Add methods with an extension

An extension adds methods to an existing structure or class without modifying its fields or identity.

struct Counter {
    var value:int
}

extend Counter {
    func increment(amount:int = 1) {
        self.value += amount
    }

    static func zero() Counter {
        return Counter(value:0)
    }
}

func main() {
    var counter = Counter.zero()
    counter.increment(2)
    print(counter.value)
}

The program prints 2. Fields come from the original type; the extension adds only increment and zero.

An extension cannot add a field, constructor, drop, override, or protected member.

Importing an extension's module activates its public methods. Placing the file next to the target type is not enough. A qualified reference can also load a child module on demand: after use STD.Math, using Math.Vec3 loads that child and activates the extensions it declares. Unused children are never loaded eagerly merely because their parent namespace is available.

Add a conformance

extend Sprite:Drawable {
    func draw() {
        print("sprite")
    }
}

The conformance is visible in files that activate the extension provider with use, directly or transitively.

An extension targets an existing non-generic structure or class. An extension method on that type may still declare its own type parameters.

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