> On Dec 31, 2015, at 7:46 PM, Susan Cheng <susan.dog...@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 <dabrah...@apple.com
> <mailto:dabrah...@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-aa64bb142...@apple.com
>
> <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 <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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