[swift-evolution] [Pitch] Remove associated type inference

[Matthew Johnson]

Sent from my iPad

> On May 26, 2016, at 1:21 AM, David Hart via swift-evolution <swift-evolution at swift.org> wrote:
> 
> We could have the generic type parameter always shadow the associated type.

I believe there has been talk of automatically introducing an associated type corresponding to generic parameters.  I'm not sure where that stands but IIRC it was someone from the core team that was considering it.

> 
>> On 26 May 2016, at 06:08, Patrick Smith <pgwsmith at gmail.com> wrote:
>> 
>> While this has been a handy feature, I don’t mind making things more explicit, as I think it helps communication. Sometimes clarity that helps the compiler will also help the reader. It’s only really convenient when writing.
>> 
>> I would be happy with this change as long as the issue with same named generics were changed. Currently in Swift 2.2 I can’t do this:
>> 
>> struct SomeGenericGenerator<Element> : GeneratorType {
>>   typealias Element = Element // Error: Type alias ‘Element’ circularly references itself
>> }
>> 
>> It’s a real pain, as now I have to think up some alternate name for the generic parameter ‘Element’.
>> 
>> Whereas, with automatic inference, I can do this without the compiler complaining:
>> 
>> struct SomeGenericGenerator<Element> : GeneratorType {
>>   mutating func next() -> Element? {
>>     return nil
>>   }
>> }
>> 
>> So I’d like to see the above `typealias Element = Element` allowed if possible.
>> 
>> 
>>> On 26 May 2016, at 7:43 AM, David Hart via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> Here’s a pitch for removing associated type inference as per the Generics Manifesto. If we want to do it, we’d better do it before Swift 3:
>>> 
>>> Remove associated type inference
>>> Proposal: SE-XXXX
>>> Author: David Hart, Douglas Gregor
>>> Status: TBD
>>> Review manager: TBD
>>> Introduction
>>> 
>>> This proposal seeks to remove the inference of associated types in types conforming to protocols.
>>> 
>>> Motivation
>>> 
>>> Even if associated types inference in a useful feature, it is also a big source of bugs in the compiler. This proposal argues that the usefulness does not outweight its architectural complexity. As per the Generics Manifesto:
>>> 
>>> associated type inference is the only place in Swift where we have a global type inference problem: it has historically been a major source of bugs, and implementing it fully and correctly requires a drastically different architecture to the type checker.
>>> Because this is a breaking change, it would be beneficial to implement it for Swift 3. 
>>> 
>>> Detailed Design
>>> 
>>> The proposal would remove associated type inference and make code which relied on it invalid:
>>> 
>>> protocol IteratorProtocol {
>>>   associatedtype Element
>>>   mutating func next() -> Element?
>>> }
>>> 
>>> struct IntIterator : IteratorProtocol {
>>>   mutating func next() -> Int? { ... }  // used to infer Element = Int
>>> }
>>> The compiler would generate an error message stating: error: IntIterator is missing its Element associated type declaration. The code would have to be modified as follows to fix the error:
>>> 
>>> struct IntIterator : IteratorProtocol {
>>>     typealias Element = Int
>>>     mutating func next() -> Int? { return nil }  // used to infer Element = Int
>>> }
>>> Impact on Existing Code
>>> 
>>> This is a breaking change that will require conforming types which relied on the inference, including in the Standard Library, to explicitly declare associated types. A Fix-It could be introduced to add the typealias and leave the type to be filled in. That way, all the type inference could be removed from the compiler.
>>> 
>>> Alternatives Considered
>>> 
>>> The only alternative is to keep the inference with the known consequences on the compiler.
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
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