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

[Douglas Gregor]

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."
>> 
>> 
>> 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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