> On Dec 31, 2015, at 1:02 PM, Dave Abrahams via swift-evolution
> <[email protected]> wrote:
>
>>
>> On Dec 31, 2015, at 10:52 AM, Donnacha Oisín Kidney <[email protected]
>> <mailto:[email protected]>> wrote:
>>
>> Just to add to that, it’s always seemed strange to me that to signify your
>> sequence is multi-pass (i.e., to make it conform to CollectionType) you have
>> to have it conform to Indexable.
>
> FWIW, Indexable is an implementation artifact that will go away when Swift’s
> generics system is improved.
>
> But if your real objection is that you have to come up with an Index and a
> subscripting operator, I can understand that. Part of the reason for this is
> our reluctance to create any distinct protocols with identical syntactic
> requirements
> <http://news.gmane.org/find-root.php?message_id=2a3e0c76-1c88-4752-8a70-aa64bb142...@apple.com>.
> To justify having a separate multi-pass sequence protocol, there would have
> to be a significant/important class of multi-pass sequences for which
> CollectionType was unimplementable without serious costs.
>
> In principle there’s a way to ease the pain of creating CollectionType
> conformances for multipass SequenceTypes…if only it didn’t crash the compiler
> <https://bugs.swift.org/browse/SR-427> ;-). Here’s a variation that uses a
> generic adapter instead of a protocol conformance declaration:
I probably failed to make clear that the code below works today, and doesn’t
crash the compiler:
>
> /// A `CollectionType` containing the same elements as `Base`, without
> storing them.
> ///
> /// - Requires: `Base` supports multiple passes (traversing it does not
> /// consume the sequence), and `Base.Generator` has value semantics
> public struct Multipass<Base: SequenceType where Base.Generator: Equatable> :
> CollectionType {
> public var startIndex: MultipassIndex<Base> {
> var g = _base.generate()
> return MultipassIndex(buffer: g.next(), generator: g)
> }
>
> public var endIndex: MultipassIndex<Base> {
> return MultipassIndex(buffer: nil, generator: _base.generate())
> }
>
> public subscript(position: MultipassIndex<Base>) -> Base.Generator.Element {
> return position.buffer!
> }
>
> public init(_ base: Base) {
> _base = base
> }
>
> var _base: Base
> }
>
> // Note: Requires T.Generator has value semantics
> public struct MultipassIndex<T: SequenceType where T.Generator: Equatable> :
> ForwardIndexType {
> public func successor() -> MultipassIndex {
> var r = self
> r.buffer = r.generator.next()
> return r
> }
> var buffer: T.Generator.Element?
> var generator: T.Generator
> }
>
> public func == <T>(x: MultipassIndex<T>, y: MultipassIndex<T>) -> Bool {
> return x.buffer == nil && y.buffer == nil || x.generator == y.generator
> }
>
> //===--- An example fibonacci sequence
> ------------------------------------===//
> struct FibGenerator : GeneratorType {
> mutating func next() -> Int? {
> let c = a + b
> a = b
> b = c
> return a < limit ? a : nil
> }
> var a, b, limit: Int
> }
>
>
> struct Fib : SequenceType {
> var limit = 1000
>
> func generate() -> FibGenerator {
> return Generator(a: 0, b: 1, limit: limit)
> }
> }
>
> //===--- Adapt Fib for use with Multipass
> ---------------------------------===//
> extension FibGenerator : Equatable {}
> func == (x: Fib.Generator, y: Fib.Generator) -> Bool {
> return x.a == y.a
> }
>
> //===--- Demonstration
> ----------------------------------------------------===//
> let c = Multipass(Fib())
> print(c.first)
> print(c.count)
> print(c.lazy.map { $0 + 1 })
>
>
>>
>>> On 31 Dec 2015, at 17:52, Erica Sadun via swift-evolution
>>> <[email protected] <mailto:[email protected]>> wrote:
>>>
>>> I'm trying to work them out, so it's still muddled.
>>>
>>> Right now, I think SequenceType is better described as CollectionWalkType
>>> but that's kind of (1) a mouthful and (2) not entirely accurate.
>>>
>>> Moving back a step: SequenceType is defined as: "A type that can be
>>> iterated with a `for`...`in` loop." But it says nothing about whether that
>>> loop ever terminates and many stdlib sequence functions currently don't
>>> make sense (at least if they're not lazy) with respect to infinite
>>> sequences, which should probably be "StreamType" not sequences. A couple of
>>> examples:
>>> Here's my fib: http://swiftstub.com/189513594/
>>> <http://swiftstub.com/189513594/>
>>> And here's Oisin's user-input sequence:
>>> https://gist.github.com/oisdk/2c7ac33bf2188528842a
>>> <https://gist.github.com/oisdk/2c7ac33bf2188528842a>
>>> Both of these are theoretically filterable, but they aren't dropLast-able,
>>> suffix-able, properly split-able, etc.
>>>
>>> Hopefully that's enough of a starting point to indicate where my thinking
>>> is at and what I'm trying to think through when it comes to this. -- E
>>>
>>>
>>>> On Dec 31, 2015, at 10:09 AM, Dave Abrahams <[email protected]
>>>> <mailto:[email protected]>> wrote:
>>>>
>>>>
>>>>> On Dec 31, 2015, at 9:05 AM, Erica Sadun via swift-evolution
>>>>> <[email protected] <mailto:[email protected]>> wrote:
>>>>>
>>>>> It does seem that in Swift the concepts of collection, sequence,
>>>>> permutation, stream, etc are a bit muddled.
>>>>
>>>> This is a pretty vague critique. Do you have specifics, and suggestions
>>>> that address them?
>>>>
>>>>>
>>>>> -- E
>>>>>
>>>>>
>>>>>> On Dec 31, 2015, at 6:51 AM, Tino Heth via swift-evolution
>>>>>> <[email protected] <mailto:[email protected]>> wrote:
>>>>>>
>>>>>>> Those are collections. Collections can be iterated over multiple times.
>>>>>> Speaking of the Fibonacci-numbers:
>>>>>> Sure we can write an algorithm that iterates over them several times —
>>>>>> it just won't ever finish the first iteration ;-)
>>>>>> (only nitpicking — I just couldn't resist)
>>>>>>
>>>>>> Happy new year!
>>>>>> _______________________________________________
>>>>>> swift-evolution mailing list
>>>>>> [email protected] <mailto:[email protected]>
>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>>> <https://lists.swift.org/mailman/listinfo/swift-evolution>
>>>>>
>>>>> _______________________________________________
>>>>> swift-evolution mailing list
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>>>>> <https://lists.swift.org/mailman/listinfo/swift-evolution>
>>>>
>>>> -Dave
>>>>
>>>
>>> _______________________________________________
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>
> -Dave
>
> _______________________________________________
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-Dave
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