[swift-evolution] About the PermutationGenerator
Dave Abrahams
dabrahams at apple.com
Fri Jan 1 20:30:02 CST 2016
I see what you’ve done, now, after diffing your code with mine. That’s a very nice improvement; it generalizes to cover more Sequences and doesn’t require an Equatable generator. Great job!
> On Dec 31, 2015, at 9:30 PM, Susan Cheng <susan.doggie at gmail.com> wrote:
>
> I didn't explain correctly.
>
> let's take this:
>
> let c = Multipass(Fib(a: 1, b: -1, limit: 10))
>
> this sequences should have results with [1, -1, 0, -1, -1, ...]
> So is c.startIndex.successor() equal to c.startIndex.successor().successor().successor()??
>
> Dave Abrahams <dabrahams at apple.com <mailto:dabrahams at apple.com>> 於 2016年1月1日星期五 寫道：
>
>> On Dec 31, 2015, at 8:22 PM, 鄭齊峯 via swift-evolution <swift-evolution at swift.org <javascript:_e(%7B%7D,'cvml','swift-evolution at swift.org');>> wrote:
>>
>> if you try my modification, it will crash.
>
> Only because your Sequence genereates an arithmetic underflow. I don’t understand what point you’re trying to make
>>
>> struct Fib : SequenceType {
>>
>> var a: Int
>> var b: Int
>>
>> var limit: Int
>>
>> func generate() -> FibGenerator {
>> return Generator(a: a, b: b, limit: limit)
>> }
>> }
>>
>> let c = Multipass(Fib(a: 1, b: -1, limit: 10))
>>
>> A SequenceType becomes a CollectionType only work by adding a Index. that the most significantly different of sequence and collection.
>
> I think I understand that pretty well, having designed both of those abstractions, but still don’t get your point. Sorry, I’m trying here, but I can’t figure out what you’re trying to tell me.
>
>>
>> /// 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> : CollectionType {
>> public var startIndex: MultipassIndex<Base> {
>> var g = _base.enumerate().generate()
>> if let (idx, val) = g.next() {
>> return MultipassIndex(index: idx, buffer: val, generator: g)
>> }
>> return MultipassIndex(index: nil, buffer: nil, generator: g)
>> }
>>
>> public var endIndex: MultipassIndex<Base> {
>> return MultipassIndex(index: nil, buffer: nil, generator: _base.enumerate().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> : ForwardIndexType {
>> public func successor() -> MultipassIndex {
>> var r = self
>> if let (idx, val) = r.generator.next() {
>> r.index = idx
>> r.buffer = val
>> } else {
>> r.index = nil
>> r.buffer = nil
>> }
>> return r
>> }
>> var index: Int?
>> var buffer: T.Generator.Element?
>> var generator: EnumerateSequence<T>.Generator
>> }
>>
>> public func == <T>(x: MultipassIndex<T>, y: MultipassIndex<T>) -> Bool {
>> return x.index == y.index
>> }
>>
>> //===--- 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 = 100
>>
>> func generate() -> FibGenerator {
>> return Generator(a: 0, b: 1, limit: limit)
>> }
>> }
>>
>> It's true that ForwardIndexType is also run-through the base one by one but it also correct that getting specific value with specific index.
>>
>> c[c.startIndex.successor().successor().successor()] // it's a defined behave
>
> Again, not sure what you’re trying to say here.
>
>>
>>
>>> Dave Abrahams <dabrahams at apple.com <javascript:_e(%7B%7D,'cvml','dabrahams at apple.com');>> 於 2016年1月1日 下午12:00 寫道：
>>>
>>>>
>>>> On Dec 31, 2015, at 7:46 PM, Susan Cheng <susan.doggie at gmail.com <javascript:_e(%7B%7D,'cvml','susan.doggie at gmail.com');>> wrote:
>>>>
>>>>
>>>> How GeneratorType confirm to Equatable??
>>>
>>> I don’t understand the question. In the code I posted there’s a working example of how a GeneratorType model can conform to Equatable..
>>>
>>>>
>>>> struct Fib : SequenceType {
>>>>
>>>> var a: Int
>>>> var b: Int
>>>>
>>>> var limit: Int
>>>>
>>>> func generate() -> FibGenerator {
>>>> return Generator(a: a, b: b, limit: limit)
>>>> }
>>>> }
>>>>
>>>> let c = Multipass(Fib(a: 1, b: -1, limit: 10))
>>>>
>>>> -Susan
>>>>
>>>>
>>>> 2016-01-01 11:17 GMT+08:00 Dave Abrahams <dabrahams at apple.com <javascript:_e(%7B%7D,'cvml','dabrahams at apple.com');>>:
>>>> 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-AA64BB14223A@apple.com <http://news.gmane.org/find-root.php?message_id=2A3E0C76-1C88-4752-8A70-AA64BB14223A@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 <https://bugs.swift.org/browse/SR-427>> ;-). Here’s a variation that uses a generic adapter instead of a protocol conformance declaration:
>>>>
>>>> /// 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 })
>>>>
>>>
>>> -Dave
>>
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> -Dave
>
-Dave
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