rhai/doc/src/plugins/function.md

Export a Rust Function to Rhai

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Sometimes only a few ad hoc functions are required and it is simpler to register individual functions instead of a full-blown [plugin module].

Macros

Macro	Apply to	Description
#[export_fn]	rust function defined in a Rust module	exports the function
register_exported_fn!	[Engine] instance, register name string, use path to function	registers the function into an [Engine] under specific name
set_exported_fn!	[Module] instance, register name string, use path to function	registers the function into an [Module] under specific name

#[export_fn] and register_exported_fn!

Apply #[export_fn] onto a function defined at module level to convert it into a Rhai plugin function.

The function cannot be nested inside another function - it can only be defined directly under a module.

To register the plugin function, simply call register_exported_fn!. The name of the function can be any text string, so it is possible to register overloaded functions as well as operators.

use rhai::plugins::*;       // import macros

#[export_fn]
fn increment(num: &mut i64) {
    *num += 1;
}

fn main() {
    let mut engine = Engine::new();

    // 'register_exported_fn!' registers the function as 'inc' with the Engine.
    register_exported_fn!(engine, "inc", increment);
}

Fallible Functions

To register [fallible functions] (i.e. functions that may return errors), apply the #[rhai_fn(return_raw)] attribute on plugin functions that return Result<Dynamic, Box<EvalAltResult>>.

A syntax error is generated if the function with #[rhai_fn(return_raw)] does not have the appropriate return type.

use rhai::plugins::*;       // import macros

#[export_fn]
#[rhai_fn(return_raw)]
pub fn double_and_divide(x: i64, y: i64) -> Result<Dynamic, Box<EvalAltResult>> {
    if y == 0 {
        Err("Division by zero!".into())
    } else {
        let result = (x * 2) / y;
        Ok(result.into())
    }
}

fn main() {
    let mut engine = Engine::new();

    // Overloads the operator '+' with the Engine.
    register_exported_fn!(engine, "+", double_and_divide);
}