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e14a99f Oct 6, 2016
@DougGregor @krilnon
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Permit where clauses to constrain associated types

Introduction

This proposal seeks to introduce a where clause to associated type declarations and improvements to protocol constraints to bring associated types the same expressive power as generic type parameters.

This proposal was discussed twice on the Swift Evolution list in the following threads:

Motivation

Currently, associated type declarations can only express simple inheritance constraints and not the more sophisticated constraints available to generic types with the where clause. Some designs, including many in the Standard Library, require more powerful constraints for associated types to be truly elegant. For example, the SequenceType protocol could be declared as follows if the current proposal was accepted:

protocol Sequence {
    associatedtype Iterator : IteratorProtocol
    associatedtype SubSequence : Sequence where SubSequence.Iterator.Element == Iterator.Element
    ...
}

Detailed Design

First of all, this proposal modifies the grammar for a protocol's associated types to the following:

protocol-associated-type-declarationattributesopt access-level-modifieropt associatedtype typealias-name ­type-inheritance-clause­opt typealias-assignment­opt requirement-clauseopt

The new requirement-clause is then used by the compiler to validate the associated types of conforming types.

Secondly, the proposal also allows protocols to use the associated types of their conforming protocols in their declaration where clause as below:

protocol IntSequence : Sequence where Iterator.Element == Int {
    ...
}

Name lookup semantics in the protocol declaration where clause only looks at associated types in the parent protocols. For example, the following code would cause an error:

protocol SomeSequence : Sequence where Counter : SomeProtocol { // error: Use of undefined associated type 'Counter'
    associatedtype Counter
}

But instead should be written on the associated type itself:

protocol IntSequence : Sequence {
    associatedtype Counter : SomeProtocol
}

Effect on ABI Stability

As mentioned previously, there are a number of places in the standard library where this feature would be adopted (such as the SubSequence.Iterator.Element == Iterator.Element example), each of which will change the mangling of any generic function/type that makes use of them.

Alternatives

Douglas Gregor argues that the proposed syntax is redundant when adding new constraints to an associated type declared in a parent protocol and proposes another syntax:

protocol Collection : Sequence {
    where SubSequence : Collection
}

But as Douglas notes himself, that syntax is ambiguous since we adopted the generic where clause at the end of declarations of the following proposal: SE-0081: Move where clause to end of declaration. For those reasons, it might be wiser not to introduce the shorthand syntax.

Acknowledgements

Thanks to Dave Abrahams and Douglas Gregor for taking the time to help me through this proposal.