Create a Module from an AST


A module can be created from a single script (or pre-compiled AST) containing global variables, functions and sub-modules via the Module::eval_ast_as_new method.

See the section on Exporting Variables, Functions and Sub-Modules for details on how to prepare a Rhai script for this purpose as well as to control which functions/variables to export.

When given an AST, it is first evaluated, then the following items are exposed as members of the new module:

  • Global variables – all variables exported via the export statement (those not exported remain hidden).

  • Functions not specifically marked private.

  • Global modules that remain in the Scope at the end of a script run (become sub-modules).

Module::eval_ast_as_new encapsulates the entire AST into each function call, merging the module namespace with the global namespace. Therefore, functions defined within the same module script can cross-call each other.


Don’t forget the export statement, otherwise there will be no variables exposed by the module other than non-private functions (unless that’s intentional).

fn main() {
use rhai::{Engine, Module};

let engine = Engine::new();

// Compile a script into an 'AST'
let ast = engine.compile(r#"
    // Functions become module functions
    fn calc(x) {
        x + 1
    fn add_len(x, y) {
        x + y.len

    // Imported modules can become sub-modules
    import "another module" as extra;

    // Variables defined at global level can become module variables
    const x = 123;
    let foo = 41;
    let hello;

    // Variable values become constant module variable values
    foo = calc(foo);
    hello = "hello, " + foo + " worlds!";

    // Finally, export the variables and modules
        x as abc,           // aliased variable name
        extra as foobar;    // export sub-module

// Convert the 'AST' into a module, using the 'Engine' to evaluate it first
// A copy of the entire 'AST' is encapsulated into each function,
// allowing functions in the module script to cross-call each other.
let module = Module::eval_ast_as_new(Scope::new(), &ast, &engine)?;

// 'module' now contains:
//   - sub-module: 'foobar' (renamed from 'extra')
//   - functions: 'calc', 'add_len'
//   - constants: 'abc' (renamed from 'x'), 'foo', 'hello'