> 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. > 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. > > -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
