The proposal looks good to me with one possible concern. I'm leaning toward types that use the defaults should still require the angle brackets, X<>. This makes it clear that you're using a generic type. That leads me to think that the examples Doug gave should be an error as the explicit types on the `let`s should either be omitted completely or fully specified (as X<>, X<Double>, X<Int>, etc.).
On Mon, Jan 23, 2017 at 1:21 PM, Joe Groff via swift-evolution < [email protected]> wrote: > > On Jan 23, 2017, at 9:51 AM, Douglas Gregor via swift-evolution < > [email protected]> wrote: > > > On Jan 23, 2017, at 7:55 AM, Srđan Rašić via swift-evolution < > [email protected]> wrote: > > Hi Everyone, > > I've opened a PR (https://github.com/apple/swift-evolution/pull/591) proposing > default generic arguments which I think would be nice addition to the > language. They are also mentioned in "Generic manifesto". > > The proposal is focusing around generic types. Generic functions are not > coved by the proposal and I don't think that we need default generic > arguments in generic functions as all the types are always part of the > function signature so the compiler can always infer them. One corner case > might be if using default argument values in which case support for default > generic arguments in functions might be useful. > > > The proposal looks fairly straightforward and reasonable. One thing to > think about is how it interacts with type inference. For example, consider > these examples: > > struct X<T = Int> { } > > func f1() -> X<Double> { return X() } > > func f2() -> X<Int> { return X() } > func f2() -> X<Double> { return X() } > > func f3<T>(_: T) -> X<T> { return X() } > > let x1: X = f1() // okay: x1 has type X<Double>? > let x2: X = f2() // ambiguous? > let x3a: X = f3(1.5) // okay: x3a has type X<Double>? > let x3b: X = f3(1) // okay: x3a has type X<Int>? > > The type checker already has some notion of “if you can’t infer a > particular type, fill in a default” that is used for literals. That rule > could be used here… or we could do something else. This should be discussed > in the proposal. > > Thanks for working on this! > > > There's an interesting parallel to the default behavior of literals. The > type of a number or string literal is inferred from type context, or falls > back to a default type like Int or String if that doesn't come up with an > answer. You could think of that of saying the 'Self' type of the protocol > constraint has a default (and maybe that's how we'd generalize the "default > type for a protocol" feature if we wanted to.) It makes sense to me to > follow a similar model for generic parameter defaults; that way, there's > one consistent rule that applies. > > -Joe > > > _______________________________________________ > swift-evolution mailing list > [email protected] > https://lists.swift.org/mailman/listinfo/swift-evolution > > -- Trent Nadeau
_______________________________________________ swift-evolution mailing list [email protected] https://lists.swift.org/mailman/listinfo/swift-evolution
