Register a Rust Function

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

To call these functions, they need to be registered with the Engine using Engine::register_fn (in the RegisterFn trait) and Engine::register_result_fn (in the RegisterResultFn trait, see fallible functions).

#![allow(unused)]
fn main() {
use rhai::{Dynamic, Engine, EvalAltResult, ImmutableString};
use rhai::RegisterFn;                       // use 'RegisterFn' trait for 'register_fn'
use rhai::RegisterResultFn;                 // use 'RegisterResultFn' trait for 'register_result_fn'

// 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_str(x: i64, s: &str) -> i64 {
    x + s.len()
}

// Function that returns a 'Dynamic' value - must return a 'Result'
fn get_any_value() -> Result<Dynamic, Box<EvalAltResult>> {
    Ok((42_i64).into())                     // standard types can use 'into()'
}

let mut engine = Engine::new();

engine
    .register_fn("add", add_len)
    .register_fn("add_str", add_len_str);

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

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

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

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

// Functions that return Dynamic values must use register_result_fn()
engine.register_result_fn("get_any_value", get_any_value);

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

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

To create a Dynamic value, use the Dynamic::from method. Standard types in Rhai can also use into().

#![allow(unused)]
fn main() {
use rhai::Dynamic;

let x = (42_i64).into();                        // 'into()' works for standard types

let y = Dynamic::from("hello!".to_string());    // remember &str is not supported by Rhai
}

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 number.

New definitions overwrite previous definitions of the same name and same number/types of parameters.