fasteval 0.1.9

A fast algebraic expression evaluation library.


A fast algebraic expression evaluation library.

fasteval is a library for parsing, compiling, and evaluating algebraic expressions. It can be used directly as a calculator language (much like python), and it is an excellent foundation for building higher-level-languages.



Add this to your Cargo.toml:

fasteval = "0.1.9"

You should always build with RUSTFLAGS="--emit=asm" because it greatly improves LLVM's compile-time optimizations.

You can build with --no-default-features to disable alphabetical keywords like and, or, NaN, inf. (These words might be important to your applications.)

You can build with --features unsafe-vars to enable Unsafe Variables.


  • Supports interpretation (i.e. parse & eval) as well as compiled execution (i.e. parse, compile, eval).
  • Variables and Custom Functions.
  • Safe for execution of untrusted expressions.
  • Good base for building higher-level languages.
  • Many built-in functions and constants.
  • Supports all the standard algebraic unary and binary operators (+ - * / ^ %), as well as comparisons (< <= == != >= >) and logical operators (&& ||) with short-circuit support.
  • Easy integration into many different types of applications, including scoped evaluation.
  • Very fast performance.

Easy Example

Here is one simple example. See the API Reference for many more!

The ez_eval() function performs the entire allocation-parse-eval process for you. It is slightly inefficient because it always allocates a fresh Slab, but it is very simple to use:

fn main() -> Result<(), fasteval::Error> {
    // This example doesn't use any variables, so just use an EmptyNamespace:
    let mut ns = fasteval::EmptyNamespace;

    let val = fasteval::ez_eval(
        "1+2*3/4^5%6 + log(100K) + log(e(),100) + [3*(3-3)/3] + (2<3) && 1.23",    &mut ns)?;
    //    |            |      |    |   |          |               |   |
    //    |            |      |    |   |          |               |   boolean logic with short-circuit support
    //    |            |      |    |   |          |               comparisons
    //    |            |      |    |   |          square-brackets act like parenthesis
    //    |            |      |    |   built-in constants: e(), pi()
    //    |            |      |    'log' can take an optional first 'base' argument, defaults to 10
    //    |            |      numeric literal with suffix: n, ยต, m, K, M, G, T
    //    |            many built-in functions: print, int, ceil, floor, abs, sign, log, round, min, max, sin, asin, ...
    //    standard binary operators

    assert_eq!(val, 1.23);


Performance Benchmarks

Here is a short summary of the performance benchmarks. For a more complete report and anlysis, see the API Reference.


Note that the following charts use logarithmic scales. Therefore, tiny visual differences actually represent very significant performance differences.

Performance of evaluation of a compiled expression:
Compiled Eval Performance

Performance of one-time interpretation (parse and eval):
Interpretation Performance

Performance of compiled Unsafe Variables, compared to the tinyexpr C library (the only other library in our test set that supports this mode):
Unsafe Compiled Eval Performance

Performance of interpreted Unsafe Variables, compared to the tinyexpr C library (the only other library in our test set that supports this mode):
Unsafe Interpretation Performance


The impressive thing about these results is that fasteval consistently achieves the fastest times across every benchmark and in every mode of operation (interpreted, compiled, and unsafe). It's easy to create a design to claim the #1 spot in any one of these metrics by sacrificing performance in another, but it is difficult to create a design that can be #1 across-the-board.

Because of the broad and robust performance advantages, fasteval is very likely to be an excellent choice for your dynamic evaluation needs.


fasteval is distributed under the terms of both the MIT license.

See LICENSE for details.