evalexpr

Evalexpr is an expression evaluator and tiny scripting language in Rust. It has a small and easy to use interface and can be easily integrated into any application. It is very lightweight and comes with no further dependencies. Evalexpr is available on crates.io, and its API Documentation is available on docs.rs.

Quickstart

Add evalexpr as dependency to your Cargo.toml:

[dependencies]
evalexpr = "5"

Add the extern crate definition to your main.rs or lib.rs:

extern crate evalexpr;

Then you can use evalexpr to evaluate expressions like this:

use evalexpr::*;

assert_eq!(eval("1 + 2 + 3"), Ok(Value::from(6)));
// `eval` returns a variant of the `Value` enum,
// while `eval_[type]` returns the respective type directly.
// Both can be used interchangeably.
assert_eq!(eval_int("1 + 2 + 3"), Ok(6));
assert_eq!(eval("1 - 2 * 3"), Ok(Value::from(-5)));
assert_eq!(eval("1.0 + 2 * 3"), Ok(Value::from(7.0)));
assert_eq!(eval("true && 4 > 2"), Ok(Value::from(true)));

You can chain expressions and assign to variables like this:

use evalexpr::*;

let mut context = HashMapContext::new();
// Assign 5 to a like this
assert_eq!(eval_empty_with_context_mut("a = 5", &mut context), Ok(EMPTY_VALUE));
// The HashMapContext is type safe, so this will fail now
assert_eq!(eval_empty_with_context_mut("a = 5.0", &mut context), Err(EvalexprError::expected_int(Value::from(5.0))));
// We can check which value the context stores for a like this
assert_eq!(context.get_value("a"), Some(&Value::from(5)));
// And use the value in another expression like this
assert_eq!(eval_int_with_context_mut("a = a + 2; a", &mut context), Ok(7));

And you can use variables and functions in expressions like this:

use evalexpr::*;

let context = context_map! {
    "five" => 5,
    "twelve" => 12,
    "f" => Function::new(Box::new(|argument| {
        if let Ok(int) = argument.as_int() {
            Ok(Value::Int(int / 2))
        } else if let Ok(float) = argument.as_float() {
            Ok(Value::Float(float / 2.0))
        } else {
            Err(EvalexprError::expected_number(argument.clone()))
        }
    })),
    "avg" => Function::new(Box::new(|argument| {
        let arguments = argument.as_tuple()?;

        if let (Value::Int(a), Value::Int(b)) = (&arguments[0], &arguments[1]) {
            Ok(Value::Int((a + b) / 2))
        } else {
            Ok(Value::Float((arguments[0].as_number()? + arguments[1].as_number()?) / 2.0))
        }
    }))
}.unwrap(); // Do proper error handling here

assert_eq!(eval_with_context("five + 8 > f(twelve)", &context), Ok(Value::from(true)));
// `eval_with_context` returns a variant of the `Value` enum,
// while `eval_[type]_with_context` returns the respective type directly.
// Both can be used interchangeably.
assert_eq!(eval_boolean_with_context("five + 8 > f(twelve)", &context), Ok(true));
assert_eq!(eval_with_context("avg(2, 4) == 3", &context), Ok(Value::from(true)));

You can also precompile expressions like this:

use evalexpr::*;

let precompiled = build_operator_tree("a * b - c > 5").unwrap(); // Do proper error handling here

let mut context = context_map! {
    "a" => 6,
    "b" => 2,
    "c" => 3
}.unwrap(); // Do proper error handling here
assert_eq!(precompiled.eval_with_context(&context), Ok(Value::from(true)));

context.set_value("c".into(), 8.into()).unwrap(); // Do proper error handling here
assert_eq!(precompiled.eval_with_context(&context), Ok(Value::from(false)));
// `Node::eval_with_context` returns a variant of the `Value` enum,
// while `Node::eval_[type]_with_context` returns the respective type directly.
// Both can be used interchangeably.
assert_eq!(precompiled.eval_boolean_with_context(&context), Ok(false));

Features

Operators

This crate offers a set of binary and unary operators for building expressions. Operators have a precedence to determine their order of evaluation. The precedence should resemble that of most common programming languages, especially Rust. The precedence of variables and values is 200, and the precedence of function literals is 190.

Supported binary operators:

Operator	Precedence	Description
^	120	Exponentiation
*	100	Product
/	100	Division
%	100	Modulo
+	95	Sum or String Concatenation
-	95	Difference
<	80	Lower than
>	80	Greater than
<=	80	Lower than or equal
>=	80	Greater than or equal
==	80	Equal
!=	80	Not equal
&&	75	Logical and
||	70	Logical or
=	50	Assignment
,	40	Aggregation
;	0	Expression Chaining

Supported unary operators:

Operator	Precedence	Description
-	110	Negation
!	110	Logical not

The Aggregation Operator

The aggregation operator aggregates two values into a tuple. If one of the values is a tuple already, the resulting tuple will be flattened. Example:

use evalexpr::*;

assert_eq!(eval("1, 2, 3"), Ok(Value::from(vec![Value::from(1), Value::from(2), Value::from(3)])));

The Assignment Operator

This crate features the assignment operator, that allows expressions to store their result in a variable in the expression context. If an expression uses the assignment operator, it must be evaluated with a mutable context. Note that assignments are type safe, meaning if an identifier is assigned a value of a type once, it cannot be assigned a value of another type.

use evalexpr::*;

let mut context = HashMapContext::new();
assert_eq!(eval_with_context("a = 5", &context), Err(EvalexprError::ContextNotManipulable));
assert_eq!(eval_empty_with_context_mut("a = 5", &mut context), Ok(EMPTY_VALUE));
assert_eq!(eval_empty_with_context_mut("a = 5.0", &mut context), Err(EvalexprError::expected_int(5.0.into())));
assert_eq!(eval_int_with_context("a", &context), Ok(5));
assert_eq!(context.get_value("a"), Some(5.into()).as_ref());

The Expression Chaining Operator

The expression chaining operator works as one would expect from programming languages that use the semicolon to end statements, like Rust, C or Java. It has the special feature that it returns the value of the last expression in the expression chain. If the last expression is terminated by a semicolon as well, then Value::Empty is returned. Expression chaining is useful together with assignment to create small scripts.

use evalexpr::*;

let mut context = HashMapContext::new();
assert_eq!(eval("1;2;3;4;"), Ok(Value::Empty));
assert_eq!(eval("1;2;3;4"), Ok(4.into()));

// Initialization of variables via script.
assert_eq!(eval_empty_with_context_mut("hp = 1; max_hp = 5; heal_amount = 3;", &mut context), Ok(EMPTY_VALUE));
// Precompile healing script.
let healing_script = build_operator_tree("hp = min(hp + heal_amount, max_hp); hp").unwrap(); // Do proper error handling here
// Execute precompiled healing script.
assert_eq!(healing_script.eval_int_with_context_mut(&mut context), Ok(4));
assert_eq!(healing_script.eval_int_with_context_mut(&mut context), Ok(5));

Contexts

An expression evaluator that just evaluates expressions would be useful already, but this crate can to more. It allows using variables, assignments, statement chaining and user-defined functions within an expression. When assigning to variables, the assignment is stored in a context. When the variable is read later on, it is read from the context. Contexts can be preserved between multiple calls to eval by creating them yourself. Here is a simple example to show the difference between preserving context between evaluations, and not preserving:

use evalexpr::*;

assert_eq!(eval("a = 5;"), Ok(Value::from(())));
// The context is not preserved between eval calls
assert_eq!(eval("a"), Err(EvalexprError::VariableIdentifierNotFound("a".to_string())));

let mut context = HashMapContext::new();
assert_eq!(eval_with_context_mut("a = 5;", &mut context), Ok(Value::from(())));
// Assignments require mutable contexts
assert_eq!(eval_with_context("a = 6", &context), Err(EvalexprError::ContextNotManipulable));
// The HashMapContext is type safe
assert_eq!(eval_with_context_mut("a = 5.5", &mut context), Err(EvalexprError::ExpectedInt { actual: Value::from(5.5) }));
// Reading a variable does not require a mutable context
assert_eq!(eval_with_context("a", &context), Ok(Value::from(5)));

Note that the assignment is forgotten between the two calls to eval in the first example. In the second part, the assignment is correctly preserved. Note here, that to assign to a variable, the context needs to be passed as a mutable reference. When passed as an immutable reference, an error is returned. Also, the HashMapContext is type safe. This means that assigning to a again with a different type yields an error as well. Type unsafe contexts may be implemented if requested. For reading a, it is enough to pass an immutable reference.

Contexts can also be manipulated in code. Take a look at the following example:

use evalexpr::*;

let mut context = HashMapContext::new();
// We can set variables in code like this...
context.set_value("a".into(), 5.into());
// ...and read from them in expressions
assert_eq!(eval_int_with_context("a", &context), Ok(5));
// We can write or overwrite variables in expressions...
assert_eq!(eval_with_context_mut("a = 10; b = 1.0;", &mut context), Ok(().into()));
// ...and read the value in code like this
assert_eq!(context.get_value("a"), Some(&Value::from(10)));
assert_eq!(context.get_value("b"), Some(&Value::from(1.0)));

Contexts are also required for user-defined functions. Those can be passed one by one with the set_function method, but it might be more convenient to use the context_map! macro instead:

use evalexpr::*;

let context = context_map!{
    "f" => Function::new(Box::new(|args| Ok(Value::from(args.as_int()? + 5)))),
}.unwrap_or_else(|error| panic!("Error creating context: {}", error));
assert_eq!(eval_int_with_context("f 5", &context), Ok(10));

For more information about user-defined functions, refer to the respective section.

Builtin Functions

This crate offers a set of builtin functions.

Identifier	Argument Amount	Argument Types	Description
min	>= 1	Numeric	Returns the minimum of the arguments
max	>= 1	Numeric	Returns the maximum of the arguments
len	1	String/Tuple	Returns the character length of a string, or the amount of elements in a tuple (not recursively)
str::regex_matches	2	String, String	Returns true if the first argument matches the regex in the second argument
str::regex_replace	3	String, String, String	Returns the first argument with all matches of the regex in the second argument replaced by the third argument
str::to_lowercase	1	String	Returns the lower-case version of the string
str::to_uppercase	1	String	Returns the upper-case version of the string
str::trim	1	String	Strips whitespace from the start and the end of the string

The min and max functions can deal with a mixture of integer and floating point arguments. If the maximum or minimum is an integer, then an integer is returned. Otherwise, a float is returned.

The regex functions require the feature flag regex_support.

Values

Operators take values as arguments and produce values as results. Values can be boolean, integer or floating point numbers, strings, tuples or the empty type. Values are denoted as displayed in the following table.

Value type	Example
Value::String	"abc", "", "a\"b\\c"
Value::Boolean	true, false
Value::Int	3, -9, 0, 135412
Value::Float	3., .35, 1.00, 0.5, 123.554
Value::Tuple	(3, 55.0, false, ()), (1, 2)
Value::Empty	()

Integers are internally represented as i64, and floating point numbers are represented as f64. Tuples are represented as Vec<Value> and empty values are not stored, but represented by rust's unit type () where necessary.

There exist type aliases for some of the types. They include IntType, FloatType, TupleType and EmptyType.

Values can be constructed either directly or using the From trait. Values can be decomposed using the Value::as_[type] methods. The type of a value can be checked using the Value::is_[type] methods.

Examples for constructing a value:

Code	Result
Value::from(4)	Value::Int(4)
Value::from(4.4)	Value::Float(4.4)
Value::from(true)	Value::Boolean(true)
Value::from(vec![Value::from(3)])	Value::Tuple(vec![Value::Int(3)])

Examples for deconstructing a value:

Code	Result
Value::from(4).as_int()	Ok(4)
Value::from(4.4).as_float()	Ok(4.4)
Value::from(true).as_int()	Err(Error::ExpectedInt {actual: Value::Boolean(true)})

Operators that take numbers as arguments can either take integers or floating point numbers. If one of the arguments is a floating point number, all others are converted to floating point numbers as well, and the resulting value is a floating point number as well. Otherwise, the result is an integer. An exception to this is the exponentiation operator that always returns a floating point number.

Values have a precedence of 200.

Variables

This crate allows to compile parameterizable formulas by using variables. A variable is a literal in the formula, that does not contain whitespace or can be parsed as value. The user needs to provide bindings to the variables for evaluation. This is done with the Context trait. Two structs implementing this trait are predefined. There is EmptyContext, that returns None for each request, and HashMapContext, that stores mappings from literals to variables in a hash map.

Variables do not have fixed types in the expression itself, but are typed by the context. The Context trait contains a function that takes a string literal and returns a Value enum. The variant of this enum decides the type on evaluation.

Variables have a precedence of 200.

User-Defined Functions

This crate also allows to define arbitrary functions to be used in parsed expressions. A function is defined as a Function instance. It contains two properties, the argument_amount and the function. The function is a boxed Fn(&[Value]) -> EvalexprResult<Value, Error>. The argument_amount determines the length of the slice that is passed to function if it is Some(_), otherwise the function is defined to take an arbitrary amount of arguments. It is verified on execution by the crate and does not need to be verified by the function.

Functions with no arguments are not allowed. Use variables instead.

Be aware that functions need to verify the types of values that are passed to them. The error module contains some shortcuts for verification, and error types for passing a wrong value type. Also, most numeric functions need to differentiate between being called with integers or floating point numbers, and act accordingly.

Functions are identified by literals, like variables as well. A literal identifies a function, if it is followed by an opening brace (, another literal, or a value.

Same as variables, function bindings are provided by the user via a Context. Functions have a precedence of 190.

Examplary variables and functions in expressions:

Expression	Valid?	Explanation
a	yes
abc	yes
a<b	no	Expression is interpreted as variable a, operator < and variable b
a b	no	Expression is interpreted as function a applied to argument b
123	no	Expression is interpreted as Value::Int
true	no	Expression is interpreted as Value::Bool
.34	no	Expression is interpreted as Value::Float

Serde

To use this crate with serde, the serde_support feature flag has to be set. This can be done like this in the Cargo.toml:

[dependencies]
evalexpr = {version = "5", features = ["serde_support"]}

This crate implements serde::de::Deserialize for its type Node that represents a parsed expression tree. The implementation expects a serde string as input. Example parsing with ron format:

extern crate ron;
use evalexpr::*;

let mut context = context_map!{
    "five" => 5
}.unwrap(); // Do proper error handling here

// In ron format, strings are surrounded by "
let serialized_free = "\"five * five\"";
match ron::de::from_str::<Node>(serialized_free) {
    Ok(free) => assert_eq!(free.eval_with_context(&context), Ok(Value::from(25))),
    Err(error) => {
        () // Handle error
    }
}

With serde, expressions can be integrated into arbitrarily complex data.

The crate also implements Serialize and Deserialize for the HashMapContext. But note that only the variables get serialized, not the functions.

License

This crate is primarily distributed under the terms of the MIT license. See LICENSE for details.

No Panicking

This crate makes extensive use of the Result pattern and is intended to never panic. The exception are panics caused by failed allocations. But unfortunately, Rust does not provide any features to prove this behavior. The developer of this crate has not found a good solution to ensure no-panic behavior in any way. Please report a panic in this crate immediately as issue on github.

Even if the crate itself is panic free, it allows the user to define custom functions that are executed by the crate. The user needs to ensure that the function he provides to the crate never panic.

Contribution

If you have any ideas for features or see any problems in the code, architecture, interface, algorithmics or documentation, please open an issue on github. If there is already an issue describing what you want to say, please add a thumbs up or whatever emoji you think fits to the issue, so I know which ones I should prioritize.

Notes for contributors:

• This crate uses the sync-readme cargo subcommand to keep the documentation in src/lib.rs and README.md in sync. The subcommand only syncs from the documentation in src/lib.rs to README.md. So please alter the documentation in the src/lib.rs rather than altering anything in between <!-- cargo-sync-readme start --> and <!-- cargo-sync-readme end --> in the README.md.