meval 0.2.0

A simple math expression parser and evaluator.

Build Status meval at


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. A typical use case is the configuration of numerical computations in Rust, think initial data and boundary conditions, via config files or command line arguments.



Simply add the corresponding entry to your Cargo.toml dependency list:

meval = "0.2"

and add this to your crate root:

extern crate meval;

Requires Rust 1.26.

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 = "sin(pi * x)".parse().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);

Custom constants and functions? Define a Context!

use meval::{Expr, Context};

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

// create a context with function definitions and variables
let mut ctx = Context::new(); // built-ins
ctx.func("phi", |x| x + y)
   .var("zeta", -1.);
// bind function with a custom context
let func = expr.bind_with_context(ctx, "x").unwrap();
assert_eq!(func(2.), -2. * -1. + 2. + 1.);

For functions of 2, 3, and N variables use Context::func2, Context::func3 and Context::funcn, respectively. See Context for more options.

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

use std::cell::Cell;
use meval::{Expr, Context};
let y = Cell::new(0.);
let expr: Expr = "phi(x)".parse().unwrap();

let mut ctx = Context::empty(); // no built-ins
ctx.func("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: +, -, *, /, % (remainder), ^ (power)
  • unary operators: +, -

It supports custom variables and functions like x, weight, C_0, f(1), etc. A variable or function name must start with [a-zA-Z_] and can contain only [a-zA-Z0-9_]. Custom functions with a variable number of arguments are also supported.

Build-ins (given by the context Context::new() and when no context provided) 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


Expr supports deserialization using the serde library to make flexible configuration easy to set up, if the feature serde is enabled (disabled by default).

extern crate serde_derive;
extern crate toml;
extern crate meval;
use meval::{Expr, Context};

struct Ode {
    #[serde(deserialize_with = "meval::de::as_f64")]
    x0: f64,
    #[serde(deserialize_with = "meval::de::as_f64")]
    t0: f64,
    f: Expr,

fn main() {
    let config = r#"
        x0 = "cos(1.)"
        t0 = 2
        f = "sin(x)"
    let ode: Ode = toml::from_str(config).unwrap();

    assert_eq!(ode.x0, 1f64.cos());
    assert_eq!(ode.t0, 2f64);
    assert_eq!(ode.f.bind("x").unwrap()(2.), 2f64.sin());

Related projects

This is a toy project of mine for learning Rust, and to be hopefully useful when writing command line scripts. There is no plan to make this anything more than math expression -> number "converter". For more advanced scripting, see:

  • dyon -- A rusty dynamically typed scripting language
  • gluon -- A static, type inferred programming language for application embedding
  • rodolf0/tox -- another shunting yard expression parser


This project is dual-licensed under the Unlicense and MIT licenses.

You may use this code under the terms of either license.