Define a contract with a protocol

A protocol declares the methods that a conforming type must provide.

protocol Drawable {
    func draw()
}

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

func main() {
    var drawable:Drawable = Sprite()
    drawable.draw()
}

Conformance is nominal: the type must name the protocol. A method with the right form is not sufficient by itself.

Require a property

A protocol may require reading, or both reading and writing, without imposing storage:

protocol Named {
    name:str { get }
}

protocol Renamable {
    name:str { get set }
}

A compatible let or var field satisfies { get }. A var field or var property satisfies { get set }; its setter may be implicit or explicit. let and var are not allowed in the protocol itself. See the complete property contract.

Store a conforming value

var drawable:Drawable = Sprite()
drawable.draw()

A protocol value exposes only the protocol requirements. A structure is copied into this dynamic value; a class preserves its shared identity. Calls through a protocol value are treated as mutable, so a direct receiver is declared with var.

Constrain a specialization

func render<T:Drawable>(value:T) {
    value.draw()
}

render(Sprite())

The compiler specializes the function for the concrete type. At this call site, the protocol therefore creates no erased container. A type parameter accepts at most one protocol constraint.

A class may name one base class first, then protocols. A structure names only protocols. A derived class inherits its base's valid conformances.

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