> On Jan 29, 2017, at 2:25 PM, Xiaodi Wu <[email protected]> wrote:
>
> On Sun, Jan 29, 2017 at 2:16 PM, Matthew Johnson <[email protected]
> <mailto:[email protected]>> wrote:
>
>> On Jan 29, 2017, at 2:01 PM, Xiaodi Wu <[email protected]
>> <mailto:[email protected]>> wrote:
>>
>> On Sun, Jan 29, 2017 at 1:37 PM, Matthew Johnson <[email protected]
>> <mailto:[email protected]>> wrote:
>>
>>
>> Sent from my iPad
>>
>> On Jan 29, 2017, at 12:58 PM, Xiaodi Wu via swift-evolution
>> <[email protected] <mailto:[email protected]>> wrote:
>>
>>> Cool. Another avenue of improvement here is relaxing the single-class
>>> spelling rule for the sake of composing typealiases.
>>>
>>> As Matthew mentioned, if I have class Base and typealiases Foo = Base &
>>> Protocol1 and Bar = Base & Protocol2, it'd be nice to allow Foo & Bar.
>>>
>>> It'd be nice to go one step further: given class Derived : Base, if I have
>>> typealiases Foo2 = Base & Protocol1 and Bar2 = Derived & Protocol2, then it
>>> could be permitted to write Foo2 & Bar2, since there is effectively only
>>> one subclass requirement (Derived).
>>>
>>> As I understand it, the rationale for allowing only one subclass
>>> requirement is that Swift supports only single inheritance. Thus, two
>>> disparate subclass requirements Base1 & Base2 would make your existential
>>> type essentially equivalent to Never. But Base1 & Base1 & Base1 is fine for
>>> the type system, the implementation burden (though greater) shouldn't be
>>> too awful, and you would measurably improve composition of typealiases.
>>
>> Yes, this is what I was indicating in my post as well.
>>
>> Are you suggesting that Base1 & Base2 compose to a type that is treated
>> identically to Never do you think it should be an immediate compiler error?
>> I remember having some discussion about this last year and think somebody
>> came up with a very interesting example of where the former might be useful.
>>
>> Last year's discussion totally eludes me for some reason. But sure, if
>> deferring the error until runtime is actually useful then why not? In the
>> absence of an interesting use case, though, I think it'd be nice for the
>> compiler to warn you that Base1 & Base2 is not going to be what you want.
>
> Deferring to runtime isn’t what I mean. If you try to actually *do* anything
> that requires an instance of `Base1 & Based` (which you almost always would)
> you would still get a compile time error.
>
> I managed to dig up the example from last year’s thread and it is definitely
> a good one:
>
> func intersection<T, U>(ts; Set<T>, us: Set<U>) -> Set<T & U>
>
> The desire is that we are always able to produce a result set. When T & U is
> uninhabitable it will simply be an empty set just like Set<Never> has a
> single value which is the empty set.
>
> Currently, Set<Never> is impossible because Never is not Hashable :)
Ahh, good point. I hadn’t tried it. It can easily be made Hashable with a
simple extension though - this code compiles today:
extension Never: Hashable {
public var hashValue: Int { return 0 }
}
public func ==(lhs: Never, rhs: Never) -> Bool { return false }
let s = Set<Never>()
> Since concrete types *can't* be used, this example seems like it'd be of
> little use currently. How widely useful would it be to have an intersection
> facility such as this when T != U even if that restriction were lifted,
> though? Seems like the only real useful thing you can do with generic Set<T &
> U> is based on the fact that it'd be Set<Hashable>. Other than those
> immediate thoughts, I'll have to think harder on this.
Sure, it’s possible that this is the only interesting example and may not have
enough value to be worthwhile. But I found it interesting enough that it stuck
around in the back of my mind for 8 months! :)
It generalizes easily to any cases where you have a generic type that is useful
despite not necessarily having access to instances of the parameterized type.
If we allow this, I *think* all uninhabitable types could be unified
semantically by making `Never` a protocol and giving them implicit conformance.
>
> This example points even more strongly in the direction of allowing *any*
> concrete type to be used, not just classes - even today we could produce
> uninhabitable existentials like this using value types.
>
> Here’s the link to the thread:
> https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160523/019463.html
>
> <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160523/019463.html>
>>
>>
>>> On Sun, Jan 29, 2017 at 12:41 Austin Zheng <[email protected]
>>> <mailto:[email protected]>> wrote:
>>> The "class comes first" requirement made more sense when the proposed
>>> syntax was still "Any<T, U, V>", intentionally mirroring how the superclass
>>> and conformances are declared on a class declaration (the archives contain
>>> more detailed arguments, both pro and con). Now that the syntax is "T & U &
>>> V", I agree that privileging the class requirement is counterintuitive and
>>> probably unhelpful.
>>>
>>> Austin
>>>
>>> > On Jan 29, 2017, at 10:37 AM, Matt Whiteside via swift-evolution
>>> > <[email protected] <mailto:[email protected]>> wrote:
>>> >
>>> > Thanks for writing this proposal David.
>>> >
>>> >> On Jan 29, 2017, at 10:13, Xiaodi Wu via swift-evolution
>>> >> <[email protected] <mailto:[email protected]>> wrote:
>>> >>
>>> >> As Matthew mentioned, the rules can certainly later be relaxed, but
>>> >> given that this proposal has the compiler generating fix-its for
>>> >> subclasses in second position, is there a reason other than stylistic
>>> >> for demanding MyClass & MyProtocol instead of MyProtocol & MyClass?
>>> >>
>>> >> From a naive perspective, it seems that if the compiler understands my
>>> >> meaning perfectly, it should just accept that spelling rather than
>>> >> complain.
>>> >
>>> > I had that thought too. Since ‘and’ is a symmetric operation, requiring
>>> > the class to be in the first position seems counter-intuitive.
>>> >
>>> > -Matt
>>> >
>>> > _______________________________________________
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>>> > [email protected] <mailto:[email protected]>
>>> > https://lists.swift.org/mailman/listinfo/swift-evolution
>>> > <https://lists.swift.org/mailman/listinfo/swift-evolution>
>>>
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