> On 28 Sep 2016, at 23:03, Kevin Ballard <ke...@sb.org> wrote:
>
> On Wed, Sep 28, 2016, at 02:02 PM, Tim Vermeulen wrote:
>>
>>> 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.
>
> Either way you write it you're dealing with reversed indices.
`collection.indexed().reversed()` will contain indices from the original
collection (but in reversed order). `collection.reversed().indexed()` will
contain indices from the collection returned by `reversed()`, which may have a
different type than `Base.Index`. It’s a distinction.
This would compile:
let characters = "Swift".characters
for (index, character) in characters.indexed().reversed() {
print(characters[index], character)
}
This wouldn’t:
let characters = "Swift".characters
for (index, character) in characters.reversed().indexed() {
print(characters[index], character)
}
>
> -Kevin
>
>>>>> 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
>>>>
>>
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