[swift-evolution] Subclass Existentials

Xiaodi Wu xiaodi.wu at gmail.com
Sun Jan 29 15:24:08 CST 2017


On Sun, Jan 29, 2017 at 3:11 PM, Matthew Johnson <matthew at anandabits.com>
wrote:

>
> On Jan 29, 2017, at 3:05 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>
> On Sun, Jan 29, 2017 at 2:40 PM, Matthew Johnson <matthew at anandabits.com>
> wrote:
>
>>
>> On Jan 29, 2017, at 2:25 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>
>> On Sun, Jan 29, 2017 at 2:16 PM, Matthew Johnson <matthew at anandabits.com>
>>  wrote:
>>
>>>
>>> On Jan 29, 2017, at 2:01 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>>
>>> On Sun, Jan 29, 2017 at 1:37 PM, Matthew Johnson <matthew at anandabits.com
>>> > wrote:
>>>
>>>>
>>>>
>>>> Sent from my iPad
>>>>
>>>> On Jan 29, 2017, at 12:58 PM, Xiaodi Wu via swift-evolution <
>>>> swift-evolution at swift.org> 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! :)
>>
>
> Hmm, it had not occurred to me: instantiating a Set<Hashable> is not
> supported (and you can substitute for Hashable any protocol you want).
> Thus, for any Set<T> and Set<U> that you can actually instantiate, unless T
> and U are both classes and one inherits from the other (in which case the
> generic `intersection<X>(a: Set<X>, b: Set<X>) -> Set<X>` already
> suffices), Set<T & U> must be the empty set. This is not a very interesting
> result.
>
>
> Yes, but this is a limitation due to the fact the existentials for a
> protocol do not conform to the protocol.  In some cases the existential
> *cannot* conform to the protocol but in many cases (especially common
> cases) it *can*.  It just doesn’t today.  There is widespread desire to see
> this situation improve.
>

Sure, but when will be the day that existentials conform to their own
protocol when they can do so, *and* we extend `&` to value types (probably
not until they can express some sort of meaningful subtyping relationship
to each other)?

At that point, I'd advocate for using compiler magic to make uninhabited
types like Never a subtype of all types conforming to all protocols. Then,
we could actually write Set<Never> without having to implement conformance
to Hashable by writing a bogus `==` function. And we could replace
EmptyCollection with Collection<Never> and simplify the standard library
API surface that way (since Array<Never>() would then be a value of type
Array<T>, etc.). And, your demonstrated use case would become interesting.
Since there is a pretty good chance that you and I won't be alive by then,
I'm happy to punt on the ideation process for this :)


> 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.or
>>> g/pipermail/swift-evolution/Week-of-Mon-20160523/019463.html
>>>
>>>
>>>
>>> On Sun, Jan 29, 2017 at 12:41 Austin Zheng <austinzheng at gmail.com>
>>>> 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 <
>>>>> swift-evolution at swift.org> wrote:
>>>>> >
>>>>> > Thanks for writing this proposal David.
>>>>> >
>>>>> >> On Jan 29, 2017, at 10:13, Xiaodi Wu via swift-evolution <
>>>>> swift-evolution at swift.org> 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
>>>>> >
>>>>> > _______________________________________________
>>>>> > swift-evolution mailing list
>>>>> > swift-evolution at swift.org
>>>>> > https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>>
>>>>> _______________________________________________
>>>> swift-evolution mailing list
>>>> swift-evolution at swift.org
>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>
>>>>
>
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