Rhai supports defining functions in script via the fn keyword.

Valid function names are the same as valid variable names.

fn add(x, y) {
    x + y

fn sub(x, y,) {     // trailing comma in parameters list is OK
    x - y

add(2, 3) == 5;

sub(2, 3,) == -1;   // trailing comma in arguments list is OK

Tip: is_def_fn

Use is_def_fn to detect if a Rhai function is defined (and therefore callable) based on its name and the number of parameters (arity).

fn foo(x) { x + 1 }

is_def_fn("foo", 1) == true;

is_def_fn("foo", 0) == false;

is_def_fn("foo", 2) == false;

is_def_fn("bar", 1) == false;

Implicit Return

The last statement of a block is always the block’s return value regardless of whether it is terminated with a semicolon ;.

fn add(x, y) {      // implicit return:
    x + y;          // value of the last statement (no need for ending semicolon)
                    // is used as the return value

fn add2(x) {
    return x + 2;   // explicit return

add(2, 3) == 5;

add2(42) == 44;

Global Definitions Only

Functions can only be defined at the global level, never inside a block or another function.

// Global level is OK
fn add(x, y) {
    x + y

// The following will not compile
fn do_addition(x) {
    fn add_y(n) {   // <- syntax error:  cannot define inside another function
        n + y


No Access to External Scope

Functions are not closures. They do not capture the calling environment and can only access their own parameters.

They cannot access variables external to the function itself.

let x = 42;

fn foo() {
    x               // <- error: variable 'x' not found

But Can Call Other Functions and Access Modules

All functions can call each other.

fn foo(x) {         // function defined in the global namespace
    x + 1

fn bar(x) {
    foo(x)          // ok! function 'foo' can be called

In addition, modules imported at global level can be accessed.

import "hello" as hey;
import "world" as woo;

    import "x" as xyz;  // <- this module is not at global level
}                       // <- it goes away here

fn foo(x) {
    hey::process(x);    // ok! imported module 'hey' can be accessed

    print(woo::value);  // ok! imported module 'woo' can be accessed

    xyz::do_work();     // <- error: module 'xyz' not found

Automatic Global Module

When a constant is declared at global scope, it is added to a special module called global.

Functions can access those constants via the special global module.

const CONSTANT = 42;        // this constant is automatically added to 'global'

let hello = 1;              // variables are not added to 'global'

    const INNER = 0;        // this constant is not at global level
}                           // <- it goes away here

fn foo(x) {
    x * global::hello       // <- error: variable 'hello' not found in 'global'

    x * global::CONSTANT    // ok! 'CONSTANT' exists in 'global'

    x * global::INNER       // <- error: constant 'INNER' not found in 'global'

Use Before Definition Allowed

Unlike C/C++, functions in Rhai can be defined anywhere at global level.

A function does not need to be defined prior to being used in a script; a statement in the script can freely call a function defined afterwards.

This is similar to Rust and many other modern languages, such as JavaScript’s function keyword.

let x = foo(41);    // <- I can do this!

fn foo(x) {         // <- define 'foo' after use
    x + 1

Arguments are Passed by Value

Functions with the same name and same number of parameters are equivalent.

All arguments are passed by value, so all Rhai script-defined functions are pure (i.e. they never modify their arguments).

Any update to an argument will not be reflected back to the caller.

fn change(s) {      // 's' is passed by value
    s = 42;         // only a COPY of 's' is changed

let x = 500;


x == 500;           // 'x' is NOT changed!

Rhai functions are pure

The only possibility for a Rhai script-defined function to modify an external variable is via the this pointer.