[swift-evolution] Enhanced existential types proposal discussion

[Austin Zheng]

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
> >
> > _______________________________________________
> > swift-evolution mailing list
> > swift-evolution at swift.org
> > https://lists.swift.org/mailman/listinfo/swift-evolution
>
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