On 17.09.2016 6:32, Xiaodi Wu via swift-evolution wrote:

Let me give a concrete example of how retroactively modeling is used.

Karl is suggesting interesting but complex and IMO too much code-breaking idea that I don't believe can be implemented at all in a reasonable amount of time to be a part of Swift as soon as possible, to address the discussed issue with protocols.

I wonder what objections could be made on the solution proposed below, which should solve a major(IMO) number of issues with protocol conformance and introduce only 1 keyword. Such solution will make Swift better as Protocol-Oriented language and later we can even improve it, but it can already solve a big number of issues:

1. As soon as possible we add 'implement' keyword which is required to mark method/property that was defined in type or extension exactly to conform to some protocol.

2. The 'implement' required only at a moment of 'direct' conformance, i.e. when you declare methods/props of the type/extension that explicitly conformed to protocol.

3. Retrospective conformance will not require this keyword and will work for now just like it is working today.

4. Later, if this will be possible at all, we can extend this model to support separate implementation of protocols with same requirements in the same type, explicit protocol name in implemented methods/props and improvements for retrospective conformance. For example some variants for *future* improvements:

4.1 Different implementation for different protocols
class Foo : ProtocolA, ProtocolB {
  implement(ProtocolA) func foo() {...}
  implement(ProtocolB) func foo() {...}
class Foo : ProtocolA, ProtocolB {
  implement ProtocolA {
        func foo() {...}
  implement ProtocolB {
        func foo() {...}

4.2 Retrospective conformance: What is the main problem with retrospective conformance? As I see it now(correct me, if I missing something), the problem arises in such situation: * we *expect* that some method(s) in type will play the role of implementation of protocol's requirements, so we retrospectively conform that type to the protocol.
* but protocol has default implementation for its requirements
* and type's methods, that we *expect* to play roles for protocol implementation, has different parameters or slightly different method name at all.

I.e. when we have this set of code logic:

type T {
  func foo()

protocol P {
  func foo(x: Int)

extension P {
  func foo(x: Int) {...}

extension T : P { // expect foo in T will play role of P.foo

I support the opinion that it is not an option to require to explicitly list conformed methods/props in type extension for retrospective conformance. But I do believe we need a way to *express our intention* regarding the retrospective conformance: do we expect that type already contains implementation for some protocol's requirements OR we are aware that protocol can have defaults for some methods and our type does not contains some implementations.

So, the solution here IMO is some syntax to express that intention. Right now I think that we can use current syntax "extension T : P" to keep it working as it now works: "I'm aware of all the names, defaults etc. Treat this as usually you did". But for example something like "extension T: implement P {..}" or "extension T: P(implement *) {..}" will say that we *expect* that all requirements of P protocol should be implemented inside T type. Or some syntax inside extension to specify the list of methods/props we expect to be implemented in T. Or "extension T : P(implement foo, bar(x:y:)) {..}".. Should be discussed.

But again, IMO this could be discussed later, after we'll have 'implement' for most important place - in type definition for method/prop that we created exactly for the conformed protocol.


Currently, there is a JIRA bug that Set does not conform to SetAlgebra. To
fix this issue, someone simply needs to write `extension Set : SetAlgebra {
}` and some tests. That's literally what the bug (filed by a core team
member) tells you to do. It's a starter bug, and if someone hasn't taken it
yet, you the reader could have a go at it. What's neat about Swift is that
it's super easy to provide the same functionality in your own project
without waiting on that bug to be fixed in Swift itself. You can simply
write a single line of code. By contrast, if your proposal were to be
implemented, this would become much more difficult.

This is actively used in Swift today. For example, in the Swift
implementation of NSScanner, you'll find the following lines:

internal protocol _BitShiftable {
    static func >>(lhs: Self, rhs: Self) -> Self
    static func <<(lhs: Self, rhs: Self) -> Self

internal protocol _IntegerLike : Integer, _BitShiftable {
    init(_ value: Int)
    static var max: Self { get }
    static var min: Self { get }

extension Int : _IntegerLike { }
extension Int32 : _IntegerLike { }
extension Int64 : _IntegerLike { }
extension UInt32 : _IntegerLike { }
extension UInt64 : _IntegerLike { }

If we adopted your proposed syntax below, it would take considerably more
lines of boilerplate code to express the same thing. The burden increases
significantly with the complexity of the retroactive modeling. For
instance, if the retroactively modeled protocol had 20 requirements and you
were retroactively conforming 20 types, that'd be at least 400 lines of

    Basically, the way I see it, if my class MyClass implements MyProtocol,
    providing someRequiredFunc(), there’s an “ownership” chain there
    (reading it backwards).

    Now what happens if MyClass implements MyOtherProtocol, which also has
    someRequiredFunc()? In that case, I want to MyClass as a
    MyOtherProtocol and get another function pointer, which just happens to
    have the same human-readable name as some other property. Just because
    they have the same function signature, absolutely doesn’t mean they’re
    the same thing.

    Now, if we strongly bind all protocol conformances to the protocol they
    implement, what happens to instance methods? They don’t belong to any
    protocol, their parent is the class itself. If you have an instance
    method called someRequiredFunc(), and you later add a conformance to
    MyProtocol, you would need to declare that it belongs to MyProtocol. If
    you don’t want it to be an API-breaking change, you have to provide a
    thunk (or we could provide a shorthand syntax which emits thunks for
    you) to let us know that MyClass::someRequiredFunc() is the same thing
    as MyClass::MyProtocol::someRequiredFunc().

Your argument is that two methods with the same name should not in any way
conflict with each other. This is a fundamental change from the status quo.
If we were to take your argument to its logical conclusion, any member A of
a type T should be capable of being designated as the implementation of a
requirement B of protocol P. In the most general case, two functions A and
B shouldn't even need to take the same number of arguments, or arguments of
the same type; you should be able to supply default arguments, or even
write custom code that takes arguments for A and computes suitable
arguments for B in order to forward A to B, and the language should allow
you to designate A as an implementation of B. But that is simply not how
Swift protocols are designed.

    Let’s take an example where retroactive modelling could go wrong.
    You’ve got different teams working on different parts of an App, and
    they’ve all got their own convention for “copy()”. Sometimes it’s a
    deep-copy, sometimes a shallow-copy, sometimes it’s used in a fragile
    way for a specific case, whatever. Now you want to go and clean that up
    by creating a “Copyable” protocol with codified guarantees. Some
    objects may already conform, others may need tweaks, and some may want
    both behaviours simultaneously (preserving the old,
    non-Copytable-compliant behaviour until the next API break), depending
    on how you look at the object. A system like this allows all of those
    different ways of looking at the object live together. You could have
    the old, non-comforming API as an extension with a FIXME to delete it
    for version 2.

Even if you design a protocol called Copyable, you still need to explicitly
extend concrete types in order to conform to Copyable. Swift does not
automagically make anything conform to your protocol. If you choose
*explicitly* to conform different types that don't guarantee the same
semantics, and then you erroneously assume that they all have the same
semantics even though you *explicitly* chose types that don't have the same
semantics, you're the one who shot yourself in the foot, so to speak. It's
not the fault of Swift at all.

    I think it’s pretty arcane that members of a type are resolved only by
    their names. If you want to provide types which allow flexible views of
    data, each view of that data needs to be completely free in its

    I would actually like to see a syntax like:

    let testObject = MyClass()
    let testMyProto = testObject.MyProtocol // the protocol-witness table
    for testObject as a MyProtocol.

    testObject.MyProtocol.someRequiredFunc() // that’s one function
    testObject.someRequiredFunc() // is a different function. May happen to
    have the same implementation as above if MyProtocol was retroactively

    I think it would fit well with the dispatch system for protocol
    extensions, too. I sometimes have code like the following:

    protocol Base {}
    protocol Derived : Base {}

    extension Base {
      func doSomething() { … }
    extension Derived {
      func doSomething() {
       (self as Base).doSomething() // Would be better if we could say
    “self.Base.doSomething()” to disambiguate instead of casting.

This is a complete redesign of protocols in Swift. With the emphasis on
minimizing source-breaking changes, I doubt such a change would be in scope
for any phase of Swift unless you could show an overwhelming benefit.

    So yeah, a big +1 to marking protocol methods with their protocol
    (whatever the syntax ends up looking like), and actually I’d take it
    further and bake them in to the ABI. That also makes it relevant for
    Swift 4 phase 1.


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