[swift-evolution] Enhanced existential types proposal discussion

Sun May 29 09:04:25 CDT 2016

Sent from my iPad

> On May 29, 2016, at 8:50 AM, Thorsten Seitz <tseitz42 at icloud.com> wrote:
> 
> 
>>> Am 28.05.2016 um 22:08 schrieb Matthew Johnson <matthew at anandabits.com>:
>>> 
>>> 
>>> On May 28, 2016, at 2:31 PM, Thorsten Seitz via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> I’m not happy with that restriction in the proposal:
>>> 
>>> Existentials cannot be used with generics in the following ways:
>>> 
>>> In generic declarations, with the requirements composed out of generic type variables:
>>> 
>>> // NOT ALLOWED
>>> func foo<A, B>(x: A, y: B) -> Any<A, B> { ... }
>>> 
>>> Why is that not allowed?
>>> 
>>> I would have hoped to be able to write something like
>>> 
>>> func union<A, B>(x: Set<A>, y: Set<B>) -> Set<Any<A, B>> { … }
>> 
>> I think what you’re looking for is an anonymous union type `A | B`, not an existential made of the two of them.  
> 
> Sorry, I of course meant
> 
> func intersection<A, B>(x: Set<A>, y: Set<B>) -> Set<Any<A, B>> { … }
> 
> I guess I will forever confuse this until we write existentials with `&`…
> 
> You are right, for the union I want the union type `A | B`. But that is stuff for another proposal.

This makes a lot more sense!  So you want to be able to use uninhabitable types in the contexts of things like collection elements (such collections must always be empty but are inhabitable by a single empty instance).  It is a great example of why it might make sense to allow types like this.  

What you are really asking for is the ability to drop the restriction that only a single superclass constraint is allowed and no value type constraints are allowed.  It might be useful in cases like your intersection example.  But it could also be a source of error confusing error messages when people form such a type and it doesn't work the way they expect.  If the type is disallowed as under the current proposal the error message might be more straightforward.  This is a topic that probably deserves further consideration.  But I have to say I find your intersection example to be reasonably compelling.

Austin, what do you think about this example?

> 
> -Thorsten
> 
> 
>> 
>> To write this `union` and have it behave in the usual way you need `Any<A, B>` to be a supertype of `A` and of `B`.  The existential doesn’t actually do that so it would not be possible for this union function to guarantee the result would have all of the members of `x` and all the members of `y` the way that a `union` usually would.  
>> 
>> The anonymous union type `A | B` *is* a supertype of `A` and a supertype of `B` so you would have no trouble writing this:
>> 
>> func union<A, B>(x: Set<A>, y: Set<B>) -> Set<A | B> { … }
>> 
>> And returning the expected result.
>> 
>>> 
>>> 
>>> -Thorsten
>>> 
>>> 
>>> 
>>>> Am 26.05.2016 um 07:53 schrieb Austin Zheng via swift-evolution <swift-evolution at swift.org>:
>>>> 
>>>> The inimitable Joe Groff provided me with an outline as to how the design could be improved. I've taken the liberty of rewriting parts of the proposal to account for his advice.
>>>> 
>>>> It turns out the runtime type system is considerably more powerful than I expected. The previous concept in which protocols with associated types' APIs were vended out selectively and using existentials has been discarded.
>>>> 
>>>> Instead, all the associated types that belong to an existential are accessible as 'anonymous' types within the scope of the existential. These anonymous types are not existentials - they are an anonymous representation of whatever concrete type is satisfying the existential's value's underlying type's associated type.
>>>> 
>>>> This is an enormous step up in power - for example, an existential can return a value of one of these anonymous associated types from one function and pass it into another function that takes the same type, maintaining perfect type safety but without ever revealing the actual type. There is no need anymore to limit the APIs exposed to the user, although there may still exist APIs that are semantically useless without additional type information.
>>>> 
>>>> A set of conversions has also been defined. At compile-time 'as' can be used to turn values of these anonymous associated types back into existentials based on the constraints defined earlier. 'as?' can also be used for conditional casting of these anonymously-typed values into potential actual types.
>>>> 
>>>> As always, the link is here, and feedback would be greatly appreciated: https://github.com/austinzheng/swift-evolution/blob/az-existentials/proposals/XXXX-enhanced-existentials.md
>>>> 
>>>> Best,
>>>> Austin
>>>> 
>>>>> On Tue, May 24, 2016 at 5:09 AM, Matthew Johnson via swift-evolution <swift-evolution at swift.org> wrote:
>>>>> 
>>>>> 
>>>>> Sent from my iPad
>>>>> 
>>>>> On May 23, 2016, at 9:52 PM, Brent Royal-Gordon via swift-evolution <swift-evolution at swift.org> wrote:
>>>>> 
>>>>> >> One initial bit of feedback -  I believe if you have existential types, I believe you can define Sequence Element directly, rather than with a type alias. e.g.
>>>>> >>
>>>>> >> protocol Sequence {
>>>>> >>  associatedtype Element
>>>>> >>  associatedtype Iterator: any<IteratorProtocol where IteratorProtocol.Element==Element>
>>>>> >>  associatedtype SubSequence: any<Sequence where Sequence.Element == Element>
>>>>> >>  …
>>>>> >> }
>>>>> >
>>>>> > That's not really the same thing. Any<IteratorProtocol> is an existential, not a protocol. It's basically an automatically-generated version of our current `AnyIterator<T>` type (though with some additional flexibility). It can't appear on the right side of a `:`, any more than AnyIterator could.
>>>>> 
>>>>> After this proposal you should be able to use these existentials anywhere you can place a constraint, so it would work.  You can do this with the protocol composition operator today and the future existential is just an extension of that capability.
>>>>> 
>>>>> >
>>>>> > What *would* work is allowing `where` clauses on associated types:
>>>>> >
>>>>> >> protocol Sequence {
>>>>> >>  associatedtype Element
>>>>> >>  associatedtype Iterator: IteratorProtocol where Iterator.Element==Element
>>>>> >>  associatedtype SubSequence: Sequence where SubSequence.Element == Element
>>>>> >>  …
>>>>> >> }
>>>>> >
>>>>> > I believe this is part of the generics manifesto.
>>>>> >
>>>>> > --
>>>>> > Brent Royal-Gordon
>>>>> > Architechies
>>>>> >
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>>>>> 
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