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

[Andrew Bennett]

I agree that this is an issue, I'd really like to see a language feature to
address it.

In some circumstances you can work around the issue with type-erasure. You
implement a generic wrapper (like AnySequence) that can take anything
conforming to that protocol and expose it with a single concrete interface.

I learnt a lot implementing a few of these. There's a good article on it
here:
https://realm.io/news/type-erased-wrappers-in-swift/

I wouldn't be too surprised if Apple formalises type erasure somehow, the
implementation is very mechanical and could be automated. For example: If
you have a protocol MyProtocol that has type requirements then perhaps
MyProtocol.Any would refer to an automatically generated type erased
concrete type. I suppose you could also do:

typealias AB = protocol<A,B>
AB.Any

The main problem with current implementations seems to be, for example:
ZipSequence is a SequenceType, but ZipSequence is not an AnySequence.

Also you cannot necessarily conform to a protocol if it has things like
initialiser requirements.





On Tue, Dec 15, 2015 at 12:45 AM, Tal Atlas via swift-evolution <
swift-evolution at swift.org> wrote:

> This is the error that gets in my way the most when trying to do protocol
> oriented programming. It can be super frustrating. That lead me to make a
> proposal a few weeks back for having generic protocols.
>
> In that proposal some swift core people expressed that they were keenly
> aware of said pain. And while they didn't deliver details in time or
> implementation they expressed that there were efforts ongoing to solve it.
>
> On Sun, Dec 13, 2015 at 6:55 PM Marc Knaup via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>> Hey guys,
>>
>> I'm looking at Swift 3.0's goal to improve generics.
>>
>> Is there any info yet if and how we will be able to refer to instances of
>> protocols that have associated types?
>> What is the difficulty in supporting this?
>>
>> Simple examples:
>> var list = [Hashable]()
>> var hashable: Hashable = 2
>>
>> Right now all we get is
>>
>>> protocol 'Hashable' can only be used as a generic constraint because it
>>> has Self or associated type requirements
>>
>>
>> Thanks,
>>   Marc
>>
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>
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