http://tpolecat.github.io/2015/04/29/f-bounds.html

currenttype.swift

import Cocoa

protocol Pet {
    var name : String { get }
    func renamed(newName: String) -> Self
}

struct Fish : Pet {
    let name : String
    func renamed(newName: String) -> Fish {
        return Fish(name: newName)
    }
}

struct Kitty : Pet {
    let name : String
    func renamed(newName: String) -> Kitty {
        return Kitty(name: newName)
    }
}

func esquire(a: Pet) -> Pet {
    return a.renamed(a.name + ", Esq.")
}

let bob = Fish(name: "Bob")
let thor = Kitty(name: "Thor")
let pets : [Pet] = [bob, thor]
let esquires = pets.map { esquire($0) }