Register any Rust Type and its Methods

Free Typing

Why “Custom”?

Rhai internally supports a number of standard data types (see this list).

Any type outside of the list is considered custom.

Rhai works seamlessly with any Rust type.

A custom type is stored in Rhai as a Rust trait object (specifically, a dyn rhai::Variant), with no restrictions other than being Clone (plus Send + Sync under the sync feature).

The type literally does not have any prerequisite other than being Clone.

It does not need to implement any other trait or use any custom #[derive].

This allows Rhai to be integrated into an existing Rust code base with as little plumbing as possible, usually silently and seamlessly.

External types that are not defined within the same crate (and thus cannot implement special Rhai traits or use special #[derive]) can also be used easily with Rhai.

Support for custom types can be turned off via the no_object feature.

Custom types are slower

Custom types run slower than built-in types due to an additional level of indirection, but for all other purposes there is no difference.

Register a Custom Type and its Methods

Tip: Working with enums

It is also possible to use Rust enums with Rhai.

See the pattern Working with Enums for more details.

Any custom type must implement the Clone trait as this allows the Engine to pass by value.

If the sync feature is used, it must also be Send + Sync.

Notice that the custom type needs to be registered using Engine::register_type or Engine::register_type_with_name.

To use native methods on custom types in Rhai scripts, it is common to register an API for the type via the Engine::register_XXX API.

use rhai::{Engine, EvalAltResult};

#[derive(Debug, Clone)]
struct TestStruct {
    field: i64

impl TestStruct {
    fn new() -> Self {
        Self { field: 1 }

    fn update(&mut self, x: i64) {      // methods take &mut as first parameter
        self.field += x;

let mut engine = Engine::new();

// Most Engine API's can be chained up.
engine.register_type::<TestStruct>()    // register custom type
      .register_fn("new_ts", TestStruct::new)
      .register_fn("update", TestStruct::update);

// Cast result back to custom type.
let result = engine.eval::<TestStruct>(
    let x = new_ts();                   // calls 'TestStruct::new'
    x.update(41);                       // calls 'TestStruct::update'
    x                                   // 'x' holds a 'TestStruct'

println!("result: {}", result.field);   // prints 42

First Parameter Must be &mut

Methods of custom types take a &mut first parameter to that type, so that invoking methods can always update it.

All other parameters in Rhai are passed by value (i.e. clones).

No support for references

Rhai does NOT support normal references (i.e. &T) as parameters. All references must be mutable (i.e. &mut T).

type_of() a Custom Type

type_of() works fine with custom types and returns the name of the type.

If Engine::register_type_with_name is used to register the custom type with a special “pretty-print” name, type_of() will return that name instead.

      .register_fn("new_ts1", TestStruct1::new)
      .register_fn("new_ts2", TestStruct2::new);

let ts1_type = engine.eval::<String>("let x = new_ts1(); x.type_of()")?;
let ts2_type = engine.eval::<String>("let x = new_ts2(); x.type_of()")?;

println!("{}", ts1_type);               // prints 'path::to::TestStruct'
println!("{}", ts1_type);               // prints 'TestStruct'

== Operator

Many standard functions (e.g. filtering, searching and sorting) expect a custom type to be comparable, meaning that the == operator must be registered for the custom type.

For example, in order to use the in operator with a custom type for an array, the == operator is used to check whether two values are the same.

// Assume 'TestStruct' implements `PartialEq`
    |item1: &mut TestStruct, item2: TestStruct| item1 == &item2

// Then this works in Rhai:
let item = new_ts();        // construct a new 'TestStruct'
item in array;              // 'in' operator uses '=='