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

[Xiaodi Wu]

On Thu, May 26, 2016 at 12:47 PM, L. Mihalkovic via swift-evolution <
swift-evolution at swift.org> wrote:

>
>
> On May 25, 2016, at 11:43 PM, 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
>    <https://github.com/apple/swift-evolution/blob/master/proposals/XXXX-remove-associated-type-inference.md>
>    - Author: David Hart <https://github.com/hartbit>, Douglas Gregor
>    <https://github.com/DougGregor>
>    - Status: TBD
>    - Review manager: TBD
>
>
> <https://github.com/hartbit/swift-evolution/blob/remove-associated-type-inference/proposals/XXXX-remove-associated-type-inference.md#introduction>
> Introduction
>
> This proposal seeks to remove the inference of associated types in types
> conforming to protocols.
>
> <https://github.com/hartbit/swift-evolution/blob/remove-associated-type-inference/proposals/XXXX-remove-associated-type-inference.md#motivation>
> 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
> <https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md>:
>
> 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.
>
> <https://github.com/hartbit/swift-evolution/blob/remove-associated-type-inference/proposals/XXXX-remove-associated-type-inference.md#detailed-design>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:
>
>
> From a purely english syntax viewpoint, i would prefer the error to be
> along the lines of
>
> IntIterator is missing a declaration for the Element associated type
>
> Or
>
> IntIterator is missing a declaration for its Element associated type
>
> Clearer still:
"IntIterator is missing a declaration for its associated type Element"


>
> struct IntIterator : IteratorProtocol {
>     typealias Element = Int
>     mutating func next() -> Int? { return nil }  // used to infer Element = Int
> }
>
>
> <https://github.com/hartbit/swift-evolution/blob/remove-associated-type-inference/proposals/XXXX-remove-associated-type-inference.md#impact-on-existing-code>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.
>
> <https://github.com/hartbit/swift-evolution/blob/remove-associated-type-inference/proposals/XXXX-remove-associated-type-inference.md#alternatives-considered>Alternatives
> Considered
> The only alternative is to keep the inference with the known consequences
> on the compiler.
>
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>
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