Combining Objects

Favor object composition over class inheritance.

— The “Gang of Four”, Design Patterns

Object Composition 

THT supports a style of object-oriented programming that relies on object composition — the technique of combining smaller objects to create bigger, more complex objects.

You can combine objects in any way — even at run-time — while maintaining a clean separation between the individual objects (i.e. encapsulation).

Example

For example, a Car object might contain an Engine object.

In turn, the Engine might be made up of sub-objects for FuelIntake, Ignition, SpinnyThing etc.

(I clearly don’t know car stuff.)

All of the sub-objects work together as one system, but each part is self-contained.

+------------------------+
|          Car           |
|                        |
|  +------------------+  |
|  |      Engine      |  |
|  |                  |  |
|  | [] FuelIntake    |  |
|  | [] Ignition      |  |
|  | [] SpinnyThing   |  |
|  |                  |  |
|  +------------------+  |
+------------------------+

How It Works 

To compose objects, simply assign a sub-object to a field.

In the car example, we will assign an Engine object to the Car object.

// modules/Car.tht
//----------------------------

class Car {

    public fields {
        engine: Engine({ acceleration: 5 })
        speed: 0
    }

    public fun drive {
        @.speed += @.engine.accelerate()
        print('Speed: ' ~ @.speed)
    }
}

// modules/Engine.tht
//----------------------------

class Engine {

    public fields {
        acceleration: 2
    }

    public fun accelerate {
        print('Vroooom!')
        return @.acceleration
    }
}

And then to use them:

$car = Car()
$car.drive()

//= 'Vroooom!'
//= Speed: 5

Because Engine is a public field, you could pass a new one in to initializer.

$fastCar = Car({
    engine: Engine({ acceleration: 20 })
})

$fastCar.drive()
//= 'Vroooom!'
//= Speed: 20

$fastCar.drive()
//= 'Vroooom!'
//= Speed: 40

What About Inheritance?