[swift-evolution] [Completing Generics] Arbitrary requirements in protocols

[Jacob Bandes-Storch]

I'm interested, but I'm by no means claiming I'll have enough time to drive
any of the discussion/proposal/implementation. :-(

Jacob

On Tue, Apr 12, 2016 at 3:07 PM, Dave Abrahams via swift-evolution <
swift-evolution at swift.org> wrote:

>
> on Tue Apr 12 2016, Douglas Gregor <swift-evolution at swift.org> wrote:
>
> >     On Apr 11, 2016, at 1:01 AM, Jacob Bandes-Storch via swift-evolution
> >     <swift-evolution at swift.org> wrote:
> >
> >     Doug wrote this in the Completing Generics manifesto, under "Minor
> >     extensions":
> >
> >         *Arbitrary requirements in protocols
> >
> >         Currently, a new protocol can inherit from other protocols,
> introduce
> >         new associated types, and add new conformance constraints to
> associated
> >         types (by redeclaring an associated type from an inherited
> protocol).
> >         However, one cannot express more general constraints. Building
> on the
> >         example from “Recursive protocol constraints”, we really want the
> >         element type of a Sequence’s SubSequence to be the same as the
> element
> >         type of the Sequence, e.g.,
> >
> >         protocol Sequence {
> >         associatedtype Iterator : IteratorProtocol
> >         …
> >         associatedtype SubSequence : Sequence where
> SubSequence.Iterator.Element
> >         == Iterator.Element
> >         }
> >
> >     +1.
> >
> >     To make it into Swift 3, would this feature require a proposal of
> its own?
> >
> > Yes. Also, be wary that the syntax above potentially conflicts with the
> syntax
> > discussed as "moving the where clauses”:
> >
> >
> http://thread.gmane.org/gmane.comp.lang.swift.evolution/13886/focus=14058
> >
> >     How feasible would it be to implement on top of the current system?
> >
> > Definitely! The archetype builder would need to learn to check these
> extra where
> > clauses, and one would need to be sure that the constraint solver is
> picking
> > them up as well.
>
> By the way, having this would enable us to massively simplify the
> standard library, and potentially lots of user-written generic code,
> too.  So I'm very excited that someone's interested!
>
> --
> Dave
>
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