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

Douglas Gregor dgregor at apple.com
Tue Apr 12 12:07:19 CDT 2016

> 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”:


> 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.

	- Doug

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