[swift-evolution] [Review] SE-0174: Change `filter` to return an associated type

Jaden Geller jaden.geller at gmail.com
Fri Apr 28 21:27:23 CDT 2017

I think that the alternatives considered section isn’t entirely correct. Even if we had a more expressive type system, it would not be possible to make the return value of `map` preserve the element type *while* still allowing types like `String`, which aren’t parameterized by an element, to conform.

I think this would require two separate mapping protocols (ignore the straw man names and syntax):

protocol Mappable: Sequence {
    func map(_ transform: (Iterator.Element) -> Iterator.Element) -> Self
    // could also add `mapInPlace`

protocol MappableFunctor: Mappable {
    kind Self<Iterator.Element>
    func map<T>(_ transform: (Iterator.Element) -> T) -> Self<T>

I think we’d also require either a way for the generic function to conform to the non-generic signature if there exists a conforming instantiation if both functions were to have the same name.

Also, we’d probably want a way to say that the `T` in the generic signature has the same type constraints as the generic parameter would need to…


Anyway, the point is that we *could* consider adding the first protocol right now (or more likely, add the requirement to `Sequence`) even though the second protocol is not possible yet. Even if/when Swift’s type system can handle that, the first protocol will still be necessary for types like `String` that cannot conform to the second.

Jaden Geller
> On Apr 28, 2017, at 5:06 PM, Douglas Gregor via swift-evolution <swift-evolution at swift.org> wrote:
> Hello Swift community,
> The review of SE-0174 "Change `filter` to return an associated type" begins now and runs through May 3, 2017. The proposal is available here:
> https://github.com/apple/swift-evolution/blob/master/proposals/0174-filter-range-replaceable.md <https://github.com/apple/swift-evolution/blob/master/proposals/0174-filter-range-replaceable.md>
> Reviews are an important part of the Swift evolution process. All reviews should be sent to the swift-evolution mailing list at
> https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
> or, if you would like to keep your feedback private, directly to the review manager. When replying, please try to keep the proposal link at the top of the message:
> Proposal link:
> https://github.com/apple/swift-evolution/blob/master/proposals/0174-filter-range-replaceable.md <https://github.com/apple/swift-evolution/blob/master/proposals/0174-filter-range-replaceable.md>
> Reply text
> Other replies
>  <https://github.com/apple/swift-evolution#what-goes-into-a-review-1>What goes into a review?
> The goal of the review process is to improve the proposal under review through constructive criticism and, eventually, determine the direction of Swift. When writing your review, here are some questions you might want to answer in your review:
> What is your evaluation of the proposal?
> Is the problem being addressed significant enough to warrant a change to Swift?
> Does this proposal fit well with the feel and direction of Swift?
> If you have used other languages or libraries with a similar feature, how do you feel that this proposal compares to those?
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> https://github.com/apple/swift-evolution/blob/master/process.md <https://github.com/apple/swift-evolution/blob/master/process.md>
> Thank you,
> -Doug Gregor
> Review Manager
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