Call Rhai Functions from Rust

Rhai also allows working backwards from the other direction – i.e. calling a Rhai-scripted function from Rust via Engine::call_fn.

│ Rhai script │

import "process" as proc;           // this is evaluated every time

fn hello(x, y) {
    // hopefully 'my_var' is in scope when this is called
    x.len + y + my_var

fn hello(x) {
    // hopefully 'my_string' is in scope when this is called
    x * my_string.len()

fn hello() {
    // hopefully 'MY_CONST' is in scope when this is called
    if MY_CONST {
        proc::process_data(42);     // can access imported module

│ Rust │

// Compile the script to AST
let ast = engine.compile(script)?;

// Create a custom 'Scope'
let mut scope = Scope::new();

// A custom 'Scope' can also contain any variables/constants available to
// the functions
scope.push("my_var", 42_i64);
scope.push("my_string", "hello, world!");
scope.push_constant("MY_CONST", true);

// Evaluate a function defined in the script, passing arguments into the
// script as a tuple.
// Beware, arguments must be of the correct types because Rhai does not
// have built-in type conversions. If arguments of the wrong types are passed,
// the Engine will not find the function.
// Variables/constants pushed into the custom 'Scope'
// (i.e. 'my_var', 'my_string', 'MY_CONST') are visible to the function.

let result = engine.call_fn::<i64>(&mut scope, &ast, "hello", ( "abc", 123_i64 ) )?;
//                            ^^^                             ^^^^^^^^^^^^^^^^^^
//              return type must be specified                 put arguments in a tuple

let result = engine.call_fn::<i64>(&mut scope, &ast, "hello", ( 123_i64, ) )?;
//                                                            ^^^^^^^^^^^^ tuple of one

let result = engine.call_fn::<i64>(&mut scope, &ast, "hello", () )?;
//                                                            ^^ unit = tuple of zero

Warning: Functions with one parameter

Functions with only one single parameter is easy to get wrong.

The proper Rust syntax is a tuple with one item:

( arg , )

Notice the comma (,) after the argument. Without it, the expression is a single value (arg) which is the same as arg and not a tuple.

A syntax error with very confusing error message will be generated by the Rust compiler if the comma is omitted.

Default behavior

When using Engine::call_fn, the AST is always evaluated before the function is called.

This is usually desirable in order to import the necessary external modules that are needed by the function.

All new variables/constants introduced are, by default, not retained inside the Scope. In other words, the Scope is rewound before each call.

If these default behaviors are not desirable, use Engine::call_fn_raw.

FuncArgs Trait


Rhai implements FuncArgs for tuples and Vec<T>.

Engine::call_fn takes a parameter of any type that implements the FuncArgs trait, which is used to parse a data type into individual argument values for the function call.

Custom types (e.g. structures) can also implement FuncArgs so they can be used for calling Engine::call_fn.

use std::iter::once;
use rhai::FuncArgs;

// A struct containing function arguments
struct Options {
    pub foo: bool,
    pub bar: String,
    pub baz: i64

impl FuncArgs for Options {
    fn parse<C: Extend<Dynamic>>(self, container: &mut C) {

let options = Options { foo: true, bar: "world", baz: 42 };

// The type 'Options' can now be used as argument to 'call_fn'
// to call a function with three parameters: fn hello(foo, bar, baz)
let result = engine.call_fn::<i64>(&mut scope, &ast, "hello", options)?;

Warning: You don’t need this

Implementing FuncArgs is almost never needed because Rhai works directly with any custom type.

It is used only in niche cases where a custom type’s fields need to be split up to pass to functions.

Low-Level API – Engine::call_fn_raw

For more control, construct all arguments as Dynamic values and use Engine::call_fn_raw, passing it anything that implements AsMut<[Dynamic]> (such as a simple array or a Vec<Dynamic>):

let result = engine.call_fn_raw(
                &mut scope,         // scope to use
                &ast,               // AST containing the functions
                false,              // false = do not evaluate the AST
                false,              // false = do not rewind the scope (i.e. keep new variables)
                "hello",            // function entry-point
                None,               // 'this' pointer, if any
                [ "abc".into(), 123_i64.into() ]    // arguments

Engine::call_fn_raw extends control to the following:

  • Whether to skip evaluation of the AST before calling the target function
  • Whether to rewind the custom Scope at the end of the function call
  • Whether to bind the this pointer to a specific value

Skip evaluation of the AST

By default, the AST is evaluated before calling the target function. A parameter can be passed to skip this evaluation.

Keep new variables/constants

By default, the Engine rewinds the custom Scope after each call to the initial size, so any new variable/constant defined are cleared and will not spill into the custom Scope.

This keeps the Scope from being continuously polluted by new variables and is usually the expected intuitive behavior.

A parameter can be passed to keep new variables/constants within the custom Scope. This allows the function to easily pass values back to the caller by leaving them inside the custom Scope.

Warning: new variables persist in Scope

If the Scope is not rewound, beware that all variables/constants defined at top level of the function or in the script body will persist inside the custom Scope.

If any of them are temporary and not intended to be retained, define them inside a statements block (see example below).

│ Rhai script │

fn initialize() {
    let x = 42;                     // 'x' is retained
    let y = x * 2;                  // 'y' is retained

    // Use a new statements block to define temp variables
        let temp = x + y;           // 'temp' is NOT retained

        foo = temp * temp;          // 'foo' is visible in the scope

let foo = 123;                      // 'foo' is retained

// Use a new statements block to define temp variables
    let bar = foo / 2;              // 'bar' is NOT retained

    foo = bar * bar;

│ Rust │

engine.call_fn_raw(&mut scope, &ast, true, false, "initialize", None, [])?;
//                                   ^^^^ evaluate AST before call
//                                         ^^^^^ do not rewind scope

// At this point, 'scope' contains these variables: 'foo', 'x', 'y'

Bind the this pointer


Engine::call_fn cannot call functions in method-call style.

Engine::call_fn_raw can also bind a value to the this pointer of a script-defined function.

It is possible, then, to call a function that uses this.

let ast = engine.compile("fn action(x) { this += x; }")?;

let mut value: Dynamic = 1_i64.into();

            &mut scope,
            Some(&mut value),       // binding the 'this' pointer
            [ 41_i64.into() ]

assert_eq!(value.as_int()?, 42);