[][src]Crate rusteval

This crate provides traits and macros that make your application's structs and functions interactive.

Annotating a struct with #[derive(Interactive)], a struct's methods with #[Methods] and a free function with #[Function] will implement a set of traits, that will allow you to access them as if Rust had a REPL.

Use this crate as an alternative for "print debugging" or as an ergonomic testing API.

This crate is no_std compatible so you can use it to interact with embedded devices and blink those LEDs from a USB or UART connection.


  • Annotate everything you want to access with Interactive, Methods and Function
  • Define a new struct that owns or holds references to the objects you want to access
  • Derive InteractiveRoot for it
  • Use the trait's methods to evaluate a string (the simplest one is eval_to_string but others allow for more custom behaviour)
  • Accessing a field will give you its Debug representation
  • Calling a function or a method will parse its arguments and give you the Debug representation of its return value

Since this crate makes a lot of use of the Debug trait the helper macro PartialDebug is provided. It implements Debug for a struct replacing all fields that do not implement Debug with a placeholder.

CLI Usage

Functions like get_all_field_names are provided. This makes it possible to implement things like auto-completion.

Have a look at the autocomplete example for how this might be done using the rustyline crate.


use rusteval::{Interactive, Methods, InteractiveRoot, Function, PartialDebug};

struct NoDebug;

#[derive(Interactive, PartialDebug, Default)]
struct ChildStruct {
    last_sum: f32,
    no_debug: NoDebug,

impl ChildStruct {
    fn add(&mut self, a: f32, b: f32) -> f32 {
        self.last_sum = a + b;

#[derive(Interactive, Debug, Default)]
struct ParentStruct {
    child: ChildStruct,

#[derive(InteractiveRoot, Debug, Default)]
struct Root {
    parent: ParentStruct,

fn split_str_at(s: &str, mid: usize) -> (&str, &str) {

let mut root = Root::default();
assert_eq!(root.eval_to_string("parent.child.add(4.2, 6.9)"), "11.1");
assert_eq!(root.eval_to_string("parent.child"), "ChildStruct { last_sum: 11.1, no_debug: Unknown }");
// split_str_at("foobar", 3) => ("foo", "bar")
assert_eq!(root.eval_to_string("split_str_at(\"foobar\", 3)"), "(\"foo\", \"bar\")");

How it works

This crate makes use of the unstable specialization feature, so it is only available on nightly.

Methods like try_as_interactive are implemented on all types. The method normally returns an error but in the specialized case a trait object (&dyn Interactive in this case) is returned.

The macros then implement getters that look something like this:

fn get_field<'a>(&'a self, field_name: &'a str) -> Result<'_, &dyn Interactive> {
    match field_name {
        "field1" => self.field1.try_as_interactive(),
        "field2" => self.field2.try_as_interactive(),
        _ => Err(InteractiveError::FieldNotFound {
            type_name: "Struct",

See the macro's documentation for more details.

Current limitations:

  • Methods and functions can only be made interactive if their argument types are supported
  • Enums are not supported



Implementation details.


Implementation details.



Contains information about function or method argument parsing errors.


The main error type of this crate.



A trait that allows to interactively evaluate a function and pass its result to the given closure.


A trait that gives interactive access to its fields as dyn Interactive or dyn Debug.


The main entry point to everything interactive.


A trait that allows to interactively evaluate a structs methods and pass their result to the given closure.

Type Definitions


The result type of most interactive methods.

Attribute Macros


Gives interactive access to a function.


Gives interactive access to a structs methods.

Derive Macros


Gives interactive access to a structs fields.


Derive this on a struct to make it an interactive access point for your application.


Implements Debug for a struct replacing all fields that do not implement Debug with a placeholder.