Export a Rust Module to Rhai

Import Prelude

When using the plugins system, the entire rhai::plugin module must be imported as a prelude because code generated will need these imports.

use rhai::plugin::*;

#[export_module]

When applied to a Rust module, the #[export_module] attribute generates the necessary code and metadata to allow Rhai access to its public (i.e. marked pub) functions, constants, type aliases, and sub-modules.

This code is exactly what would need to be written by hand to achieve the same goal, and is custom fit to each exported item.

All pub functions become registered functions, constants become module constants, type aliases become custom types, and sub-modules become Rhai sub-modules.

use rhai::plugin::*;        // a "prelude" import for macros

// My custom type
pub struct TestStruct {
    pub value: i64
}

#[export_module]
mod my_module {
    // This type alias will register the friendly name 'ABC' for the
    // custom type 'TestStruct'.
    pub type ABC = TestStruct;

    // This constant will be registered as the constant variable 'MY_NUMBER'.
    // Ignored when registered as a global module.
    pub const MY_NUMBER: i64 = 42;

    // This function will be registered as 'greet'
    // but is only available with the 'greetings' feature.
    #[cfg(feature = "greetings")]
    pub fn greet(name: &str) -> String {
        format!("hello, {}!", name)
    }
    /// This function will be registered as 'get_num'.
    ///
    /// If this is a Rust doc-comment, then it is included in the metadata.
    pub fn get_num() -> i64 {
        mystic_number()
    }
    /// This function will be registered as 'create_abc'.
    pub fn create_abc(value: i64) -> ABC {
        ABC { value }
    }
    /// This function will be registered as the 'value' property of type 'ABC'.
    #[rhai_fn(get = "value")]
    pub fn get_value(ts: &mut ABC) -> i64 {
        ts.value
    }
    // This function will be registered as 'increment'.
    // It will also be exposed to the global namespace since 'global' is set.
    #[rhai_fn(global)]
    pub fn increment(ts: &mut ABC) {
        ts.value += 1;
    }
    // This function is not 'pub', so NOT registered.
    fn mystic_number() -> i64 {
        42
    }

    // Sub-modules are ignored when the module is registered globally.
    pub mod my_sub_module {
        // This function is ignored when registered globally.
        // Otherwise it is a valid registered function under a sub-module.
        pub fn get_info() -> String {
            "hello".to_string()
        }
    }

    // Sub-modules are commonly used to put feature gates on a group of
    // functions because feature gates cannot be put on function definitions.
    // This is currently a limitation of the plugin procedural macros.
    #[cfg(feature = "advanced_functions")]
    pub mod advanced {
        // This function is ignored when registered globally.
        // Otherwise it is a valid registered function under a sub-module
        // which only exists when the 'advanced_functions' feature is used.
        pub fn advanced_calc(input: i64) -> i64 {
            input * 2
        }
    }
}

If the [`metadata`] feature is active, doc-comments (i.e. comments starting with `///` or wrapped
with `/**` ... `*/`) on plugin functions are extracted into [_metadata_][functions metadata].

It is always a good idea to put [doc-comments] onto [plugin modules] and [plugin functions],
as they can be used to auto-generate documentation later on.

Usage

The plugin module can be registered into an Engine as a normal module.

This is usually done via the exported_module! macro.

The macro combine_with_exported_module! can also be used to combine all the functions and variables into an existing module, flattening the namespace – i.e. all sub-modules are eliminated and their contents promoted to the top level. This is typical for developing custom packages.

Register with Engine::register_global_module

The simplest way to register the plugin module into an Engine is:

1. use the exported_module! macro to turn it into a normal Rhai module,
2. call Engine::register_global_module to register it

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

    // The macro call creates a Rhai module from the plugin module.
    let module = exported_module!(my_module);

    // A module can simply be registered into the global namespace.
    engine.register_global_module(module.into());
}

The functions contained within the module definition (i.e. greet, get_num, create_abc and increment, the value property getter), and the TestStruct custom type (with friendly name ABC) are automatically registered into the Engine when Engine::register_global_module is called.

let x = greet("world");
x == "hello, world!";

let x = greet(get_num().to_string());
x == "hello, 42!";

let x = get_num();
x == 42;

let abc = create_abc(x);

type_of(abc) == "ABC";

abc.value == 42;

abc.increment();

abc.value == 43;

When using a [module] as a [package], only functions registered at the _top level_ can be accessed.

Variables as well as sub-modules are **ignored**.

Register with Engine::register_static_module

Another simple way to register the plugin module into an Engine is, again:

1. use the exported_module! macro to turn it into a normal Rhai module,
2. call Engine::register_static_module to register it under a particular module namespace

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

    // The macro call creates a Rhai module from the plugin module.
    let module = exported_module!(my_module);

    // A module can simply be registered as a static module namespace.
    engine.register_static_module("service", module.into());
}

The functions contained within the module definition (i.e. greet, get_num and increment), plus the constant MY_NUMBER, are automatically registered under the module namespace service:

let x = service::greet("world");
x == "hello, world!";

service::MY_NUMBER == 42;

let x = service::greet(service::get_num().to_string());
x == "hello, 42!";

let x = service::get_num();
x == 42;

let abc = service::create_abc(x);

type_of(abc) == "ABC";

abc.value == 42;

service::increment(abc);

abc.value == 43;

The default for all [getters/setters] and [indexers] defined in a [plugin module] is
`#[rhai_fn(global)]` unless specifically overridden by `#[rhai_fn(internal)]`.

All functions (usually methods) defined in the module and marked with #[rhai_fn(global)], all type iterators and all custom types are automatically exposed to the global namespace, so iteration, getters/setters and indexers for custom types can work as expected.

Therefore, in the example above, the increment method (defined with #[rhai_fn(global)]) works fine when called in method-call style:

let x = 42;
x.increment();
x == 43;

Load Dynamically

See the [module] section for more information.

Using this directly as a dynamically-loadable Rhai module is almost the same, except that a module resolver must be used to serve the module, and the module is loaded via import statements.

Combine into Custom Package

Finally, the plugin module can also be used to develop a custom package, using combine_with_exported_module! which automatically flattens the module namespace so that all functions in sub-modules are promoted to the top level namespace, all sub-modules are eliminated, and all variables are ignored.

Due to _flattening_, sub-modules are often used conveniently as a _grouping_ mechanism, especially to
put _feature gates_ or _compile-time gates_ (i.e. `#[cfg(...)]`) on a large collection of functions
without having to duplicate the gates onto each individual function.

#[export_module]
mod my_module {
    // Always available
    pub fn func0() {}

    // The following functions are only available under 'foo'.
    // Use a sub-module for convenience, since all functions underneath
    // will be flattened into the namespace.
    #[cfg(feature = "foo")]
    pub mod group_foo {
        pub fn func1() {}
        pub fn func2() {}
        pub fn func3() {}
    }

    // The following functions are only available under 'bar'
    #[cfg(feature = "bar")]
    pub mod group_bar {
        pub fn func4() {}
        pub fn func5() {}
        pub fn func6() {}
    }
}

// The above is equivalent to:
#[export_module]
mod my_module_alternate {
    pub fn func0() {}

    #[cfg(feature = "foo")]
    pub fn func1() {}
    #[cfg(feature = "foo")]
    pub fn func2() {}
    #[cfg(feature = "foo")]
    pub fn func3() {}

    #[cfg(feature = "bar")]
    pub fn func4() {}
    #[cfg(feature = "bar")]
    pub fn func5() {}
    #[cfg(feature = "bar")]
    pub fn func6() {}
}

// Registered functions:
//   func0 - always available
//   func1, func2, func3 - available under 'foo'
//   func4, func5, func6 - available under 'bar'
//   func0, func1, func2, func3, func4, func5, func6 - available under 'foo' and 'bar'
combine_with_exported_module!(module, "my_module_ID", my_module);

Functions Overloading and Operators

The _first_ parameter of a function can also be [`NativeCallContext`].

Operators and overloaded functions can be specified via applying the #[rhai_fn(name = "...")] attribute to individual functions.

The text string given as the name parameter to #[rhai_fn] is used to register the function with the Engine, disregarding the actual name of the function.

With #[rhai_fn(name = "...")], multiple functions may be registered under the same name in Rhai, so long as they have different parameters.

Operators (which require function names that are not valid for Rust) can also be registered this way.

Registering the same function name with the same parameter types will cause a parse error.

use rhai::plugin::*;        // a "prelude" import for macros

#[export_module]
mod my_module {
    // This is the '+' operator for 'TestStruct'.
    #[rhai_fn(name = "+")]
    pub fn add(obj: &mut TestStruct, value: i64) {
        obj.prop += value;
    }
    // This function is 'calc (i64)'.
    pub fn calc(num: i64) -> i64 {
        ...
    }
    // This function is 'calc (i64, bool)'.
    #[rhai_fn(name = "calc")]
    pub fn calc_with_option(num: i64, option: bool) -> i64 {
        ...
    }
}

Getters, Setters and Indexers

[Getters/setters] and [indexers] default to `#[rhai_fn(global)]` unless overridden by `#[rhai_fn(internal)]`.

Functions can be marked as getters/setters and indexers for custom types via the #[rhai_fn] attribute, which is applied on a function level.

use rhai::plugin::*;        // a "prelude" import for macros

#[export_module]
mod my_module {
    // This is a normal function 'greet'.
    pub fn greet(name: &str) -> String {
        format!("hello, {}!", name)
    }
    // This is a getter for 'TestStruct::prop'.
    #[rhai_fn(get = "prop", pure)]
    pub fn get_prop(obj: &mut TestStruct) -> i64 {
        obj.prop
    }
    // This is a setter for 'TestStruct::prop'.
    #[rhai_fn(set = "prop")]
    pub fn set_prop(obj: &mut TestStruct, value: i64) {
        obj.prop = value;
    }
    // This is an index getter for 'TestStruct'.
    #[rhai_fn(index_get)]
    pub fn get_index(obj: &mut TestStruct, index: i64) -> bool {
        obj.list[index]
    }
    // This is an index setter for 'TestStruct'.
    #[rhai_fn(index_set)]
    pub fn get_index(obj: &mut TestStruct, index: i64, state: bool) {
        obj.list[index] = state;
    }
}

Multiple Registrations

Parameters to the #[rhai_fn(...)] attribute can be applied multiple times, separated by commas.

Multiple registrations is useful for `name = "..."`, `get = "..."` and `set = "..."` to give
multiple alternative names to the same function.

use rhai::plugin::*;        // a "prelude" import for macros

#[export_module]
mod my_module {
    // This function can be called in five ways
    #[rhai_fn(name = "get_prop_value", name = "prop", name = "+", set = "prop", index_get)]
    pub fn prop_function(obj: &mut TestStruct, index: i64) -> i64 {
        obj.prop[index]
    }
}

The above function can be called in five ways:

Pure Functions

Apply the #[rhai_fn(pure)] attribute on a method function (i.e. one taking a &mut first parameter) to mark it as pure – i.e. it does not modify the &mut parameter.

This is often done to avoid expensive cloning for methods or property getters that return information about a custom type and does not modify it.

Pure functions _MUST NOT_ modify the `&mut` parameter.
There is no checking.

Non-pure functions raise a runtime error when passed a [constant] value as the first `&mut` parameter.

Pure functions can be passed a [constant] value as the first `&mut` parameter.

use rhai::plugin::*;        // a "prelude" import for macros

#[export_module]
mod my_module {
    // This function can be passed a constant
    #[rhai_fn(name = "add1", pure)]
    pub fn add_scaled(array: &mut rhai::Array, x: i64) -> i64 {
        array.iter().map(|v| v.as_int().unwrap()).fold(0, |(r, v)| r += v * x)
    }
    // This function CANNOT be passed a constant
    #[rhai_fn(name = "add2")]
    pub fn add_scaled2(array: &mut rhai::Array, x: i64) -> i64 {
        array.iter().map(|v| v.as_int().unwrap()).fold(0, |(r, v)| r += v * x)
    }
    // This getter can be applied to a constant
    #[rhai_fn(get = "first1", pure)]
    pub fn get_first(array: &mut rhai::Array) -> i64 {
        array[0]
    }
    // This getter CANNOT be applied to a constant
    #[rhai_fn(get = "first2")]
    pub fn get_first2(array: &mut rhai::Array) -> i64 {
        array[0]
    }
    // The following is a syntax error because a setter is SUPPOSED to
    // mutate the object.  Therefore the 'pure' attribute cannot be used.
    #[rhai_fn(get = "values", pure)]
    pub fn set_values(array: &mut rhai::Array, value: i64) {
        // ...
    }
}

When applied to a Rhai script:

// Constant
const VECTOR = [1, 2, 3, 4, 5, 6, 7];

let r = VECTOR.add1(2);     // ok!

let r = VECTOR.add2(2);     // runtime error: constant modified

let r = VECTOR.first1;      // ok!

let r = VECTOR.first2;      // runtime error: constant modified

Fallible Functions

To register fallible functions (i.e. functions that may return errors), apply the #[rhai_fn(return_raw)] attribute on functions that return Result<T, Box<EvalAltResult>> where T is any clonable type.

use rhai::plugin::*;        // a "prelude" import for macros

#[export_module]
mod my_module {
    /// This overloads the '/' operator for i64.
    #[rhai_fn(name = "/", return_raw)]
    pub fn double_and_divide(x: i64, y: i64) -> Result<i64, Box<EvalAltResult>> {
        if y == 0 {
            Err("Division by zero!".into())
        } else {
            Ok((x * 2) / y)
        }
    }
}

A compilation error — usually something that says `Result` does not implement
`Clone` — is generated if a fallible function is missing `#[rhai_fn(return_raw)]`.

It is another compilation error for the reverse — a function with
`#[rhai_fn(return_raw)]` does not have the appropriate return type.

#[export_module] Parameters

Parameters can be applied to the #[export_module] attribute to override its default behavior.

Inner Attributes

`#[rhai_fn]` is applied to functions, while `#[rhai_mod]` is applied to sub-modules.

Inner attributes can be applied to the inner items of a module to tweak the export process.

Parameters should be set on inner attributes to specify the desired behavior.