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

Dave Abrahams dabrahams at apple.com
Tue Apr 12 17:07:50 CDT 2016

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!


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