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:
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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