            source_position: SourcePosition::from(node.range()),
         })
     }
 
-    fn run(&self, source: &str, context: &mut Map) -> Result<Value> {
+    fn expected_type(&self, context: &Context) -> Result<Type, ValidationError> {
-        let collection = self.collection.run(source, context)?;
+        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),
+        }
+    }
 
-        match collection {
+    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();
-                let item = if let Some(index_end) = &self.index_end {
-                    let index_end = index_end.run(source, context)?.as_integer()? as usize;
-                    let sublist = list.items()[index..=index_end].to_vec();
-
-                    Value::List(List::with_items(sublist))
-                } else {
-                    list.items().get(index).cloned().unwrap_or_default()
-                };
 
                 Ok(item)
             }
             Value::Map(map) => {
-                let value = if let Expression::Identifier(identifier) = &self.index {
+                let map = map.inner();
-                    let key = identifier.inner();
 
-                    map.variables()?.get(key).cloned().unwrap_or(Value::Empty)
+                let value = if let IndexExpression::Identifier(identifier) = &self.index {
+                    if let Some(value) = map.get(identifier) {
+                        value
                     } else {
-                    let value = self.index.run(source, context)?;
+                        return Err(RuntimeError::ValidationFailure(
-                    let key = value.as_string()?;
+                            ValidationError::VariableIdentifierNotFound(identifier.clone()),
+                        ));
+                    }
+                } else {
+                    let index_value = self.index.run(source, context)?;
+                    let identifier = Identifier::new(index_value.as_string()?);
 
-                    map.variables()?.get(key).cloned().unwrap_or(Value::Empty)
+                    if let Some(value) = map.get(&identifier) {
+                        value
+                    } else {
+                        return Err(RuntimeError::ValidationFailure(
+                            ValidationError::VariableIdentifierNotFound(identifier.clone()),
+                        ));
+                    }
                 };
 
-                Ok(value)
+                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()))
+                Ok(Value::string(item.to_string()))
             }
-            _ => Err(Error::ExpectedCollection { actual: collection }),
+            _ => Err(RuntimeError::ValidationFailure(
+                ValidationError::ExpectedCollection { actual: value },
+            )),
         }
     }
 }
 
-#[cfg(test)]
+impl Format for Index {
-mod tests {
+    fn format(&self, output: &mut String, indent_level: u8) {
-    use super::*;
+        self.collection.format(output, indent_level);
-    use crate::evaluate;
+        output.push(':');
-
+        self.index.format(output, indent_level);
-    #[test]
-    fn evaluate_list_index() {
-        let test = evaluate("x = [1 [2] 3] x:1:0").unwrap();
-
-        assert_eq!(Value::Integer(2), test);
-    }
-
-    #[test]
-    fn evaluate_map_index() {
-        let test = evaluate("x = {y = {z = 2}} x:y:z").unwrap();
-
-        assert_eq!(Value::Integer(2), test);
-    }
-
-    #[test]
-    fn evaluate_complex_index() {
-        let test = evaluate("x = [1 2 3]; y = || <int> { 0 } x:((y))").unwrap();
-
-        assert_eq!(Value::Integer(1), test);
     }
 }

src/abstract_tree/index_assignment.rs

@@ -1,93 +1,96 @@
 use serde::{Deserialize, Serialize};
-use tree_sitter::Node;
 
-use crate::{AbstractTree, Error, Index, Map, Result, Statement, Value};
+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,
-
-#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
-pub enum AssignmentOperator {
-    Equal,
-    PlusEqual,
-    MinusEqual,
 }
 
 impl AbstractTree for IndexAssignment {
-    fn from_syntax_node(source: &str, node: Node) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
-        Error::expect_syntax_node(source, "index_assignment", node)?;
+        SyntaxError::expect_syntax_node("index_assignment", node)?;
 
         let index_node = node.child(0).unwrap();
-        let index = Index::from_syntax_node(source, index_node)?;
+        let index = Index::from_syntax(index_node, source, context)?;
 
-        let operator_node = node.child(1).unwrap().child(0).unwrap();
+        let operator_node = node.child(1).unwrap();
-        let operator = match operator_node.kind() {
+        let operator = AssignmentOperator::from_syntax(operator_node, source, context)?;
-            "=" => AssignmentOperator::Equal,
-            "+=" => AssignmentOperator::PlusEqual,
-            "-=" => AssignmentOperator::MinusEqual,
-            _ => {
-                return Err(Error::UnexpectedSyntaxNode {
-                    expected: "=, += or -=",
-                    actual: operator_node.kind(),
-                    location: operator_node.start_position(),
-                    relevant_source: source[operator_node.byte_range()].to_string(),
-                })
-            }
-        };
 
         let statement_node = node.child(2).unwrap();
-        let statement = Statement::from_syntax_node(source, statement_node)?;
+        let statement = Statement::from_syntax(statement_node, source, context)?;
 
         Ok(IndexAssignment {
             index,
             operator,
             statement,
+            position: node.range().into(),
         })
     }
 
-    fn run(&self, source: &str, context: &mut Map) -> Result<Value> {
+    fn expected_type(&self, _context: &Context) -> Result<Type, ValidationError> {
-        let index_collection = self.index.collection.run(source, context)?;
+        Ok(Type::None)
-        let index_context = index_collection.as_map().unwrap_or(&context);
+    }
-        let index_key = if let crate::Expression::Identifier(identifier) = &self.index.index {
+
-            identifier.inner()
+    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 {
-            return Err(Error::VariableIdentifierNotFound(
+            let index_run = self.index.index.run(source, context)?;
-                self.index.index.run(source, context)?.to_string(),
+            let expected_identifier = Identifier::new(index_run.as_string()?);
+
+            return Err(RuntimeError::ValidationFailure(
+                ValidationError::VariableIdentifierNotFound(expected_identifier),
             ));
         };
 
-        let value = self.statement.run(source, &mut context.clone())?;
+        let value = self.statement.run(source, context)?;
 
         let new_value = match self.operator {
             AssignmentOperator::PlusEqual => {
-                if let Some(mut previous_value) = index_context.variables()?.get(index_key).cloned()
+                if let Some(previous_value) = context.get_value(index_identifier)? {
-                {
+                    previous_value.add(value, self.position)?
-                    previous_value += value;
-                    previous_value
                 } else {
-                    Value::Empty
+                    return Err(RuntimeError::ValidationFailure(
+                        ValidationError::VariableIdentifierNotFound(index_identifier.clone()),
+                    ));
                 }
             }
             AssignmentOperator::MinusEqual => {
-                if let Some(mut previous_value) = index_context.variables()?.get(index_key).cloned()
+                if let Some(previous_value) = context.get_value(index_identifier)? {
-                {
+                    previous_value.subtract(value, self.position)?
-                    previous_value -= value;
-                    previous_value
                 } else {
-                    Value::Empty
+                    Value::none()
                 }
             }
             AssignmentOperator::Equal => value,
         };
 
-        index_context
+        context.set_value(index_identifier.clone(), new_value)?;
-            .variables_mut()?
-            .insert(index_key.clone(), new_value);
 
-        Ok(Value::Empty)
+        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/insert.rs

@@ -1,44 +0,0 @@
-use serde::{Deserialize, Serialize};
-use tree_sitter::Node;
-
-use crate::{AbstractTree, Expression, Identifier, Map, Result, Value};
-
-#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
-pub struct Insert {
-    identifier: Identifier,
-    expression: Expression,
-}
-
-impl AbstractTree for Insert {
-    fn from_syntax_node(source: &str, node: Node) -> Result<Self> {
-        let identifier_node = node.child(2).unwrap();
-        let identifier = Identifier::from_syntax_node(source, identifier_node)?;
-        let expression_node = node.child(3).unwrap();
-        let expression = Expression::from_syntax_node(source, expression_node)?;
-
-        Ok(Insert {
-            identifier,
-            expression,
-        })
-    }
-
-    fn run(&self, source: &str, context: &mut Map) -> Result<Value> {
-        let table_name = self.identifier.inner().clone();
-        let mut table = self.identifier.run(source, context)?.as_table()?.clone();
-        let new_rows = self.expression.run(source, context)?.into_inner_list()?;
-        let values = new_rows.items();
-
-        table.reserve(values.len());
-
-        for row in values.iter() {
-            let row_values = row.clone().into_inner_list()?;
-            table.insert(row_values.items().clone())?;
-        }
-
-        context
-            .variables_mut()?
-            .insert(table_name, Value::Table(table));
-
-        Ok(Value::Empty)
-    }
-}

src/abstract_tree/logic.rs

@@ -1,8 +1,11 @@
 use serde::{Deserialize, Serialize};
-use tree_sitter::Node;
 
-use crate::{AbstractTree, Error, Expression, Map, Result, Value};
+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,
@@ -11,32 +14,24 @@ pub struct Logic {
 }
 
 impl AbstractTree for Logic {
-    fn from_syntax_node(source: &str, node: Node) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
-        let left_node = node.child(0).unwrap();
+        SyntaxError::expect_syntax_node("logic", node)?;
-        let left = Expression::from_syntax_node(source, left_node)?;
 
-        let operator_node = node.child(1).unwrap().child(0).unwrap();
+        let first_node = node.child(0).unwrap();
-        let operator = match operator_node.kind() {
+        let (left_node, operator_node, right_node) = {
-            "==" => LogicOperator::Equal,
+            if first_node.is_named() {
-            "!=" => LogicOperator::NotEqual,
+                (first_node, node.child(1).unwrap(), node.child(2).unwrap())
-            "&&" => LogicOperator::And,
+            } else {
-            "||" => LogicOperator::Or,
+                (
-            ">" => LogicOperator::Greater,
+                    node.child(1).unwrap(),
-            "<" => LogicOperator::Less,
+                    node.child(2).unwrap(),
-            ">=" => LogicOperator::GreaterOrEqual,
+                    node.child(3).unwrap(),
-            "<=" => LogicOperator::LessOrEqaul,
+                )
-            _ => {
-                return Err(Error::UnexpectedSyntaxNode {
-                    expected: "==, !=, &&, ||, >, <, >= or <=",
-                    actual: operator_node.kind(),
-                    location: operator_node.start_position(),
-                    relevant_source: source[operator_node.byte_range()].to_string(),
-                })
             }
         };
-
+        let left = Expression::from_syntax(left_node, source, context)?;
-        let right_node = node.child(2).unwrap();
+        let operator = LogicOperator::from_syntax(operator_node, source, context)?;
-        let right = Expression::from_syntax_node(source, right_node)?;
+        let right = Expression::from_syntax(right_node, source, context)?;
 
         Ok(Logic {
             left,
@@ -45,9 +40,23 @@ impl AbstractTree for Logic {
         })
     }
 
-    fn run(&self, source: &str, context: &mut Map) -> Result<Value> {
+    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()) {
@@ -68,21 +77,19 @@ impl AbstractTree for Logic {
             LogicOperator::Greater => left > right,
             LogicOperator::Less => left < right,
             LogicOperator::GreaterOrEqual => left >= right,
-            LogicOperator::LessOrEqaul => left <= right,
+            LogicOperator::LessOrEqual => left <= right,
         };
 
         Ok(Value::Boolean(result))
     }
 }
 
-#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+impl Format for Logic {
-pub enum LogicOperator {
+    fn format(&self, output: &mut String, indent_level: u8) {
-    Equal,
+        self.left.format(output, indent_level);
-    NotEqual,
+        output.push(' ');
-    And,
+        self.operator.format(output, indent_level);
-    Or,
+        output.push(' ');
-    Greater,
+        self.right.format(output, indent_level);
-    Less,
+    }
-    GreaterOrEqual,
-    LessOrEqaul,
 }

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

@@ -2,21 +2,144 @@
 //!
 //! 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;
+use tree_sitter::Node as SyntaxNode;
 
-use crate::{AbstractTree, Map, Result, Value};
+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 {}
+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(_source: &str, _node: Node) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
-        todo!()
+        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)?);
             }
 
-    fn run(&self, _source: &str, _context: &mut Map) -> Result<Value> {
+            if child.kind() == "statement" {
-        todo!()
+                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

@@ -1,67 +1,74 @@
 use serde::{Deserialize, Serialize};
-use tree_sitter::Node;
 
-use crate::{AbstractTree, Error, Expression, Map, Result, Value};
+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(source: &str, node: Node) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
-        let left_node = node.child(0).unwrap();
+        SyntaxError::expect_syntax_node("math", node)?;
-        let left = Expression::from_syntax_node(source, left_node)?;
 
-        let operator_node = node.child(1).unwrap().child(0).unwrap();
+        let left_node = node.child(0).unwrap();
-        let operator = match operator_node.kind() {
+        let left = Expression::from_syntax(left_node, source, context)?;
-            "+" => MathOperator::Add,
+
-            "-" => MathOperator::Subtract,
+        let operator_node = node.child(1).unwrap();
-            "*" => MathOperator::Multiply,
+        let operator = MathOperator::from_syntax(operator_node, source, context)?;
-            "/" => MathOperator::Divide,
-            "%" => MathOperator::Modulo,
-            _ => {
-                return Err(Error::UnexpectedSyntaxNode {
-                    expected: "+, -, *, / or %",
-                    actual: operator_node.kind(),
-                    location: operator_node.start_position(),
-                    relevant_source: source[operator_node.byte_range()].to_string(),
-                })
-            }
-        };
 
         let right_node = node.child(2).unwrap();
-        let right = Expression::from_syntax_node(source, right_node)?;
+        let right = Expression::from_syntax(right_node, source, context)?;
 
         Ok(Math {
             left,
             operator,
             right,
+            position: node.range().into(),
         })
     }
 
-    fn run(&self, source: &str, context: &mut Map) -> Result<Value> {
+    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 + right,
+            MathOperator::Add => left.add(right, self.position)?,
-            MathOperator::Subtract => left - right,
+            MathOperator::Subtract => left.subtract(right, self.position)?,
-            MathOperator::Multiply => left * right,
+            MathOperator::Multiply => left.multiply(right, self.position)?,
-            MathOperator::Divide => left / right,
+            MathOperator::Divide => left.divide(right, self.position)?,
-            MathOperator::Modulo => left % right,
+            MathOperator::Modulo => left.modulo(right, self.position)?,
-        }?;
+        };
 
         Ok(value)
     }
 }
 
-#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+impl Format for Math {
-pub enum MathOperator {
+    fn format(&self, output: &mut String, indent_level: u8) {
-    Add,
+        self.left.format(output, indent_level);
-    Subtract,
+        output.push(' ');
-    Multiply,
+        self.operator.format(output, indent_level);
-    Divide,
+        output.push(' ');
-    Modulo,
+        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

@@ -1,52 +1,149 @@
 //! Abstract, executable representations of corresponding items found in Dust
 //! source code. The types that implement [AbstractTree] are inteded to be
-//! created by an [Evaluator].
+//! created by an [Interpreter].
-//!
+pub mod r#as;
-//! When adding new lanugage features, first extend the grammar to recognize new
-//! syntax nodes. Then add a new AbstractTree type using the existing types as
-//! examples.
-
 pub mod assignment;
+pub mod assignment_operator;
 pub mod block;
-pub mod built_in_function;
+pub mod command;
+pub mod enum_defintion;
+pub mod enum_pattern;
 pub mod expression;
-pub mod filter;
-pub mod find;
 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 insert;
+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 remove;
+pub mod math_operator;
-pub mod select;
 pub mod statement;
-pub mod transform;
+pub mod struct_definition;
 pub mod r#type;
-pub mod r#use;
+pub mod type_definition;
+pub mod type_specification;
 pub mod value_node;
 pub mod r#while;
-pub mod r#yield;
 
 pub use {
-    assignment::*, block::*, built_in_function::*, expression::*, filter::*, find::*,
+    assignment::*, assignment_operator::*, block::*, command::*, enum_defintion::*,
-    function_call::*, identifier::*, if_else::*, index::*, index_assignment::IndexAssignment,
+    enum_pattern::*, expression::*, function_call::*, function_expression::*, function_node::*,
-    insert::*, logic::*, math::*, r#for::*, r#match::*, r#type::*, r#use::*, r#while::*,
+    identifier::*, if_else::*, index::*, index_assignment::IndexAssignment, index_expression::*,
-    r#yield::*, remove::*, select::*, statement::*, transform::*, value_node::*,
+    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 tree_sitter::Node;
+use serde::{Deserialize, Serialize};
 
-use crate::{Map, Result, Value};
+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 {
+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
@@ -55,8 +152,27 @@ pub trait AbstractTree: Sized {
     ///
     /// If necessary, the source code can be accessed directly by getting the
     /// node's byte range.
-    fn from_syntax_node(source: &str, node: Node) -> Result<Self>;
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError>;
 
-    /// Execute dust code by traversing the tree.
+    /// Return the type of the value that this abstract node will create when
-    fn run(&self, source: &str, context: &mut Map) -> Result<Value>;
+    /// 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/remove.rs

@@ -1,75 +0,0 @@
-use std::sync::RwLock;
-
-use rayon::prelude::*;
-use serde::{Deserialize, Serialize};
-use tree_sitter::Node;
-
-use crate::{AbstractTree, Block, Error, Expression, Identifier, Map, Result, Value};
-
-#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
-pub struct Remove {
-    item_id: Identifier,
-    collection: Expression,
-    predicate: Block,
-}
-
-impl AbstractTree for Remove {
-    fn from_syntax_node(source: &str, node: Node) -> Result<Self> {
-        let identifier_node = node.child(1).unwrap();
-        let item_id = Identifier::from_syntax_node(source, identifier_node)?;
-
-        let expression_node = node.child(3).unwrap();
-        let collection = Expression::from_syntax_node(source, expression_node)?;
-
-        let block_node = node.child(4).unwrap();
-        let predicate = Block::from_syntax_node(source, block_node)?;
-
-        Ok(Remove {
-            item_id,
-            collection,
-            predicate,
-        })
-    }
-
-    fn run(&self, source: &str, context: &mut Map) -> Result<Value> {
-        let value = self.collection.run(source, context)?;
-        let values = value.as_list()?;
-        let key = self.item_id.inner();
-        let should_remove_index = RwLock::new(None);
-
-        values
-            .items()
-            .par_iter()
-            .enumerate()
-            .try_for_each(|(index, value)| {
-                if should_remove_index.read()?.is_some() {
-                    return Ok(());
-                }
-
-                let iter_context = Map::clone_from(context)?;
-
-                iter_context
-                    .variables_mut()?
-                    .insert(key.clone(), value.clone());
-
-                let should_remove = self
-                    .predicate
-                    .run(source, &mut iter_context.clone())?
-                    .as_boolean()?;
-
-                if should_remove {
-                    let _ = should_remove_index.write()?.insert(index);
-                }
-
-                Ok::<(), Error>(())
-            })?;
-
-        let index = should_remove_index.read()?;
-
-        if let Some(index) = *index {
-            Ok(values.items_mut().remove(index))
-        } else {
-            Ok(Value::Empty)
-        }
-    }
-}

src/abstract_tree/select.rs

@@ -1,108 +0,0 @@
-use serde::{Deserialize, Serialize};
-use tree_sitter::Node;
-
-use crate::{AbstractTree, Block, Expression, Identifier, Map, Result, Table, Value};
-
-#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
-pub struct Select {
-    identifiers: Vec<Identifier>,
-    expression: Expression,
-    predicate: Option<Block>,
-}
-
-impl AbstractTree for Select {
-    fn from_syntax_node(source: &str, node: Node) -> Result<Self> {
-        let child_count = node.child_count();
-        let mut identifiers = Vec::new();
-
-        let identifier_list = node.child(1).unwrap();
-
-        for index in 1..identifier_list.child_count() - 1 {
-            let node = identifier_list.child(index).unwrap();
-
-            if node.is_named() {
-                let identifier = Identifier::from_syntax_node(source, node)?;
-                identifiers.push(identifier);
-            }
-        }
-
-        let expression_node = node.child(3).unwrap();
-        let expression = Expression::from_syntax_node(source, expression_node)?;
-
-        let final_node = node.child(child_count - 1).unwrap();
-
-        let predicate = if final_node.kind() == "block" {
-            Some(Block::from_syntax_node(source, final_node)?)
-        } else {
-            None
-        };
-
-        Ok(Select {
-            identifiers,
-            expression,
-            predicate,
-        })
-    }
-
-    fn run(&self, source: &str, context: &mut Map) -> Result<Value> {
-        let value = self.expression.run(source, context)?;
-        let old_table = value.as_table()?;
-        let column_names = if !self.identifiers.is_empty() {
-            self.identifiers
-                .iter()
-                .cloned()
-                .map(|identifier| identifier.take_inner())
-                .collect()
-        } else {
-            old_table.headers().clone()
-        };
-        let mut new_table = Table::new(column_names.to_vec());
-
-        for row in old_table.rows() {
-            let mut new_row = Vec::new();
-            let row_context = Map::new();
-
-            for (i, value) in row.iter().enumerate() {
-                let column_name = old_table.headers().get(i).unwrap();
-
-                row_context
-                    .variables_mut()?
-                    .insert(column_name.clone(), value.clone());
-
-                let new_table_column_index =
-                    new_table
-                        .headers()
-                        .iter()
-                        .enumerate()
-                        .find_map(|(index, new_column_name)| {
-                            if new_column_name == column_name {
-                                Some(index)
-                            } else {
-                                None
-                            }
-                        });
-
-                if let Some(index) = new_table_column_index {
-                    while new_row.len() < index + 1 {
-                        new_row.push(Value::Empty);
-                    }
-                    new_row[index] = value.clone();
-                }
-            }
-
-            if let Some(where_clause) = &self.predicate {
-                let should_include = where_clause
-                    .run(source, &mut row_context.clone())?
-                    .as_boolean()?;
-
-                if should_include {
-                    new_table.insert(new_row)?;
-                }
-            } else {
-                new_table.insert(new_row)?;
-            }
-        }
-
-        Ok(Value::Table(new_table))
-    }
-}

src/abstract_tree/statement.rs

@@ -1,114 +1,202 @@
 use serde::{Deserialize, Serialize};
-use tree_sitter::Node;
 
 use crate::{
-    AbstractTree, Assignment, Block, Error, Expression, Filter, Find, For, IfElse, IndexAssignment,
+    error::{RuntimeError, SyntaxError, ValidationError},
-    Insert, Map, Match, Remove, Result, Select, Transform, Use, Value, While,
+    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 enum Statement {
+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>),
-    Return(Expression),
     Expression(Expression),
     IfElse(Box<IfElse>),
     Match(Match),
     While(Box<While>),
     Block(Box<Block>),
     For(Box<For>),
-    Transform(Box<Transform>),
-    Filter(Box<Filter>),
-    Find(Box<Find>),
-    Remove(Box<Remove>),
-    Use(Use),
-    Select(Box<Select>),
-    Insert(Box<Insert>),
     IndexAssignment(Box<IndexAssignment>),
+    TypeDefinition(TypeDefinition),
 }
 
-impl AbstractTree for Statement {
+impl AbstractTree for StatementKind {
-    fn from_syntax_node(source: &str, node: Node) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
-        Error::expect_syntax_node(source, "statement", node)?;
+        SyntaxError::expect_syntax_node("statement_kind", node)?;
 
         let child = node.child(0).unwrap();
 
         match child.kind() {
-            "assignment" => Ok(Statement::Assignment(Box::new(
+            "assignment" => Ok(StatementKind::Assignment(Box::new(
-                Assignment::from_syntax_node(source, child)?,
+                Assignment::from_syntax(child, source, context)?,
             ))),
-            "return" => {
+            "expression" => Ok(StatementKind::Expression(Expression::from_syntax(
-                let expression_node = child.child(1).unwrap();
+                child, source, context,
-
-                Ok(Statement::Return(Expression::from_syntax_node(source, expression_node)?))
-            },
-            "expression" => Ok(Self::Expression(Expression::from_syntax_node(
-                source, child,
             )?)),
-            "if_else" => Ok(Statement::IfElse(Box::new(IfElse::from_syntax_node(
+            "if_else" => Ok(StatementKind::IfElse(Box::new(IfElse::from_syntax(
-                source, child,
+                child, source, context,
             )?))),
-            "tool" => Ok(Statement::IfElse(Box::new(IfElse::from_syntax_node(
+            "while" => Ok(StatementKind::While(Box::new(While::from_syntax(
-                source, child,
+                child, source, context,
             )?))),
-            "while" => Ok(Statement::While(Box::new(While::from_syntax_node(
+            "block" => Ok(StatementKind::Block(Box::new(Block::from_syntax(
-                source, child,
+                child, source, context,
             )?))),
-            "block" => Ok(Statement::Block(Box::new(Block::from_syntax_node(
+            "for" => Ok(StatementKind::For(Box::new(For::from_syntax(
-                source, child,
+                child, source, context,
             )?))),
-            "for" => Ok(Statement::For(Box::new(For::from_syntax_node(
+            "index_assignment" => Ok(StatementKind::IndexAssignment(Box::new(
-                source, child,
+                IndexAssignment::from_syntax(child, source, context)?,
-            )?))),
+            ))),
-            "transform" => Ok(Statement::Transform(Box::new(Transform::from_syntax_node(
+            "match" => Ok(StatementKind::Match(Match::from_syntax(
-                source, child,
+                child, source, context,
-            )?))),
+            )?)),
-            "filter" => Ok(Statement::Filter(Box::new(Filter::from_syntax_node(
+            "type_definition" => Ok(StatementKind::TypeDefinition(TypeDefinition::from_syntax(
-                source, child,
+                child, source, context
-            )?))),
+            )?)),
-            "find" => Ok(Statement::Find(Box::new(Find::from_syntax_node(
+            _ => Err(SyntaxError::UnexpectedSyntaxNode {
-                source, child,
+                expected:
-            )?))),
+                    "assignment, index assignment, expression, type_definition, block, return, if...else, while, for or match".to_string(),
-            "remove" => Ok(Statement::Remove(Box::new(Remove::from_syntax_node(
+                actual: child.kind().to_string(),
-                source, child,
+                position: node.range().into(),
-            )?))),
-            "select" => Ok(Statement::Select(Box::new(Select::from_syntax_node(
-                source, child,
-            )?))),
-            "use" => Ok(Statement::Use(Use::from_syntax_node(source, child)?)),
-            "insert" => Ok(Statement::Insert(Box::new(Insert::from_syntax_node(
-                source, child,
-            )?))),
-            "index_assignment" => Ok(Statement::IndexAssignment(Box::new(IndexAssignment::from_syntax_node(
-                source, child,
-            )?))),
-            _ => Err(Error::UnexpectedSyntaxNode {
-                expected: "assignment, expression, if...else, while, for, transform, filter, tool, async, find, remove, select, insert, index_assignment or yield",
-                actual: child.kind(),
-                location: child.start_position(),
-                relevant_source: source[child.byte_range()].to_string(),
             }),
         }
     }
 
-    fn run(&self, source: &str, context: &mut Map) -> Result<Value> {
+    fn expected_type(&self, _context: &Context) -> Result<Type, ValidationError> {
         match self {
-            Statement::Assignment(assignment) => assignment.run(source, context),
+            StatementKind::Assignment(assignment) => assignment.expected_type(_context),
-            Statement::Return(expression) => expression.run(source, context),
+            StatementKind::Expression(expression) => expression.expected_type(_context),
-            Statement::Expression(expression) => expression.run(source, context),
+            StatementKind::IfElse(if_else) => if_else.expected_type(_context),
-            Statement::IfElse(if_else) => if_else.run(source, context),
+            StatementKind::Match(r#match) => r#match.expected_type(_context),
-            Statement::Match(r#match) => r#match.run(source, context),
+            StatementKind::While(r#while) => r#while.expected_type(_context),
-            Statement::While(r#while) => r#while.run(source, context),
+            StatementKind::Block(block) => block.expected_type(_context),
-            Statement::Block(block) => block.run(source, context),
+            StatementKind::For(r#for) => r#for.expected_type(_context),
-            Statement::For(r#for) => r#for.run(source, context),
+            StatementKind::IndexAssignment(index_assignment) => {
-            Statement::Transform(transform) => transform.run(source, context),
+                index_assignment.expected_type(_context)
-            Statement::Filter(filter) => filter.run(source, context),
+            }
-            Statement::Find(find) => find.run(source, context),
+            StatementKind::TypeDefinition(type_definition) => {
-            Statement::Remove(remove) => remove.run(source, context),
+                type_definition.expected_type(_context)
-            Statement::Use(run) => run.run(source, context),
+            }
-            Statement::Select(select) => select.run(source, context),
+        }
-            Statement::Insert(insert) => insert.run(source, context),
+    }
-            Statement::IndexAssignment(index_assignment) => index_assignment.run(source, 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/transform.rs

@@ -1,55 +0,0 @@
-use rayon::prelude::*;
-use serde::{Deserialize, Serialize};
-use tree_sitter::Node;
-
-use crate::{AbstractTree, Block, Expression, Identifier, List, Map, Result, Value};
-
-#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
-pub struct Transform {
-    identifier: Identifier,
-    expression: Expression,
-    item: Block,
-}
-
-impl AbstractTree for Transform {
-    fn from_syntax_node(source: &str, node: Node) -> Result<Self> {
-        let identifier_node = node.child(1).unwrap();
-        let identifier = Identifier::from_syntax_node(source, identifier_node)?;
-
-        let expression_node = node.child(3).unwrap();
-        let expression = Expression::from_syntax_node(source, expression_node)?;
-
-        let item_node = node.child(4).unwrap();
-        let item = Block::from_syntax_node(source, item_node)?;
-
-        Ok(Transform {
-            identifier,
-            expression,
-            item,
-        })
-    }
-
-    fn run(&self, source: &str, context: &mut Map) -> Result<Value> {
-        let expression_run = self.expression.run(source, context)?;
-        let values = expression_run.as_list()?.items();
-        let key = self.identifier.inner();
-        let new_values = values
-            .par_iter()
-            .map(|value| {
-                let iter_context = Map::clone_from(context).unwrap();
-
-                iter_context
-                    .variables_mut()
-                    .unwrap()
-                    .insert(key.clone(), value.clone());
-
-                self.item
-                    .run(source, &mut iter_context.clone())
-                    .unwrap_or_default()
-            })
-            .filter(|value| !value.is_empty())
-            .collect();
-
-        Ok(Value::List(List::with_items(new_values)))
-    }
-}

src/abstract_tree/type.rs

@@ -1,93 +1,265 @@
-use std::fmt::{self, Display, Formatter};
+use std::{
+    collections::BTreeMap,
+    fmt::{self, Display, Formatter},
+};
 
 use serde::{Deserialize, Serialize};
-use tree_sitter::Node;
+use tree_sitter::Node as SyntaxNode;
 
-use crate::{AbstractTree, Error, Map, Result, Value};
+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,
+    Function {
+        parameter_types: Vec<Type>,
+        return_type: Box<Type>,
+    },
     Integer,
     List,
-    Map,
+    ListOf(Box<Type>),
+    ListExact(Vec<Type>),
+    Map(Option<BTreeMap<Identifier, Type>>),
+    None,
+    Number,
     String,
-    Table,
+    Range,
 }
 
 impl Type {
-    pub fn check(&self, value: &Value) -> Result<()> {
+    pub fn custom(name: Identifier, arguments: Vec<Type>) -> Self {
-        match (self, value.r#type()) {
+        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::Function, Type::Function)
             | (Type::Integer, Type::Integer)
             | (Type::List, Type::List)
-            | (Type::Map, Type::Map)
+            | (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::Table, Type::Table) => Ok(()),
+            | (Type::None, Type::None) => true,
-            (Type::Boolean, _) => Err(Error::TypeCheck {
+            (Type::Map(left_types), Type::Map(right_types)) => left_types == right_types,
-                expected: Type::Boolean,
+            (
-                actual: value.clone(),
+                Type::Custom {
-            }),
+                    name: left_name,
-            (Type::Float, _) => Err(Error::TypeCheck {
+                    arguments: left_arguments,
-                expected: Type::Float,
+                },
-                actual: value.clone(),
+                Type::Custom {
-            }),
+                    name: right_name,
-            (Type::Function, _) => Err(Error::TypeCheck {
+                    arguments: right_arguments,
-                expected: Type::Function,
+                },
-                actual: value.clone(),
+            ) => left_name == right_name && left_arguments == right_arguments,
-            }),
+            (Type::ListOf(self_item_type), Type::ListOf(other_item_type)) => {
-            (Type::Integer, _) => Err(Error::TypeCheck {
+                self_item_type.accepts(&other_item_type)
-                expected: Type::Integer,
-                actual: value.clone(),
-            }),
-            (Type::List, _) => Err(Error::TypeCheck {
-                expected: Type::List,
-                actual: value.clone(),
-            }),
-            (Type::Map, _) => Err(Error::TypeCheck {
-                expected: Type::Map,
-                actual: value.clone(),
-            }),
-            (Type::String, _) => Err(Error::TypeCheck {
-                expected: Type::String,
-                actual: value.clone(),
-            }),
-            (Type::Table, _) => Err(Error::TypeCheck {
-                expected: Type::Table,
-                actual: value.clone(),
-            }),
             }
+            (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(source: &str, node: Node) -> Result<Self> {
+    fn from_syntax(
-        Error::expect_syntax_node(source, "type", node)?;
+        node: SyntaxNode,
+        _source: &str,
+        context: &Context,
+    ) -> Result<Self, SyntaxError> {
+        SyntaxError::expect_syntax_node("type", node)?;
 
-        let range_without_punctuation = node.start_byte() + 1..node.end_byte() - 1;
+        let type_node = node.child(0).unwrap();
 
-        let r#type = match &source[range_without_punctuation] {
+        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,
-            "fn" => Type::Function,
+            "(" => {
+                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,
-            "list" => Type::List,
+            "map" => Type::Map(None),
-            "map" => Type::Map,
+            "num" => Type::Number,
+            "none" => Type::None,
             "str" => Type::String,
-            "table" => Type::Table,
             _ => {
-                return Err(Error::UnexpectedSyntaxNode {
+                return Err(SyntaxError::UnexpectedSyntaxNode {
-                    expected: "any, bool, float, fn, int, list, map, str or table",
+                    expected: "any, bool, float, int, num, str, list, map, custom type, (, [ or {"
-                    actual: node.kind(),
+                        .to_string(),
-                    location: node.start_position(),
+                    actual: type_node.kind().to_string(),
-                    relevant_source: source[node.byte_range()].to_string(),
+                    position: node.range().into(),
                 })
             }
         };
@@ -95,23 +267,128 @@ impl AbstractTree for Type {
         Ok(r#type)
     }
 
-    fn run(&self, _source: &str, _context: &mut Map) -> Result<Value> {
+    fn expected_type(&self, _context: &Context) -> Result<Type, ValidationError> {
-        Ok(Value::Empty)
+        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 {
+    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 => write!(f, "function"),
+            Type::Function {
-            Type::Integer => write!(f, "integer"),
+                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::Map => write!(f, "map"),
+            Type::ListOf(item_type) => write!(f, "[{item_type}]"),
-            Type::String => write!(f, "string"),
+            Type::ListExact(types) => {
-            Type::Table => write!(f, "table"),
+                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/use.rs

@@ -1,32 +0,0 @@
-use std::fs::read_to_string;
-
-use serde::{Deserialize, Serialize};
-
-use crate::{evaluate_with_context, AbstractTree, Error, Map, Result, Value};
-
-#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
-pub struct Use {
-    path: String,
-}
-
-impl AbstractTree for Use {
-    fn from_syntax_node(source: &str, node: tree_sitter::Node) -> crate::Result<Self> {
-        Error::expect_syntax_node(source, "use", node)?;
-
-        let string_node = node.child(1).unwrap();
-        let path = source[string_node.start_byte() + 1..string_node.end_byte() - 1].to_string();
-
-        println!("{path}");
-
-        Ok(Use { path })
-    }
-
-    fn run(&self, _source: &str, _context: &mut Map) -> Result<Value> {
-        let file_contents = read_to_string(&self.path)?;
-        let mut file_context = Map::new();
-
-        evaluate_with_context(&file_contents, &mut file_context)?;
-
-        Ok(Value::Map(file_context))
-    }
-}

src/abstract_tree/value_node.rs

@@ -1,37 +1,48 @@
-use std::collections::BTreeMap;
+use std::{cmp::Ordering, ops::RangeInclusive};
 
 use serde::{Deserialize, Serialize};
-use tree_sitter::Node;
+use tree_sitter::Node as SyntaxNode;
 
 use crate::{
-    AbstractTree, Error, Expression, Function, Identifier, List, Map, Result, Statement, Table,
+    error::{RuntimeError, SyntaxError, ValidationError},
-    Value,
+    AbstractTree, Context, Expression, Format, Function, FunctionNode,
+    Identifier, List, Type, Value, TypeDefinition, MapNode,
 };
 
-#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
+#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq)]
 pub enum ValueNode {
     Boolean(String),
     Float(String),
+    Function(Function),
     Integer(String),
     String(String),
     List(Vec<Expression>),
-    Empty,
+    Map(MapNode),
-    Map(BTreeMap<String, Statement>),
+    Range(RangeInclusive<i64>),
-    Table {
+    Struct {
-        column_names: Vec<Identifier>,
+        name: Identifier,
-        rows: Box<Expression>,
+        properties: MapNode,
+    },
+    Enum {
+        name: Identifier,
+        variant: Identifier,
+        expression: Option<Box<Expression>>,
     },
-    Function(Function),
 }
 
 impl AbstractTree for ValueNode {
-    fn from_syntax_node(source: &str, node: Node) -> Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
-        debug_assert_eq!("value", node.kind());
+        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;
@@ -45,66 +56,59 @@ impl AbstractTree for ValueNode {
                     let current_node = child.child(index).unwrap();
 
                     if current_node.is_named() {
-                        let expression = Expression::from_syntax_node(source, current_node)?;
+                        let expression = Expression::from_syntax(current_node, source, context)?;
+
                         expressions.push(expression);
                     }
                 }
 
                 ValueNode::List(expressions)
             }
-            "table" => {
-                let identifier_list_node = child.child(1).unwrap();
-                let identifier_count = identifier_list_node.child_count();
-                let mut column_names = Vec::with_capacity(identifier_count);
-
-                for index in 0..identifier_count {
-                    let identifier_node = identifier_list_node.child(index).unwrap();
-
-                    if identifier_node.is_named() {
-                        let identifier = Identifier::from_syntax_node(source, identifier_node)?;
-
-                        column_names.push(identifier)
-                    }
-                }
-
-                let expression_node = child.child(2).unwrap();
-                let expression = Expression::from_syntax_node(source, expression_node)?;
-
-                ValueNode::Table {
-                    column_names,
-                    rows: Box::new(expression),
-                }
-            }
             "map" => {
-                let mut child_nodes = BTreeMap::new();
+                ValueNode::Map(MapNode::from_syntax(child, source, context)?)
-                let mut current_key = "".to_string();
-
-                for index in 0..child.child_count() - 1 {
-                    let child_syntax_node = child.child(index).unwrap();
-
-                    if child_syntax_node.kind() == "identifier" {
-                        current_key =
-                            Identifier::from_syntax_node(source, child_syntax_node)?.take_inner();
             }
+            "range" => {
+                let mut split = source[child.byte_range()].split("..");
+                let start = split.next().unwrap().parse().unwrap();
+                let end = split.next().unwrap().parse().unwrap();
 
-                    if child_syntax_node.kind() == "statement" {
+                ValueNode::Range(start..=end)
-                        let key = current_key.clone();
+            }
-                        let statement = Statement::from_syntax_node(source, child_syntax_node)?;
+            "enum_instance" => {
+                let name_node = child.child(0).unwrap();
+                let name = Identifier::from_syntax(name_node, source, context)?;
                 
-                        child_nodes.insert(key, statement);
+                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
                 }
             }
-
-                ValueNode::Map(child_nodes)
-            }
-            "function" => ValueNode::Function(Function::from_syntax_node(source, child)?),
             _ => {
-                return Err(Error::UnexpectedSyntaxNode {
+                return Err(SyntaxError::UnexpectedSyntaxNode {
                     expected:
-                        "string, integer, float, boolean, list, table, map, function or empty",
+                        "string, integer, float, boolean, range, list, map, option, function, struct or enum"
-                    actual: child.kind(),
+                            .to_string(),
-                    location: child.start_position(),
+                    actual: child.kind().to_string(),
-                    relevant_source: source[child.byte_range()].to_string(),
+                    position: node.range().into(),
                 })
             }
         };
@@ -112,12 +116,93 @@ impl AbstractTree for ValueNode {
         Ok(value_node)
     }
 
-    fn run(&self, source: &str, context: &mut Map) -> Result<Value> {
+    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) => Value::Float(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.parse().unwrap()),
+            ValueNode::String(value_source) => Value::string(value_source.clone()),
             ValueNode::List(expressions) => {
                 let mut values = Vec::with_capacity(expressions.len());
 
@@ -129,51 +214,144 @@ impl AbstractTree for ValueNode {
 
                 Value::List(List::with_items(values))
             }
-            ValueNode::Empty => Value::Empty,
+            ValueNode::Map(map_node) => map_node.run(source, context)?,
-            ValueNode::Map(key_statement_pairs) => {
+            ValueNode::Range(range) => Value::Range(range.clone()),
-                let map = Map::new();
+            ValueNode::Struct { name, properties } => {
-
+                let instance = if let Some(definition) = context.get_definition(name)? {
-                {
+                    if let TypeDefinition::Struct(struct_definition) = definition {
-                    let mut variables = map.variables_mut()?;
+                        struct_definition.instantiate(properties, source, context)?
-
+                    } else {
-                    for (key, statement) in key_statement_pairs {
+                        return Err(RuntimeError::ValidationFailure(ValidationError::ExpectedStructDefintion { actual: definition.clone() }))
-                        let value = statement.run(source, context)?;
-
-                        variables.insert(key.clone(), value);
                     }
+                } else {
+                    return Err(RuntimeError::ValidationFailure(
+                        ValidationError::TypeDefinitionNotFound(name.clone())
+                    ));
+                };
+
+                Value::Struct(instance)
+
             }
-
+            ValueNode::Enum { name, variant, expression } => {
-                Value::Map(map)
+                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()
                             }
-            ValueNode::Table {
+                        ));
-                column_names,
-                rows: row_expression,
-            } => {
-                let mut headers = Vec::with_capacity(column_names.len());
-                let mut rows = Vec::new();
-
-                for identifier in column_names {
-                    let name = identifier.inner().clone();
-
-                    headers.push(name)
                     }
+                } else {
+                    return Err(RuntimeError::ValidationFailure(
+                        ValidationError::TypeDefinitionNotFound(name.clone())
+                    ));
+                };
 
-                let _row_values = row_expression.run(source, context)?;
+                Value::Enum(instance)                
-                let row_values = _row_values.as_list()?.items();
+            },
-
-                for value in row_values.iter() {
-                    let row = value.as_list()?.items().clone();
-
-                    rows.push(row)
-                }
-
-                let table = Table::from_raw_parts(headers, rows);
-
-                Value::Table(table)
-            }
-            ValueNode::Function(function) => Value::Function(function.clone()),
         };
 
         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

@@ -1,8 +1,13 @@
 use serde::{Deserialize, Serialize};
-use tree_sitter::Node;
 
-use crate::{AbstractTree, Block, Expression, Map, Result, Value};
+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,
@@ -10,33 +15,50 @@ pub struct While {
 }
 
 impl AbstractTree for While {
-    fn from_syntax_node(source: &str, node: Node) -> crate::Result<Self> {
+    fn from_syntax(node: SyntaxNode, source: &str, context: &Context) -> Result<Self, SyntaxError> {
-        debug_assert_eq!("while", node.kind());
+        SyntaxError::expect_syntax_node("while", node)?;
 
         let expression_node = node.child(1).unwrap();
-        let expression = Expression::from_syntax_node(source, expression_node)?;
+        let expression = Expression::from_syntax(expression_node, source, context)?;
 
         let block_node = node.child(2).unwrap();
-        let block = Block::from_syntax_node(source, block_node)?;
+        let block = Block::from_syntax(block_node, source, context)?;
 
         Ok(While { expression, block })
     }
 
-    fn run(&self, source: &str, context: &mut Map) -> Result<Value> {
+    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)?;
         }
 
-        Ok(Value::Empty)
+        log::info!("RUN while loop end");
+
+        Ok(Value::none())
     }
 }
 
-#[cfg(test)]
+impl Format for While {
-mod tests {
+    fn format(&self, output: &mut String, indent_level: u8) {
-    use crate::evaluate;
+        output.push('\n');
-
+        While::indent(output, indent_level);
-    #[test]
+        output.push_str("while ");
-    fn evalualate_while_loop() {
+        self.expression.format(output, indent_level);
-        assert_eq!(evaluate("while false { 'foo' }"), Ok(crate::Value::Empty))
+        output.push(' ');
+        self.block.format(output, indent_level);
+        output.push('\n');
     }
 }

src/abstract_tree/yield.rs

@@ -1,47 +0,0 @@
-use serde::{Deserialize, Serialize};
-use tree_sitter::Node;
-
-use crate::{AbstractTree, BuiltInFunction, Expression, FunctionCall, Result, Value};
-
-#[derive(Debug, Clone, Serialize, Deserialize, Eq, PartialEq, PartialOrd, Ord)]
-pub struct Yield {
-    call: FunctionCall,
-}
-
-impl AbstractTree for Yield {
-    fn from_syntax_node(source: &str, node: Node) -> Result<Self> {
-        let input_node = node.child(0).unwrap();
-        let input = Expression::from_syntax_node(source, input_node)?;
-
-        let function_node = node.child(3).unwrap();
-        let mut arguments = Vec::new();
-
-        arguments.push(input);
-
-        for index in 4..node.child_count() - 1 {
-            let node = node.child(index).unwrap();
-
-            if node.is_named() {
-                let expression = Expression::from_syntax_node(source, node)?;
-
-                arguments.push(expression);
-            }
-        }
-
-        let call = if function_node.kind() == "built_in_function" {
-            let function = BuiltInFunction::from_syntax_node(source, function_node)?;
-
-            FunctionCall::BuiltIn(Box::new(function))
-        } else {
-            let name = Expression::from_syntax_node(source, function_node)?;
-
-            FunctionCall::ContextDefined { name, arguments }
-        };
-
-        Ok(Yield { call })
-    }
-
-    fn run(&self, source: &str, context: &mut crate::Map) -> Result<Value> {
-        self.call.run(source, context)
-    }
-}

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

@@ -1,365 +0,0 @@
-//! 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.
-
-use tree_sitter::{Node, Point};
-
-use crate::{value::Value, Identifier, Type};
-
-use std::{fmt, io, num::ParseFloatError, string::FromUtf8Error, sync::PoisonError, time};
-
-pub type Result<T> = std::result::Result<T, Error>;
-
-#[derive(Clone, PartialEq)]
-pub enum Error {
-    UnexpectedSyntaxNode {
-        expected: &'static str,
-        actual: &'static str,
-        location: Point,
-        relevant_source: String,
-    },
-
-    TypeCheck {
-        expected: Type,
-        actual: 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.
-    ExpectedToolArgumentAmount {
-        tool_name: &'static str,
-        expected: usize,
-        actual: usize,
-    },
-
-    /// A function was called with the wrong amount of arguments.
-    ExpectedAtLeastFunctionArgumentAmount {
-        identifier: String,
-        minimum: usize,
-        actual: usize,
-    },
-
-    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,
-    },
-
-    ExpectedEmpty {
-        actual: Value,
-    },
-
-    ExpectedMap {
-        actual: Value,
-    },
-
-    ExpectedTable {
-        actual: Value,
-    },
-
-    ExpectedFunction {
-        actual: Value,
-    },
-
-    /// A string, list, map or table value was expected.
-    ExpectedCollection {
-        actual: Value,
-    },
-
-    /// 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(Identifier),
-
-    /// The function failed due to an external error.
-    ToolFailure(String),
-
-    /// A custom error explained by its message.
-    CustomMessage(String),
-
-    /// Invalid user input.
-    Syntax {
-        source: String,
-        location: Point,
-    },
-}
-
-impl Error {
-    pub fn expect_syntax_node(source: &str, expected: &'static str, actual: Node) -> Result<()> {
-        if expected == actual.kind() {
-            Ok(())
-        } else if actual.is_error() {
-            Err(Error::Syntax {
-                source: source[actual.byte_range()].to_string(),
-                location: actual.start_position(),
-            })
-        } else {
-            Err(Error::UnexpectedSyntaxNode {
-                expected,
-                actual: actual.kind(),
-                location: actual.start_position(),
-                relevant_source: source[actual.byte_range()].to_string(),
-            })
-        }
-    }
-
-    pub fn expect_tool_argument_amount(
-        tool_name: &'static str,
-        expected: usize,
-        actual: usize,
-    ) -> Result<()> {
-        if expected == actual {
-            Ok(())
-        } else {
-            Err(Error::ExpectedToolArgumentAmount {
-                tool_name,
-                expected,
-                actual,
-            })
-        }
-    }
-}
-
-impl<T> From<PoisonError<T>> for Error {
-    fn from(value: PoisonError<T>) -> Self {
-        Error::ToolFailure(value.to_string())
-    }
-}
-
-impl From<FromUtf8Error> for Error {
-    fn from(value: FromUtf8Error) -> Self {
-        Error::ToolFailure(value.to_string())
-    }
-}
-
-impl From<ParseFloatError> for Error {
-    fn from(value: ParseFloatError) -> Self {
-        Error::ToolFailure(value.to_string())
-    }
-}
-
-impl From<csv::Error> for Error {
-    fn from(value: csv::Error) -> Self {
-        Error::ToolFailure(value.to_string())
-    }
-}
-
-impl From<io::Error> for Error {
-    fn from(value: std::io::Error) -> Self {
-        Error::ToolFailure(value.to_string())
-    }
-}
-
-impl From<reqwest::Error> for Error {
-    fn from(value: reqwest::Error) -> Self {
-        Error::ToolFailure(value.to_string())
-    }
-}
-
-impl From<serde_json::Error> for Error {
-    fn from(value: serde_json::Error) -> Self {
-        Error::ToolFailure(value.to_string())
-    }
-}
-
-impl From<time::SystemTimeError> for Error {
-    fn from(value: time::SystemTimeError) -> Self {
-        Error::ToolFailure(value.to_string())
-    }
-}
-
-impl From<toml::de::Error> for Error {
-    fn from(value: toml::de::Error) -> Self {
-        Error::ToolFailure(value.to_string())
-    }
-}
-
-impl std::error::Error for Error {}
-
-impl fmt::Debug for Error {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "{self}")
-    }
-}
-
-impl fmt::Display for Error {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        use Error::*;
-
-        match self {
-            AssertEqualFailed { expected, actual } => {
-                write!(f, "Equality assertion failed")?;
-
-                if expected.is_table() {
-                    write!(f, "\n{expected}\n")?;
-                } else {
-                    write!(f, " {expected} ")?;
-                }
-
-                write!(f, "does not equal")?;
-
-                if actual.is_table() {
-                    write!(f, "\n{actual}")
-                } else {
-                    write!(f, " {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
-            ),
-            ExpectedToolArgumentAmount {
-                tool_name,
-                expected,
-                actual,
-            } => write!(
-                f,
-                "{tool_name} 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 string but got {:?}.", actual)
-            }
-            ExpectedInteger { actual } => write!(f, "Expected an integer, but got {:?}.", actual),
-            ExpectedFloat { actual } => write!(f, "Expected a float, but got {:?}.", actual),
-            ExpectedNumber { actual } => {
-                write!(f, "Expected a float or integer but got {:?}.", actual)
-            }
-            ExpectedNumberOrString { actual } => {
-                write!(f, "Expected a number or string, but got {:?}.", actual)
-            }
-            ExpectedBoolean { actual } => {
-                write!(f, "Expected a boolean, but got {:?}.", actual)
-            }
-            ExpectedList { actual } => write!(f, "Expected a list, but got {:?}.", actual),
-            ExpectedMinLengthList {
-                minimum_len,
-                actual_len,
-            } => write!(
-                f,
-                "Expected a list of at least {minimum_len} values, but got one with {actual_len}.",
-            ),
-            ExpectedFixedLenList {
-                expected_len,
-                actual,
-            } => write!(
-                f,
-                "Expected a list of len {}, but got {:?}.",
-                expected_len, actual
-            ),
-            ExpectedEmpty { actual } => write!(f, "Expected an empty value, but got {:?}.", actual),
-            ExpectedMap { actual } => write!(f, "Expected a map, but got {:?}.", actual),
-            ExpectedTable { actual } => write!(f, "Expected a table, but got {:?}.", actual),
-            ExpectedFunction { actual } => {
-                write!(f, "Expected function, but got {:?}.", actual)
-            }
-            ExpectedCollection { actual } => {
-                write!(
-                    f,
-                    "Expected a string, list, map or table, but got {:?}.",
-                    actual
-                )
-            }
-            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.inner()
-            ),
-            UnexpectedSyntaxNode {
-                expected,
-                actual,
-                location,
-                relevant_source,
-            } => write!(
-                f,
-                "Expected {expected}, but got {actual} at {location}. Code: {relevant_source} ",
-            ),
-            WrongColumnAmount { expected, actual } => write!(
-                f,
-                "Wrong column amount. Expected {expected} but got {actual}."
-            ),
-            ToolFailure(message) => write!(f, "{message}"),
-            CustomMessage(message) => write!(f, "{message}"),
-            Syntax { source, location } => {
-                write!(f, "Syntax error at {location}, this is not valid: {source}")
-            }
-            TypeCheck { expected, actual } => write!(
-                f,
-                "Type check error. Expected a {expected} but got {actual}."
-            ),
-        }
-    }
-}

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/evaluator.rs

@@ -1,274 +0,0 @@
-//! The top level of Dust's API with functions to interpret Dust code.
-//!
-//! You can use this library externally by calling either of the "eval"
-//! functions or by constructing your own Evaluator.
-use std::fmt::{self, Debug, Formatter};
-
-use tree_sitter::{Parser, Tree as TSTree};
-
-use crate::{language, AbstractTree, Map, Result, Statement, Value};
-
-/// Evaluate the given source code.
-///
-/// Returns a vector of results from evaluating the source code. Each comment
-/// and statemtent will have its own result.
-///
-/// # Examples
-///
-/// ```rust
-/// # use dust_lang::*;
-/// assert_eq!(evaluate("1 + 2 + 3"), Ok(Value::Integer(6)));
-/// ```
-pub fn evaluate(source: &str) -> Result<Value> {
-    let mut context = Map::new();
-
-    evaluate_with_context(source, &mut context)
-}
-
-/// Evaluate the given source code with the given context.
-///
-/// # Examples
-///
-/// ```rust
-/// # use dust_lang::*;
-/// let mut context = Map::new();
-///
-/// {
-///     let mut variables = context.variables_mut().unwrap();
-///
-///     variables.insert("one".into(), 1.into());
-///     variables.insert("two".into(), 2.into());
-///     variables.insert("three".into(), 3.into());
-/// }
-///
-/// let dust_code = "four = 4 one + two + three + four";
-///
-/// assert_eq!(
-///     evaluate_with_context(dust_code, &mut context),
-///     Ok(Value::Integer(10))
-/// );
-/// ```
-pub fn evaluate_with_context(source: &str, context: &mut Map) -> Result<Value> {
-    let mut parser = Parser::new();
-    parser.set_language(language()).unwrap();
-
-    Evaluator::new(parser, context, source).run()
-}
-
-/// A collection of statements and comments interpreted from a syntax tree.
-///
-/// The Evaluator turns a tree sitter concrete syntax tree into a vector of
-/// abstract trees called [Item][]s that can be run to execute the source code.
-pub struct Evaluator<'c, 's> {
-    _parser: Parser,
-    context: &'c mut Map,
-    source: &'s str,
-    syntax_tree: TSTree,
-}
-
-impl Debug for Evaluator<'_, '_> {
-    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
-        write!(f, "Evaluator context: {}", self.context)
-    }
-}
-
-impl<'c, 's> Evaluator<'c, 's> {
-    pub fn new(mut parser: Parser, context: &'c mut Map, source: &'s str) -> Self {
-        let syntax_tree = parser.parse(source, None).unwrap();
-
-        Evaluator {
-            _parser: parser,
-            context,
-            source,
-            syntax_tree,
-        }
-    }
-
-    pub fn run(self) -> Result<Value> {
-        let root_node = self.syntax_tree.root_node();
-
-        let mut prev_result = Value::Empty;
-
-        for index in 0..root_node.child_count() {
-            let statement_node = root_node.child(index).unwrap();
-            let statement = Statement::from_syntax_node(self.source, statement_node)?;
-
-            prev_result = statement.run(self.source, self.context)?;
-        }
-
-        Ok(prev_result)
-    }
-
-    pub fn syntax_tree(&self) -> String {
-        self.syntax_tree.root_node().to_sexp()
-    }
-}
-
-#[cfg(test)]
-mod tests {
-    use crate::{List, Table};
-
-    use super::*;
-
-    #[test]
-    fn evaluate_empty() {
-        assert_eq!(evaluate("x = 9"), Ok(Value::Empty));
-        assert_eq!(evaluate("x = 1 + 1"), Ok(Value::Empty));
-    }
-
-    #[test]
-    fn evaluate_integer() {
-        assert_eq!(evaluate("1"), Ok(Value::Integer(1)));
-        assert_eq!(evaluate("123"), Ok(Value::Integer(123)));
-        assert_eq!(evaluate("-666"), Ok(Value::Integer(-666)));
-    }
-
-    #[test]
-    fn evaluate_float() {
-        assert_eq!(evaluate("0.1"), Ok(Value::Float(0.1)));
-        assert_eq!(evaluate("12.3"), Ok(Value::Float(12.3)));
-        assert_eq!(evaluate("-6.66"), Ok(Value::Float(-6.66)));
-    }
-
-    #[test]
-    fn evaluate_string() {
-        assert_eq!(evaluate("\"one\""), Ok(Value::String("one".to_string())));
-        assert_eq!(evaluate("'one'"), Ok(Value::String("one".to_string())));
-        assert_eq!(evaluate("`one`"), Ok(Value::String("one".to_string())));
-        assert_eq!(evaluate("`'one'`"), Ok(Value::String("'one'".to_string())));
-        assert_eq!(evaluate("'`one`'"), Ok(Value::String("`one`".to_string())));
-        assert_eq!(
-            evaluate("\"'one'\""),
-            Ok(Value::String("'one'".to_string()))
-        );
-    }
-
-    #[test]
-    fn evaluate_list() {
-        assert_eq!(
-            evaluate("[1, 2, 'foobar']"),
-            Ok(Value::List(List::with_items(vec![
-                Value::Integer(1),
-                Value::Integer(2),
-                Value::String("foobar".to_string()),
-            ])))
-        );
-    }
-
-    #[test]
-    fn evaluate_map() {
-        let map = Map::new();
-
-        {
-            let mut variables = map.variables_mut().unwrap();
-
-            variables.insert("x".to_string(), Value::Integer(1));
-            variables.insert("foo".to_string(), Value::String("bar".to_string()));
-        }
-
-        assert_eq!(evaluate("{ x = 1, foo = 'bar' }"), Ok(Value::Map(map)));
-    }
-
-    #[test]
-    fn evaluate_table() {
-        let mut table = Table::new(vec!["messages".to_string(), "numbers".to_string()]);
-
-        table
-            .insert(vec![Value::String("hiya".to_string()), Value::Integer(42)])
-            .unwrap();
-        table
-            .insert(vec![Value::String("foo".to_string()), Value::Integer(57)])
-            .unwrap();
-        table
-            .insert(vec![Value::String("bar".to_string()), Value::Float(99.99)])
-            .unwrap();
-
-        assert_eq!(
-            evaluate(
-                "
-                table |messages numbers| [
-                    ['hiya', 42]
-                    ['foo', 57]
-                    ['bar', 99.99]
-                ]
-                "
-            ),
-            Ok(Value::Table(table))
-        );
-    }
-
-    #[test]
-    fn evaluate_if() {
-        assert_eq!(
-            evaluate("if true { 'true' }"),
-            Ok(Value::String("true".to_string()))
-        );
-    }
-
-    #[test]
-    fn evaluate_if_else() {
-        assert_eq!(evaluate("if false { 1 } else { 2 }"), Ok(Value::Integer(2)));
-        assert_eq!(
-            evaluate("if true { 1.0 } else { 42.0 }"),
-            Ok(Value::Float(1.0))
-        );
-    }
-
-    #[test]
-    fn evaluate_if_else_else_if_else() {
-        assert_eq!(
-            evaluate(
-                "
-                    if false {
-                        'no'
-                    } else if 1 + 1 == 3 {
-                        'nope'
-                    } else {
-                        'ok'
-                    }
-                "
-            ),
-            Ok(Value::String("ok".to_string()))
-        );
-    }
-
-    #[test]
-    fn evaluate_if_else_if_else_if_else_if_else() {
-        assert_eq!(
-            evaluate(
-                "
-                    if false {
-                        'no'
-                    } else if 1 + 1 == 1 {
-                        'nope'
-                    } else if 9 / 2 == 4 {
-                        'nope'
-                    } else if 'foo' == 'bar' {
-                        'nope'
-                    } else {
-                        'ok'
-                    }
-                "
-            ),
-            Ok(Value::String("ok".to_string()))
-        );
-    }
-
-    #[test]
-    fn evaluate_function_call() {
-        assert_eq!(
-            evaluate(
-                "
-                foobar = |message <str>| <str> { message }
-                (foobar 'Hiya')
-                ",
-            ),
-            Ok(Value::String("Hiya".to_string()))
-        );
-    }
-
-    #[test]
-    fn evaluate_built_in_function_call() {
-        assert_eq!(evaluate("(output 'Hiya')"), Ok(Value::Empty));
-    }
-}

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

@@ -1,20 +1,26 @@
-//! The Dust library is used to implement the Dust language, `src/main.rs` implements the command
+//! The Dust library is used to parse, format and run dust source code.
-//! line binary.
 //!
-//! Using this library is simple and straightforward, see the [inferface] module for instructions on
+//! See the [interpret] module for more information.
-//! interpreting Dust code. Most of the language's features are implemented in the [tools] module.
+//!
-
+//! You can use this library externally by calling either of the "interpret"
+//! functions or by constructing your own Interpreter.
 pub use crate::{
-    abstract_tree::*,
+    abstract_tree::*, built_in_functions::BuiltInFunction, context::*, error::Error, interpret::*,
-    error::*,
+    value::*,
-    evaluator::*,
-    value::{function::Function, list::List, map::Map, table::Table, Value},
 };
 
-mod abstract_tree;
+pub use tree_sitter::Node as SyntaxNode;
-mod error;
+
-mod evaluator;
+pub mod abstract_tree;
-mod value;
+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;
 
@@ -29,18 +35,6 @@ pub fn language() -> Language {
     unsafe { tree_sitter_dust() }
 }
 
-/// The content of the [`node-types.json`][] file for this grammar.
-///
-/// [`node-types.json`]: https://tree-sitter.github.io/tree-sitter/using-parsers#static-node-types
-pub const NODE_TYPES: &str = include_str!("../tree-sitter-dust/src/node-types.json");
-
-// Uncomment these to include any queries that this grammar contains
-
-// pub const HIGHLIGHTS_QUERY: &'static str = include_str!("../../queries/highlights.scm");
-// pub const INJECTIONS_QUERY: &'static str = include_str!("../../queries/injections.scm");
-// pub const LOCALS_QUERY: &'static str = include_str!("../../queries/locals.scm");
-// pub const TAGS_QUERY: &'static str = include_str!("../../queries/tags.scm");
-
 #[cfg(test)]
 mod tests {
     #[test]

src/main.rs

@@ -1,18 +1,20 @@
 //! Command line interface for the dust programming language.
-use clap::Parser;
+
-use rustyline::{
+use clap::{Parser, Subcommand};
-    completion::FilenameCompleter,
+use colored::Colorize;
-    error::ReadlineError,
+use crossterm::event::{KeyCode, KeyModifiers};
-    highlight::Highlighter,
+use nu_ansi_term::{Color, Style};
-    hint::{Hint, Hinter, HistoryHinter},
+use reedline::{
-    history::DefaultHistory,
+    default_emacs_keybindings, ColumnarMenu, Completer, DefaultHinter, EditCommand, Emacs, Prompt,
-    Completer, Context, Editor, Helper, Validator,
+    Reedline, ReedlineEvent, ReedlineMenu, Signal, Span, SqliteBackedHistory, Suggestion,
 };
-use tree_sitter::Parser as TSParser;
 
-use std::{borrow::Cow, fs::read_to_string};
+use std::{borrow::Cow, fs::read_to_string, io::Write, path::PathBuf, process::Command};
 
-use dust_lang::{evaluate_with_context, language, Evaluator, Map, Value};
+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)]
@@ -22,181 +24,384 @@ struct Args {
     #[arg(short, long)]
     command: Option<String>,
 
-    /// Data to assign to the "input" variable.
+    /// Command for alternate functionality besides running the source.
-    #[arg(short, long)]
+    #[command(subcommand)]
-    input: Option<String>,
+    cli_command: Option<CliCommand>,
-
-    /// A path to file whose contents will be assigned to the "input" variable.
-    #[arg(short = 'p', long)]
-    input_path: Option<String>,
-
-    /// A path to file whose contents will be assigned to the "input" variable.
-    #[arg(short = 't', long = "tree")]
-    show_syntax_tree: bool,
 
     /// 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() {
-        return run_cli_shell();
+        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 {
+    } else if let Some(command) = args.command {
-        command.clone()
+        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()
         };
 
-    let mut context = Map::new();
+        self.left = left_prompt;
+        self.right = right_prompt;
+    }
+}
 
-    if let Some(input) = args.input {
+impl Prompt for StarshipPrompt {
-        context
+    fn render_prompt_left(&self) -> Cow<str> {
-            .variables_mut()
+        Cow::Borrowed(&self.left)
-            .unwrap()
-            .insert("input".to_string(), Value::String(input));
     }
 
-    if let Some(path) = args.input_path {
+    fn render_prompt_right(&self) -> Cow<str> {
-        let file_contents = read_to_string(path).unwrap();
+        Cow::Borrowed(&self.right)
-
-        context
-            .variables_mut()
-            .unwrap()
-            .insert("input".to_string(), Value::String(file_contents));
     }
 
-    let mut parser = TSParser::new();
+    fn render_prompt_indicator(&self, _prompt_mode: reedline::PromptEditMode) -> Cow<str> {
-    parser.set_language(language()).unwrap();
+        Cow::Borrowed(" ")
-
-    let evaluator = Evaluator::new(parser, &mut context, &source);
-
-    if args.show_syntax_tree {
-        println!("{}", evaluator.syntax_tree());
     }
 
-    let eval_result = evaluator.run();
+    fn render_prompt_multiline_indicator(&self) -> Cow<str> {
+        Cow::Borrowed("")
+    }
 
-    match eval_result {
+    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_empty() {
+                        if !value.is_none() {
                             println!("{value}")
                         }
                     }
-        Err(error) => eprintln!("{error}"),
+                    Err(error) => println!("{error}"),
                 }
-}
+
-
+                prompt.reload();
-#[derive(Helper, Completer, Validator)]
+            }
-struct DustReadline {
+            Ok(Signal::CtrlD) | Ok(Signal::CtrlC) => {
-    #[rustyline(Completer)]
+                println!("\nLeaving the Dust shell.");
-    completer: FilenameCompleter,
+                break;
-
+            }
-    tool_hints: Vec<ToolHint>,
+            x => {
-
+                println!("Unknown event: {:?}", x);
-    #[rustyline(Hinter)]
+            }
-    _hinter: HistoryHinter,
+        }
-}
+    }
-
+
-impl DustReadline {
+    Ok(())
-    fn new() -> Self {
-        Self {
-            completer: FilenameCompleter::new(),
-            _hinter: HistoryHinter {},
-            tool_hints: Vec::new(),
-        }
-    }
-}
-
-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 = Map::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 = evaluate_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/value/enum_instance.rs

@@ -0,0 +1,40 @@
+use std::fmt::{self, Display, Formatter};
+
+use serde::{Deserialize, Serialize};
+
+use crate::{Identifier, Value};
+
+#[derive(Debug, Clone, Eq, PartialEq, PartialOrd, Ord, Serialize, Deserialize)]
+pub struct EnumInstance {
+    name: Identifier,
+    variant: Identifier,
+    value: Option<Box<Value>>,
+}
+
+impl EnumInstance {
+    pub fn new(name: Identifier, variant_name: Identifier, value: Option<Value>) -> Self {
+        Self {
+            name,
+            variant: variant_name,
+            value: value.map(|value| Box::new(value)),
+        }
+    }
+
+    pub fn name(&self) -> &Identifier {
+        &self.name
+    }
+
+    pub fn variant(&self) -> &Identifier {
+        &self.variant
+    }
+
+    pub fn value(&self) -> &Option<Box<Value>> {
+        &self.value
+    }
+}
+
+impl Display for EnumInstance {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        write!(f, "{}::{}({:?})", self.name, self.variant, self.value)
+    }
+}