[swift-evolution] protocol can only be used as a generic constraint because it has Self or associated type requirements

[Dave Abrahams]

> On Dec 13, 2015, at 11:03 PM, Paul Cantrell <cantrell at pobox.com> wrote:
> 
>> On Dec 13, 2015, at 11:56 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>>> On Dec 13, 2015, at 3:55 PM, Marc Knaup via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> Is there any info yet if and how we will be able to refer to instances of protocols that have associated types?
>>> 
>>> As far as I know, this isn't a solvable problem.
> 
>>> 
>>> What is the difficulty in supporting this?
>> 
>> Here's a simple example:
>> 
>> protocol P {
>>   typealias A
>>   var x: A { get set }
>> }
>> 
>> struct Y : P {
>>   var x: Int
>> }
>> 
>> struct Z : P {
>>   var x: String
>> }
>> 
>> func f(p1: P, p2: P) {
>>   p1.x = p2.x // assigning an Int to a String?
>> }
> 
> p1.x = p2.x should be a compiler error, because there’s not enough type information.

> But that shouldn’t stop a programmer from doing this:
> 
> protocol P {
>   typealias A
>   var x: A { get set }
>   var y: Int
> }
> 
> struct Y : P {
>   var x: Int
>   var y: Int
> }
> 
> struct Z : P {
>   var x: String
>   var y: Int
> }
> 
> func maxY(p1: P, p2: P) -> Int {
>   return max(p1.y, p2.y)   // No problems here!
> }
> 
> …right?

Ah, but then you have the situation that P doesn't conform to P (it doesn't have an x that you can access).  In my opinion that is just too weird an idea to be usable.
Personally, I have always thought that protocols intended to be used as unbound existentials (not P<A: Int>, just plain P) are different beasts entirely from the kind used as generic constraints and should be explicitly declared as such—I don't think I've ever seen a protocol that is well-used in both ways; if you have counterexamples I'd love to see them.  In "existential protocols," declaring an associated type or creating a self requirement would be an error at the point of declaration.  

-Dave



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