[swift-evolution] [Proposal]Allow constraints on associatedtype and shorten type constraints list in function

[Anders Ha]

It is one of the items in the Generics Manifesto, and we had a discussion thread with a proposal on this already, however halted for being an addictive feature. Anyway, aren't discussions on post Swift 3 matters preferred to start on Aug 1?



Hart's proposal
https://github.com/hartbit/swift-evolution/blob/master/proposals/XXXX-powerful-constraints-associated-types.md <https://github.com/hartbit/swift-evolution/blob/master/proposals/XXXX-powerful-constraints-associated-types.md>

[swift-evolution] [Proposal] More Powerful Constraints for Associated Types
https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160502/016354.html <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160502/016354.html>



Regards,
Anders


> On 29 Jul 2016, at 10:17 AM, Susan Cheng via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Hello swift community,
> 
> I want to introduce a proposal to allow constraints on associatedtype. 
> I found a bug report(https://bugs.swift.org/browse/SR-1466 <https://bugs.swift.org/browse/SR-1466>) and it's due to without constraints on associatedtype itself.
> 
> This force us always have to write the redundant constraints like Indices.Iterator.Element == Index or Indices.SubSequence.Iterator.Element == Index on type constraints in function:
> 
> public extension MutableCollection where Self : RandomAccessCollection, Indices.Index == Index, Indices.SubSequence : RandomAccessCollection, Indices.SubSequence.Iterator.Element == Index {
>     
>     /// Shuffle `self` in-place.
>     mutating func shuffle() {
>         for i in self.indices.dropLast() {
>             let j = self.indices.suffix(from: i).random()!
>             if i != j {
>                 swap(&self[i], &self[j])
>             }
>         }
>     }
> }
> 
> Besides this, we also can write some odd definitions but allowed by swift compiler.
> 
> struct MyArray : Collection {
>     
>     typealias Indices = CountableRange<Int32>
>     
>     var base: [Int]
>     
>     var startIndex: Int {
>         return base.startIndex
>     }
>     var endIndex: Int {
>         return base.endIndex
>     }
> 
>     func index(after: Int) -> Int {
>         return after + 1
>     }
>     
>     var indices: CountableRange<Int32> {
>         return CountableRange(uncheckedBounds: (lower: Int32(startIndex), upper: Int32(endIndex)))
>     }
>     
>     subscript(position: Int) -> Int {
>         get {
>             return base[position]
>         }
>         set {
>             base[position] = newValue
>         }
>     }
> }
> 
> as a reference: http://stackoverflow.com/questions/37581234/can-an-associated-type-be-restricted-by-protocol-conformance-and-a-where-clause <http://stackoverflow.com/questions/37581234/can-an-associated-type-be-restricted-by-protocol-conformance-and-a-where-clause>
> 
> it's clearly that we need a syntax like this:
> 
> public protocol Collection : Indexable, Sequence {
>     
>     /// A sequence that represents a contiguous subrange of the collection's
>     /// elements.
>     ///
>     /// This associated type appears as a requirement in the `Sequence`
>     /// protocol, but it is restated here with stricter constraints. In a
>     /// collection, the subsequence should also conform to `Collection`.
>     associatedtype SubSequence : IndexableBase, Sequence where SubSequence.Iterator.Element == Iterator.Element = Slice<Self>
>     
>     /// A type that can represent the indices that are valid for subscripting the
>     /// collection, in ascending order.
>     associatedtype Indices : IndexableBase, Sequence where Indices.Iterator.Element == Index = DefaultIndices<Self>
> 
> }
> 
> This  harmless and brings huge benefits to swift.
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20160729/51790df7/attachment.html>