[swift-evolution] Default Generic Arguments

Srđan Rašić srdan.rasic at gmail.com
Mon Jan 23 14:43:55 CST 2017 

@doug:

> 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>?

Agreed. When type is explicitly defined in the context, that type should
override the default type. In this example type is explicitly defined by
the return argument so we infer that type instead of using the default one.

> let x2: X = f2()   // ambiguous?

Uncertain. I might even lean to `X<Int>` because defining a property as
`let x1: X` would be considered the same as defining it as `let x1:
X<Int>`. In that case I would expect that the correct overload is inferable.

> let x3a: X = f3(1.5)   // okay: x3a has type X<Double>?
> let x3b: X = f3(1)   // okay: x3a has type X<Int>?

Agreed. These two are related to x1. Type is defined in the context (by
inferring it from the literal) so it overrides the default argument.

I like the parallel with the default behaviour of literals - “if you can’t
infer a particular type, fill in a default". We should aim for that.


On Mon, Jan 23, 2017 at 9:18 PM, Srđan Rašić <srdan.rasic at gmail.com> wrote:

> I too agree that empty angle brackets are redundant because the
> information they convey is not useful enough to justify the syntax clutter.
> I would value clean syntax in this case more than explicitness and I think
> that would go along with Swift's philosophy. I would compare this to
> Swift's type inference where one can omit type information that's already
> known to the compiler in order to increase readability.
>
> > For example, if you had a struct that used a T (defaulted to Int) for a
> field, and that field's range should become a Double in your use case, you
> know that there's something you can change to get that behavior, while just
> X might look like you'd need to create your own type.
>
> I think such cases would be extremely rare and one would have to be very
> ignorant about the types he/she works with. Additionally, that syntax is
> useful only for types with one generic argument. Say we have `Promise<T, E
> = Error>` and declare property as `let p: Promise<Int>`. How would you
> convey the information that there is a second argument that could be
> changed? Keeping the comma would be very ugly :)
>
>
>
> On Mon, Jan 23, 2017 at 8:51 PM, Sean Heber <sean at fifthace.com> wrote:
>
>> I agree. I don’t think empty angle brackets convey anything useful to the
>> reader.
>>
>> l8r
>> Sean
>>
>>
>> > On Jan 23, 2017, at 1:25 PM, T.J. Usiyan via swift-evolution <
>> swift-evolution at swift.org> wrote:
>> >
>> > I am against requiring empty angle brackets. I could live with it
>> either way, but I think that one reason to provide default types is to hide
>> the detail that there is a type parameter until such a time as it is
>> needed.  Empty angle brackets call attention to the feature in a manner
>> that discards any possible gains on this front. Empty angle brackets would
>> be confusing to explain to someone new to the language and–more
>> importantly–shouldn't be necessary to explain in the "falling back to
>> defaults" case.
>> >
>> > On Mon, Jan 23, 2017 at 1:41 PM, Trent Nadeau via swift-evolution <
>> swift-evolution at swift.org> wrote:
>> > 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 <
>> swift-evolution at swift.org> wrote:
>> >
>> >> On Jan 23, 2017, at 9:51 AM, Douglas Gregor via swift-evolution <
>> swift-evolution at swift.org> wrote:
>> >>
>> >>
>> >>> On Jan 23, 2017, at 7:55 AM, Srđan Rašić via swift-evolution <
>> swift-evolution at swift.org> 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
>> > swift-evolution at swift.org
>> > https://lists.swift.org/mailman/listinfo/swift-evolution
>> >
>> >
>> >
>> >
>> > --
>> > Trent Nadeau
>> >
>> > _______________________________________________
>> > swift-evolution mailing list
>> > swift-evolution at swift.org
>> > https://lists.swift.org/mailman/listinfo/swift-evolution
>> >
>> >
>> > _______________________________________________
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>> > swift-evolution at swift.org
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>>
>>
>
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