> On 28 Sep 2016, at 22:57, Kevin Ballard <ke...@sb.org> wrote: > > On Wed, Sep 28, 2016, at 01:54 PM, Tim Vermeulen wrote: >> >>> On 28 Sep 2016, at 22:46, Kevin Ballard <ke...@sb.org> wrote: >>> >>> That's a bunch of complexity for no benefit. Why would you ever use this as >>> a collection? >> >> I think there is a benefit. Something like `collection.indexed().reversed()` >> would benefit from that, and I think that could be useful. > > Perhaps, though you could just say `collection.reversed().indexed()` instead.
This isn’t necessarily the same though, is it? The reversed collection might use different indices than the original collection. > >>> The whole point is to be used in a for loop. If it was a collection then >>> you'd need to have an index for that collection, so now you have an index >>> that lets you get the index for another collection, which is pretty useless >>> because you could just be using that underlying index to begin with. >> >> Rather than introducing a new index for this, we can simply use the index of >> the base collection for subscripting. > > That's actually a good idea, and if we do make it a collection this is > probably how we should handle it. But I still argue that the ability to make > something a collection doesn't mean it should be a collection, if there's no > good reason for anyone to actually try to use it as such. > > -Kevin > >>> On Wed, Sep 28, 2016, at 01:38 PM, Tim Vermeulen via swift-evolution wrote: >>>> +1 for `indexed()`, but I’m not sure about `IndexedSequence`. Why not >>>> `IndexedCollection`, which could also conform to Collection? With >>>> conditional conformances to BidirectionalCollection and >>>> RandomAccessCollection. This wouldn’t penalise the performance with >>>> respect to a simple `IndexedSequence`, would it? >>>> >>>>> Gist here:https://gist.github.com/erica/2b2d92e6db787d001c689d3e37a7c3f2 >>>>> >>>>> Introducingindexed()collections >>>>> Proposal: TBD >>>>> Author:Erica Sadun(https://github.com/erica),Nate >>>>> Cook(https://github.com/natecook1000),Jacob >>>>> Bandes-Storch(https://github.com/jtbandes),Kevin >>>>> Ballard(https://github.com/kballard) >>>>> Status: TBD >>>>> Review manager: TBD >>>>> >>>>> Introduction >>>>> >>>>> This proposal introducesindexed()to the standard library, a method on >>>>> collections that returns an (index, element) tuple sequence. >>>>> >>>>> >>>>> Swift-evolution thread:TBD(https://gist.github.com/erica/tbd) >>>>> >>>>> Motivation >>>>> >>>>> The standard library'senumerated()method returns a sequence of pairs >>>>> enumerating a sequence. The pair's first member is a monotonically >>>>> incrementing integer starting at zero, and the second member is the >>>>> corresponding element of the sequence. When working with arrays, the >>>>> integer is coincidentally the same type and value as anArrayindex but the >>>>> enumerated value is not generated with index-specific semantics. This may >>>>> lead to confusion when developers attempt to subscript a non-array >>>>> collection with enumerated integers. It can introduce serious bugs when >>>>> developers useenumerated()-based integer subscripting with non-zero-based >>>>> array slices. >>>>> >>>>> >>>>> Indices have a specific, fixed meaning in Swift, which are used to create >>>>> valid collection subscripts. This proposal introducesindexed()to produce >>>>> a more semantically relevant sequence by pairing a >>>>> collection'sindiceswith its members. While it is trivial to create a >>>>> solution in Swift, the most common developer approach shown here >>>>> calculates indexes twice: >>>>> >>>>> extension Collection { /// Returns a sequence of pairs (*idx*, *x*), >>>>> where *idx* represents a /// consecutive collection index, and *x* >>>>> represents an element of /// the sequence. func indexed() >>>>> ->Zip2Sequence<Self.Indices, Self>{ return zip(indices, self) } } >>>>> >>>>> Incrementing an index in some collections can be unnecessarily costly. In >>>>> a lazy filtered collection, an index increment is potentially O(N). We >>>>> feel this is better addressed introducing a new function into the >>>>> Standard Library to provide a more efficient design that avoids the >>>>> attractive nuisance of the "obvious" solution. >>>>> >>>>> Detailed Design >>>>> >>>>> Our vision ofindexed()bypasses duplicated index generation with their >>>>> potentially high computation costs. We'd create an iterator that >>>>> calculates each index once and then applies that index to subscript the >>>>> collection. Implementation would take place throughIndexedSequence, >>>>> similar toEnumeratedSequence. >>>>> >>>>> Impact on Existing Code >>>>> >>>>> This proposal is purely additive and has no impact on existing code. >>>>> >>>>> Alternatives Considered >>>>> Not yet >>>>> >>>>> >>>>> >>>>> >>>> _______________________________________________ >>>> swift-evolution mailing list >>>> swift-evolution@swift.org >>>> https://lists.swift.org/mailman/listinfo/swift-evolution >> _______________________________________________ swift-evolution mailing list swift-evolution@swift.org https://lists.swift.org/mailman/listinfo/swift-evolution
