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

[Douglas Gregor]

> On Apr 24, 2016, at 1:34 PM, David Hart via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 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.
> 
Believe it or not, I support this direction…

>  <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.
> 
The only thing that bothers me about this syntax is that I have to introduce an associated type to add requirements. For example, what if I want my inheriting protocol to add a requirement to an existing associated type?

protocol P { }

protocol Q {
  typealias AssocType
}

protocol R : Q {
  // I want to just add “AssocType : P”, but I have to redeclare AssocType to do so
  typealias AssocType where AssocType : P
}

Did you consider an alternate syntax that puts the where clause outside the braces, e.g.,

protocol R : Q where AssocType : P {
  // …
}

There are two things I like about this. First, it breaks the unnecessary link between an associated type and a (possibly unrelated) where clause, eliminating the need to redeclare associated types in inheriting protocols. Second, it’s effectively the same syntax as constrained extensions, which have a similar feel.

Note that, if we do the above, I’d love to make it an error to define a new associated type with the same name as an associated type in an inherited protocol. It’s odd that we do so, and IIRC the only use case for it is to add requirement to an “existing” associated type.

	- Doug


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