[swift-users] Swift's method dispatch

[Jordan Rose]

Consider this example:

protocol P {
    associatedtype T
    func f() // *
}
extension P {
    func f() { print("T is unknown") }
}
extension P where T == Int {
    func f() { print("T is Int") }
}

struct X<T> : P {
    func f() { print("this one is actually best") }
}

struct Y<U> where U: P, U.T == Int {
    typealias T = U.T
    var a: U
    func g() { a.f() }
}

let x = X<Int>()
x.f() // "this one is actually best"

let y = Y(a: X<Int>())
y.g() // "this one is actually best"

I can't think of any other choice for 'a.f()' that would preserve this behavior, which means we're doing the best thing: prefer dynamic dispatch over static dispatch when operating on a generic value. (Or at least the "least bad" thing.) And of course this reasoning has to be local; Y.g can't look and say "oh, I know nothing else conforms to P, and X {does, doesn't} have a custom implementation, so I should pick the constrained extension instead".

The real answer might be "we should have had a different syntax for default implementations vs. mixin operations", but that's a much bigger can of worms.

Jordan


> On Dec 8, 2017, at 13:07, Jens Persson via swift-users <swift-users at swift.org> wrote:
> 
> Thanks Slava and Greg,
> 
> (
> I'm aware that it prints "T is Int" from both calls if I remove func f() from P itself, that's why I wrote "... unless * is commented out." in the comment of the last line
> Note that the "U.T == Int"-part of 
>   struct Y<U> where U: P, U.T == Int {
> is key here. If it had been only
>   struct Y<U> where U: P {
> then I hadn't been surprised that it printed "T is unknown".
> )
> 
> Filed https://bugs.swift.org/browse/SR-6564 <https://bugs.swift.org/browse/SR-6564> since I think it is just strange that the compiler should not use its knowledge of U.T == Int when choosing between the two f()-implementations.
> I think I will be a little disappointed if the solution is to deem it an ambiguity
> : )
> 
> /Jens
> 
> 
> 
> On Fri, Dec 8, 2017 at 9:19 PM, Greg Parker <gparker at apple.com <mailto:gparker at apple.com>> wrote:
> Evidence in favor of Slava's analysis: if you remove `func f()` from P itself, leaving it in the extensions only, then you get "T is Int" from both calls.
> 
> 
> > On Dec 8, 2017, at 12:12 PM, Slava Pestov via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
> >
> > Hi Jens,
> >
> > I think the problem is that overload ranking always prefers a protocol requirement to a protocol extension member, because usually you want the dynamic dispatch through the requirement instead of calling the default implementation. But it appears that this heuristic does not take into account the fact that the protocol extension member could be more constrained than the requirement.
> >
> > Please file a bug, but it is unclear what the desired behavior actually is here. Perhaps it should just diagnose an ambiguity.
> >
> > Slava
> >
> >> On Dec 8, 2017, at 6:25 AM, Jens Persson via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
> >>
> >> Hi all!
> >>
> >> Can someone please explain the rationale behind the last line printing
> >> "T is unknown"
> >> rather than (what I would expect):
> >> "T is Int"
> >> in the following program?
> >>
> >>
> >> protocol P {
> >>    associatedtype T
> >>    func f() // *
> >> }
> >> extension P {
> >>    func f() { print("T is unknown") }
> >> }
> >> extension P where T == Int {
> >>    func f() { print("T is Int") }
> >> }
> >>
> >> struct X<T> : P {}
> >>
> >> struct Y<U> where U: P, U.T == Int {
> >>    // NOTE: The compiler/type-checker knows that U.T == Int here so ...
> >>    typealias T = U.T
> >>    var a: U
> >>    func g() { a.f() } // ... how/why could this print anything but "T is Int"?
> >> }
> >>
> >> let x = X<Int>()
> >> x.f() // Prints "T is Int", no matter if * is commented out or not.
> >>
> >> let y = Y(a: X<Int>())
> >> y.g() // Prints "T is unknown" unless * is commented out. Why?
> >>
> >>
> >> IMHO this looks like the compiler simply ignores that struct Y<U> has the constraint  U.T == Int.
> >> How else to explain this behavior?
> >> /Jens
> >>
> >> _______________________________________________
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> >> swift-users at swift.org <mailto:swift-users at swift.org>
> >> https://lists.swift.org/mailman/listinfo/swift-users <https://lists.swift.org/mailman/listinfo/swift-users>
> >
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> 
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