[swift-evolution] [Pitch/plea] Recursive protocol constraints

[Austin Zheng]

I'm still working on this. When you mention where clauses on associated
types, how should I handle that in the context of the proposal?

* Consider where clauses part of the proposal (so that the proposal becomes
"recursive associated type constraints + where clauses on associated types")
* Assume that where clauses will definitely be accepted, and write the
stdlib changes assuming that
* Assume that where clauses might not be accepted, and write the stdlib
changes assuming that, but also have a section containing further stdlib
changes conditional on where clauses being accepted

Best,
Austin

On Sun, Nov 13, 2016 at 7:55 PM, Douglas Gregor <dgregor at apple.com> wrote:

>
>
> Sent from my iPhone
>
> On Nov 13, 2016, at 4:03 PM, Austin Zheng <austinzheng at gmail.com> wrote:
>
> I'd be happy to put something together, unless someone else wants to take
> it on.
>
>
> Great, thanks!
>
>
> Doug, I also owe you a PR adding a minor amendment to one of the accepted
> proposals. I'll get to that this week.
>
>
> Sounds great.
>
>   - Doug
>
> Austin
>
> On Sun, Nov 13, 2016 at 10:13 PM, Douglas Gregor via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>> Recursive protocol constraints is one small-looking feature that could
>> greatly improve the standard library. The generics manifesto describes it
>> this way:
>>
>> "Currently, an associated type cannot be required to conform to its
>> enclosing protocol (or any protocol that inherits that protocol). For
>> example, in the standard library SubSequence type of a Sequence should
>> itself be a Sequence:
>>
>> protocol Sequence {
>>   associatedtype Iterator : IteratorProtocol
>>   ...
>>   associatedtype SubSequence : Sequence   // currently ill-formed, but should be possible}
>>
>> The compiler currently rejects this protocol, which is unfortunate: it
>> effectively pushes the SubSequence-must-be-a-Sequence requirement into
>> every consumer of SubSequence, and does not communicate the intent of
>> this abstraction well."
>>
>> <https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md#nested-generics>It's
>> actually slightly worse than the above implies: the standard library has a
>> pile of underscore-prefixed protocols (e.g., _Sequence) specifically to
>> dodge this restriction. They are ugly, and we want them to go away. Many of
>> these places are marked with an ABI FIXME in the standard library sources.
>>
>> Would someone like to write up a proposal for this feature? The syntax
>> and basic semantics are pretty direct, but a proposal should also capture
>> the expected effects on the standard library, particularly when combined
>> with where clauses on associated types.
>>
>> I also have a nagging feeling that we will need some form of restrictions
>> on this feature for implementation reasons, e.g., because some recursive
>> constraints will form unsolvable systems.
>>
>> For reference, we've already been implementing this feature. Some
>> information about the compiler internal issues is captured at:
>>
>>   https://gist.github.com/DougGregor/e7c4e7bb4465d6f5fa2b59be72dbdba6
>>
>>   - Doug
>>
>>
>>
>> Sent from my iPhone
>>
>> _______________________________________________
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>> swift-evolution at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>
>>
>
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