[swift-evolution] [swift-evolution-announce] [Review] SE-0142: Permit where clauses to constrain associated types

[Karl]

> On 24 Sep 2016, at 21:58, Drew Crawford via swift-evolution <swift-evolution at swift.org> wrote:
> 
> This is the #1 Swift feature I have wanted for the past 2 years.
> 
> One thing I would like to see (perhaps out of scope for this proposal) is the natural extension for generic parameters:
> 
> struct Box<T> {
>     var t: T
> }
> 
> extension Box where T == Int { //error: Same-type requirement makes generic parameter T non-generic
> }
> 
> There is a great deal of overlap between associated types and generic parameters, and it seems to me that it would be favorable to allow `==` with both generic parameter operands and associatedtype operands for consistency.
> 
> Drew
> 


I think this is already part of the Generics Manifesto: https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md

> Concrete same-type requirements
> 
> Currently, a constrained extension cannot use a same-type constraint to make a type parameter equivalent to a concrete type. For example:
> 
> extension Array where Element == String {
>   func makeSentence() -> String {
>     // uppercase first string, concatenate with spaces, add a period, whatever
>   }
> }
> This is a highly-requested feature that fits into the existing syntax and semantics. Note that one could imagine introducing new syntax, e.g., extending Array<String>, which gets into new-feature territory: see the section on "Parameterized extensions".
> 

- Karl
> On September 23, 2016 at 6:51:51 PM, Douglas Gregor (dgregor at apple.com <mailto:dgregor at apple.com>) wrote:
> 
>> Hello Swift community,
>> 
>> The review of SE-0142: "Permit where clauses to constrain associated types" begins now and runs through September 30, 2016. The proposal is available here:
>> 
>> https://github.com/apple/swift-evolution/blob/master/proposals/0142-associated-types-constraints.md <https://github.com/apple/swift-evolution/blob/master/proposals/0142-associated-types-constraints.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/0142-associated-types-constraints.md <https://github.com/apple/swift-evolution/blob/master/proposals/0142-associated-types-constraints.md>
>> Reply text
>> 
>> Other replies
>>  <https://github.com/apple/swift-evolution/pull/532#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?
>> How much effort did you put into your review? A glance, a quick reading, or an in-depth study?
>> More information about the Swift evolution process is available at
>> 
>> 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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