> 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] <mailto:[email protected]>> wrote: >> >> Hi Everyone, >> >> I've opened a PR (https://github.com/apple/swift-evolution/pull/591 >> <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
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