Register a Rust Function for Use in Rhai Scripts

Rhai’s scripting engine is very lightweight. It gets most of its abilities from functions.

To call these functions, they need to be registered via Engine::register_fn.

Tip: Function overloading

Functions registered with the Engine can be overloaded as long as the signature is unique, i.e. different functions can have the same name as long as their parameters are of different types or different numbers (i.e. arity).

New definitions overwrite previous definitions of the same name, same arity and same parameter types.

use rhai::{Dynamic, Engine, ImmutableString};

// Normal function that returns a standard type
// Remember to use 'ImmutableString' and not 'String'
fn add_len(x: i64, s: ImmutableString) -> i64 {
    x + s.len()
// Alternatively, '&str' maps directly to 'ImmutableString'
fn add_len_count(x: i64, s: &str, c: i64) -> i64 {
    x + s.len() * c
// Function that returns a 'Dynamic' value
fn get_any_value() -> Dynamic {
    42_i64.into()                       // standard types can use '.into()'

let mut engine = Engine::new();

engine.register_fn("add", add_len)
      .register_fn("add", add_len_count)
      .register_fn("add", get_any_value)
      .register_fn("inc", |x: i64| {    // closure is also OK!
          x + 1
      .register_fn("log", |label: &str, x: i64| {
          println!("{label} = {x}");

let result = engine.eval::<i64>(r#"add(40, "xx")"#)?;

println!("Answer: {result}");           // prints 42

let result = engine.eval::<i64>(r#"add(40, "x", 2)"#)?;

println!("Answer: {result}");           // prints 42

let result = engine.eval::<i64>("add()")?;

println!("Answer: {result}");           // prints 42

let result = engine.eval::<i64>("inc(41)")?;

println!("Answer: {result}");           // prints 42"log("value", 42)"#)?;     // prints "value = 42"

Tip: Use closures

It is common for short functions to be registered via a closure.

engine.register_fn("foo", |x: i64, y: bool| ...);

Tip: Create a Dynamic

To create a Dynamic value, use Dynamic::from.

Standard types in Rhai can also use .into().

use rhai::Dynamic;

let obj = TestStruct::new();

let x = Dynamic::from(obj);

// '.into()' works for standard types

let x = 42_i64.into();

let y = "hello!".into();