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

[Austin Zheng]

I'd be happy to put something together, unless someone else wants to take
it on.

Doug, I also owe you a PR adding a minor amendment to one of the accepted
proposals. I'll get to that this week.

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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>
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