Object-Oriented Programming (OOP)

Rhai does not have objects per se and is not object-oriented (in the traditional sense), but it is possible to simulate object-oriented programming.

Use Object Maps to Simulate OOP

Rhai’s object maps has special support for OOP.

When a property of an object map is called like a method function, and if it happens to hold a valid function pointer (perhaps defined via an anonymous function or more commonly as a closure), then the call will be dispatched to the actual function with this binding to the object map itself.

Use Closures to Define Methods

Closures defined as values for object map properties take on a syntactic shape which resembles very closely that of class methods in an OOP language.

Closures also capture variables from the defining environment, which is a very common language feature. It can be turned off via the no_closure feature.

let factor = 1;

// Define the object
let obj = #{
        data: 0,                            // object field
        increment: |x| this.data += x,      // 'this' binds to 'obj'
        update: |x| this.data = x * factor, // 'this' binds to 'obj', 'factor' is captured
        action: || print(this.data)         // 'this' binds to 'obj'
    };

// Use the object
obj.increment(1);
obj.action();                               // prints 1

obj.update(42);
obj.action();                               // prints 42

factor = 2;

obj.update(42);
obj.action();                               // prints 84

Simulating Inheritance with Polyfills

The fill_with method of object maps can be conveniently used to polyfill default method implementations from a base class, as per OOP lingo.

Do not use the mixin method because it overwrites existing fields.

// Define base class
let BaseClass = #{
    factor: 1,
    data: 42,

    get_data: || this.data * 2,
    update: |x| this.data += x * this.factor
};

let obj = #{
    // Override base class field
    factor: 100,

    // Override base class method
    // Notice that the base class can also be accessed, if in scope
    get_data: || this.call(BaseClass.get_data) * 999,
}

// Polyfill missing fields/methods
obj.fill_with(BaseClass);

// By this point, 'obj' has the following:
//
// #{
//      factor: 100
//      data: 42,
//      get_data: || this.call(BaseClass.get_data) * 999,
//      update: |x| this.data += x * this.factor
// }

// obj.get_data() => (this.data (42) * 2) * 999
obj.get_data() == 83916;

obj.update(1);

obj.data == 142

Prototypical Inheritance via Mixin

Some languages like JavaScript has prototypical inheritance, which bases inheritance on a prototype object.

It is possible to simulate this form of inheritance using object maps, leveraging the fact that, in Rhai, all values are cloned and there are no pointers. This significantly simplifies coding logic.

// Define prototype 'class'

const PrototypeClass = #{
    field: 42,

    get_field: || this.field,
    set_field: |x| this.field = x
};

// Create instances of the 'class'

let obj1 = PrototypeClass;                  // a copy of 'PrototypeClass'

obj1.get_field() == 42;

let obj2 = PrototypeClass;                  // a copy of 'PrototypeClass'

obj2.mixin(#{                               // override fields and methods
    field: 1,
    get_field: || this.field * 2
};

obj2.get_field() == 2;

let obj2 = PrototypeClass + #{              // compact syntax with '+'
    field: 1,
    get_field: || this.field * 2
};

obj2.get_field() == 2;

// Inheritance chain

const ParentClass = #{
    field: 123,
    new_field: 0,
    action: || print(this.new_field * this.field)
};

const ChildClass = #{
    action: || {
        this.field = this.new_field;
        this.new_field = ();
    }
}

let obj3 = PrototypeClass + ParentClass + ChildClass;

// Alternate formulation

const ParentClass = PrototypeClass + #{
    field: 123,
    new_field: 0,
    action: || print(this.new_field * this.field)
};

const ChildClass = ParentClass + #{
    action: || {
        this.field = this.new_field;
        this.new_field = ();
    }
}

let obj3 = ChildClass;                      // a copy of 'ChildClass'