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

( arg , )

FuncArgs Trait

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) {
        container.extend(once(self.foo.into()));
        container.extend(once(self.bar.into()));
        container.extend(once(self.baz.into()));
    }
}

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)?;

Engine::call_fn_with_options

For more control, use Engine::call_fn_with_options, which takes a type CallFnOptions:

use rhai::{Engine, CallFnOptions};

let options = CallFnOptions::new()
                .eval_ast(false)            // do not evaluate the AST
                .rewind_scope(false)        // do not rewind the scope (i.e. keep new variables)
                .bind_this_ptr(&mut state); // 'this' pointer

let result = engine.call_fn_with_options::<i64>(
                options,                    // options
                &mut scope,                 // scope to use
                &ast,                       // AST containing the functions
                "hello",                    // function entry-point
                ( "abc", 123_i64 )          // arguments
             )?;

CallFnOptions allows control of the following:

Skip evaluation of the AST

By default, the AST is evaluated before calling the target function.

Setting eval_ast to false skips 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 prevents the Scope from being continuously polluted by new variables and is usually the intuitively expected behavior.

Setting rewind_scope to false retains 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.

┌─────────────┐
│ 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 │
└──────┘

let options = CallFnOptions::new().rewind_scope(false);

engine.call_fn_with_options(options, &mut scope, &ast, "initialize", ())?;

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

Bind the this pointer

CallFnOptions 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();

let options = CallFnOptions::new()
                .eval_ast(false)
                .rewind_scope(false)
                .bind_this_ptr(&mut value);

engine.call_fn_with_options(options, &mut scope, &ast, "action", ( 41_i64, ))?;

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