I wrote the proposal which was discussed to introduce generic constraints for
associated types. I’d like to get some feedback on it and get it ready before
submitting it:
More Powerful Constraints for Associated Types
Proposal: SE-XXXX
<https://github.com/hartbit/swift-evolution/blob/master/proposals/XXXX-powerful-constraints-associated-types.md>
Author(s): David Hart <http://github.com/hartbit>
Status: TBD
Review manager: TBD
<https://github.com/hartbit/swift-evolution/blob/master/proposals/XXXX-powerful-constraints-associated-types.md#introduction>Introduction
This proposal seeks to introduce a where expression to associated types
declarations to bring the same expressive power as generic type constraints.
This proposal was discussed on the Swift Evolution list in the
[swift-evolution] [Completing Generics] Arbitrary requirements in protocols
<http://thread.gmane.org/gmane.comp.lang.swift.evolution/14243> thread.
<https://github.com/hartbit/swift-evolution/blob/master/proposals/XXXX-powerful-constraints-associated-types.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 expression. 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/master/proposals/XXXX-powerful-constraints-associated-types.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.
<https://github.com/hartbit/swift-evolution/blob/master/proposals/XXXX-powerful-constraints-associated-types.md#issues>Issues
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 expression at the end of declarations like discussed in the
following thread:
http://thread.gmane.org/gmane.comp.lang.swift.evolution/13886/focus=14058
<http://thread.gmane.org/gmane.comp.lang.swift.evolution/13886/focus=14058>.
For those reasons, it might be wiser not to introduce the shorthand syntax.
<https://github.com/hartbit/swift-evolution/blob/master/proposals/XXXX-powerful-constraints-associated-types.md#acknowledgements>Acknowledgements
Thanks to Dave Abrahams and Douglas Gregor for taking the time to help me
through this proposal._______________________________________________
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