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

[Douglas Gregor]

> On Apr 12, 2016, at 11:19 PM, David Hart <david at hartbit.com> wrote:
> 
> Hi Doug,
> 
> I've read the discussion about moving the where clause to the right of declarations (which I full-heartedly approve) but I don't see how it would have any impact on the syntax of associated types requirements.

I think “arbitrary requirements in protocols” doesn’t necessarily require them to be attached to an associated type. For example, Collection might require that the SubSequence type itself be a Collection:

	protocol Collection : Sequence {
	  associatedtype SubSequence : Collection
	} 

Why did we have to declare the SubSequence associated type again (since it’s already down in SubSequence) just to make it conform to Collection. In fact, why are we even allowed to declare a redundant associated type, which we know is defined in Sequence and will have to be the same? Seems to me that we just want a where clause to add the constraint, e.g.,

	protocol Collection : Sequence {
	  where SubSequence : Collection
	} 

which of course would conflict with moving where clauses later, e.g.,

	protocol Collection : Sequence {
          func foo() // is the following where clause part of foo() or part of Collection?
	  where SubSequence : Collection
	} 


	- Doug


> 
> David
> 
> On 12 Apr 2016, at 19:07, Douglas Gregor via swift-evolution <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.
>> 
>> 	- Doug
>> 
>> 
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