Sent from my iPhone
> On Nov 13, 2016, at 4:03 PM, Austin Zheng <[email protected]> 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 >> <[email protected]> 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 >> >> _______________________________________________ >> swift-evolution mailing list >> [email protected] >> https://lists.swift.org/mailman/listinfo/swift-evolution >> >
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