on Tue Apr 12 2016, Douglas Gregor <[email protected]> wrote:
> On Apr 11, 2016, at 1:01 AM, Jacob Bandes-Storch via swift-evolution > <[email protected]> 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 _______________________________________________ swift-evolution mailing list [email protected] https://lists.swift.org/mailman/listinfo/swift-evolution
