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@amberstar amberstar/Copyable.swift
Last active Feb 3, 2018

What would you like to do?
protocol Copyable {
typealias CopyType
func copy() -> CopyType
class A: Copyable {
var x: Int
init(x: Int) { self.x = x }
func copy() -> A {
return A(x: self.x)
class B: A {
var y : Int
init(x: Int, y: Int) {
self.y = y
super.init(x: x)
override func copy() -> B {
return B(x: self.x, y: self.y)
let a = A(x: 1)
let b = a.copy()
let c = B(x: 1, y: 2)
let d = c.copy()

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rnapier commented Nov 8, 2015

Trouble is that CopyType can be anything. So to use Copyable in a useful way, we have to put it in a generic with a constraint of "C: Copyable where C.CopyType == C". Since it has an associated type, you can't accept a parameter of type "Copyable."

Your current code would work just as well if Copyable didn't exist; it's really just methods that happen to be on the classes. I suspect in practice this will make it not worth the trouble. Still, the closest to useful I've seen so far.

Of course I've been spending a lot of time thinking about what I'd use Copyable for. I keep feeling like I need it, but I don't think it's ever shown up in pure Swift code, and when bridging to ObjC I have to use NSCopying in practice anyway.


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amberstar commented Nov 8, 2015

For sure. It's not useful in practice. A type requirement of Self in a protocol should travel down the class inheritance chain. I don't think it makes any practical sense the way it behaves now.

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