[swift-evolution] Introduce "associated_type" keyword

[Loïc Lecrenier]

I have drafted a formal proposal here: https://gist.github.com/loiclec/22459d230a21dbcb81fc
Would love to receive feedback from the community on it, I am particularly worried about the correctness of the terms I used.
Should I make a pull request to swift-evolution now, or should we continue the conversation here?

Thanks, 

Loïc

(and sorry about emailing you on Sunday 😇 ) 

> On Dec 6, 2015, at 1:48 AM, John McCall <rjmccall at apple.com> wrote:
> 
>> On Dec 5, 2015, at 4:35 PM, Loïc Lecrenier via swift-evolution <swift-evolution at swift.org> wrote:
>> Hi everyone :)
>> 
>> I propose introducing a new "associated_type" keyword that will replace "typealias" for declaring associated types in protocols.
>> I remember being confused by associated types when I started using Swift, and I think one reason why was the use of the typealias keyword to define them.
>> One reason was that I thought I knew what typealias did, and so I didn't stop to learn what it did inside a protocol. An other reason was the difficulty of finding help when searching for "typealias" instead of "associated types".
>> Then, when I thought I understood it, I started building an excessively protocol-oriented program as an exercise. And I still lost a lot of time fighting Swift by trying to use "real" typealias-es inside of protocols.
>> 
>> Conceptually, I had something like this:
>> 
>> protocol ProtA {
>>  typealias Container : SequenceType
>> }
>> protocol ProtB {
>>  typealias AnOtherAssocType : ProtA
>>  func foo(x: AnOtherAssocType.Container.Generator.Element, y: AnOtherAssocType.Container.Generator.Element) -> AnOtherAssocType.Container.Generator.Element
>> }
>> 
>> The function foo is very difficult to read, so I wanted to use a shortcut to Element by doing this:
>> 
>> protocol ProtB {
>>  typealias A : ProtA
>>  typealias Element = A.Container.Generator.Element
>>  func foo(x: Element, y: Element) -> Element
>> }
>> 
>> But by doing so, I didn't create a shortcut to Element, but an associated type with a default value of Element. (right?)
>> Then I tried to write extensions to ProtB where Element conforms to, say, Equatable, and couldn't make it work because A.Container.Generator.Element didn't conform to Equatable.
>> 
>> So, that was a rather long explanation of the reasons I think we should replace the typealias keyword by associated_type, and allow "real" typealias-es inside protocols.
> 
> I think this is a great idea; re-using typealias for associated types was a mistake.
> 
> John.
> 
>> 
>> Ideally, I would write
>> 
>> protocol ProtB {
>>  associated_type AnOtherAssocType : ProtA
>>  typealias Element = AnOtherAssocType.Container.Generator.Element
>>  func foo(x: Element, y: Element) -> Element
>> }
>> 
>> and it would be exactly the same as
>> 
>> protocol ProtB {
>>  associated_type AnOtherAssocType : ProtA
>>  func foo(x: A.Container.Generator.Element, y: A.Container.Generator.Element) -> A.Container.Generator.Element
>> }
>> 
>> There are probably some problems created by this proposal, but right now I can't see any :/
>> 
>> Thanks,
>> 
>> Loïc
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