> On May 25, 2016, at 11:43 PM, David Hart via swift-evolution > <swift-evolution@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: >
>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 > > 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@swift.org > https://lists.swift.org/mailman/listinfo/swift-evolution
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