Open Radar

 
Summary:
I wanted a scan method, so I wrote myself a ScanSequenceView and added my scan method to an extension of SequenceType:

extension SequenceType {
	func scan<T>(initial: T, combine: (T, Generator.Element) -> T) -> ScanSequenceView<Self, T>
}

Then I wanted it to be lazy, so I wrote a LazySequence extension with a scan method:

extension LazySequence {
	func scan<T>(initial: T, combine: (T, Generator.Element) -> T) -> LazySequence<ScanSequenceView<LazySequence<S>, T>> { … }
}

But my tests of its laziness failed, because lazy([ 0, 1, 2 ]).scan(…) isn’t calling the LazySequence implementation, it’s calling the SequenceType implementation, because [Int] is a RandomAccessCollection and thus lazy([ 0, 1, 2 ]) returns a LazyRandomAccessCollection. There is no relationship between LazyRandomAccessCollection and LazySequence; they are entirely disjoint.

A LazySequenceType protocol joins them back up:

public protocol LazySequenceType: SequenceType {
	typealias Sequence: SequenceType
}

extension LazySequenceType {
	public func scan<T>(initial: T, combine: (T, Generator.Element) -> T) -> LazySequence<ScanSequenceView<Self, T>> {
		return lazy(ScanSequenceView(sequence: self, initial: initial, combine: combine))
	}
}

extension LazySequence: LazySequenceType {
	public typealias Sequence = S
}

extension LazyForwardCollection: LazySequenceType {
	public typealias Sequence = S
}

extension LazyBidirectionalCollection: LazySequenceType {
	public typealias Sequence = S
}

extension LazyRandomAccessCollection: LazySequenceType {
	public typealias Sequence = S
}


Please relate LazySequence and the various Lazy*Collections such that we can extend them once, like how extending SequenceType with scan() extended CollectionType instances too.


Steps to Reproduce:
N/A

Expected Results:
N/A

Actual Results:
N/A

Regression:
N/A

Notes:
Fortunately this is something that app & library authors can undertake for themselves, as witnessed by the code I wrote above actually functioning. I think it should be in the standard library regardless, however.