[swift-evolution] [Proposal] More Powerful Constraints for Associated Types

[David Hart]

I’ve updated the proposal with comments from the discussions and opened a pull request. Please let me know if you have any feedback before it’s merged:

More Powerful Constraints for Associated Types

Proposal: SE-XXXX <https://github.com/apple/swift-evolution/blob/master/proposals/XXXX-associated-types-constraints.md>
Author(s): David Hart <http://github.com/hartbit>, Jacob Bandes-Storch <https://github.com/hartbit/swift-evolution/blob/associated-types-constraints/proposals/jtbandes@gmail.com>, Douglas Gregor <https://github.com/hartbit/swift-evolution/blob/associated-types-constraints/proposals/dgregor@apple.com>
Status: TBD
Review manager: TBD
 <https://github.com/hartbit/swift-evolution/blob/associated-types-constraints/proposals/XXXX-associated-types-constraints.md#introduction>Introduction

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

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

[Completing Generics] Arbitrary requirements in protocols <http://thread.gmane.org/gmane.comp.lang.swift.evolution/14243>
More Powerful Constraints for Associated Types <http://thread.gmane.org/gmane.comp.lang.swift.evolution/15201>
 <https://github.com/hartbit/swift-evolution/blob/associated-types-constraints/proposals/XXXX-associated-types-constraints.md#motivation>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 can be declared as follows:

protocol Sequence {
    associatedtype Iterator : IteratorProtocol
    associatedtype SubSequence : Sequence where SubSequence.Iterator.Element == Iterator.Element
    ...
}
 <https://github.com/hartbit/swift-evolution/blob/associated-types-constraints/proposals/XXXX-associated-types-constraints.md#detail-design>Detail Design

With this proposal, the grammar for protocols associated types would be modified to:

protocol-associated-type-declaration → attributesopt 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.

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 {
    ...
}
 <https://github.com/hartbit/swift-evolution/blob/associated-types-constraints/proposals/XXXX-associated-types-constraints.md#alternatives>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 will become ambiguous if we adopt the generic where clause at the end of declarations like discussed in proposal SE-0081: Move where clause to end of declaration <https://github.com/apple/swift-evolution/blob/master/proposals/0081-move-where-expression.md>. For those reasons, it might be wiser not to introduce the shorthand syntax.

 <https://github.com/hartbit/swift-evolution/blob/associated-types-constraints/proposals/XXXX-associated-types-constraints.md#acknowledgements>Acknowledgements

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