On Thu, May 26, 2016 at 12:47 PM, L. Mihalkovic via swift-evolution < swift-evolution@swift.org> wrote:
> > > 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 > > <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. > > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org > https://lists.swift.org/mailman/listinfo/swift-evolution > > > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org > https://lists.swift.org/mailman/listinfo/swift-evolution > >
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