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

[Douglas Gregor]

> On Apr 12, 2016, at 11:23 PM, David Hart via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I wouldn't mind driving the discussion and proposal, because I'd really like to see a more complete generics system. Before I start, can David or Doug, or someone else with a high-level view of the generics system tell me if this is where to start or if there is another feature in the Complete Generics manifesto which is more urgent first?

I think this is a fine feature to focus on. It’s useful, fits well in the system, and it’s scope is small enough that it’s achievable.

	- Doug

> 
> David
> 
> On 13 Apr 2016, at 01:46, Jacob Bandes-Storch via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> 
>> 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 <mailto:swift-evolution at swift.org>> wrote:
>> 
>> on Tue Apr 12 2016, Douglas Gregor <swift-evolution at swift.org <mailto: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 <mailto: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 <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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