Crate meval [] [src]

This Rust crate provides a simple math expression parsing and evaluation. Its main goal is to be convenient to use, while allowing for some flexibility. Currently works only with f64 types.

Simple examples

extern crate meval;

fn main() {
    let r = meval::eval_str("1 + 2").unwrap();

    println!("1 + 2 = {}", r);

Need to define a Rust function from an expression? No problem, use Expr for this and more:

extern crate meval;

fn main() {
    let expr = meval::Expr::from_str("sin(pi * x)").unwrap();
    let func = expr.bind("x").unwrap();

    let vs: Vec<_> = (0..100+1).map(|i| func(i as f64 / 100.)).collect();

    println!("sin(pi * x), 0 <= x <= 1: {:?}", vs);

Expr::bind returns a boxed closure that is slightly less convenient than an unboxed closure since Box<Fn(f64) -> f64> does not implement FnOnce, Fn or FnMut. So to use it directly as a function argument where a closure is expected, it has to be manually dereferenced:

let func = meval::Expr::from_str("x").unwrap().bind("x").unwrap();
let r = Some(2.).map(&*func);

Custom constants and functions? Define a context!

let y = 1.;
let expr = meval::Expr::from_str("phi(-2 * zeta + x)").unwrap();

// create a context with function definitions and variables
let ctx = (meval::CustomFunc("phi", |x| x + y), ("zeta", -1.));
// bind function with builtins as well as custom context
let func = expr.bind_with_context((meval::builtin(), ctx), "x").unwrap();
assert_eq!(func(2.), -2. * -1. + 2. + 1.);

For functions with 2, 3, and N variables use CustomFunc2, CustomFunc3 and CustomFuncN respectively.

If you need a custom function depending on mutable parameters, you will need to use a Cell:

use std::cell::Cell;
let y = Cell::new(0.);
let expr = meval::Expr::from_str("phi(x)").unwrap();

let ctx = meval::CustomFunc("phi", |x| x + y.get());
let func = expr.bind_with_context(ctx, "x").unwrap();
assert_eq!(func(2.), 2.);
assert_eq!(func(2.), 5.);

Supported expressions

meval supports basic mathematical operations on floating point numbers:

  • binary operators: +, -, *, /, ^ (power)
  • unary operators: +, -

It supports custom variables like x, weight, C_0, etc. A variable must start with [a-zA-Z_] and can contain only [a-zA-Z0-9_].

Build-ins (given by context meval::builtin()) currently supported:

  • functions implemented using functions of the same name in Rust std library:

    • sqrt, abs
    • exp, ln
    • sin, cos, tan, asin, acos, atan, atan2
    • sinh, cosh, tanh, asinh, acosh, atanh
    • floor, ceil, round
    • signum
  • other functions:

    • max(x, ...), min(x, ...): maximum and minimumum of 1 or more numbers
  • constants:

    • pi
    • e

This is a toy project of mine for learning Rust, and to be hopefully useful when writing command line scripts. For other similar projects see:


pub use shunting_yard::RPNError;
pub use tokenizer::ParseError;



Implementation of the shunting-yard algorithm for converting an infix expression to an expression in reverse Polish notation (RPN).


Tokenizer that converts a mathematical expression in a string form into a series of Tokens.



Built-in functions and constants.


A custom function of one variable.


A custom function of two variables.


A custom function of three variables.


A custom function of N variables.


Representation of a parsed expression.



An error produced during parsing or evaluation.


Function evaluation error.



Values of variables (and constants) for substitution into an evaluated expression.



Returns the build-in constants in a form that can be used as a Context.


Evaluates a string with built-in constants.


Evaluates a string with the given context.