## Introduction
Add a few more functional sequence utilities to the standard library.
## Motivation
We have map, filter, and reduce, but we're missing a bunch of useful utilities
like scan, iterate, takeWhile, and dropWhile. Interestingly, the stdlib
includes an implementation of scan in the doc comment for LazySequenceType, it
just doesn't actually provide it as API.
## Proposed solution
We extend SequenceType with 3 new methods scan, takeWhile, and dropWhile. We
also add a single global function iterate.
## Detailed design
We add the following extension to SequenceType:
extension SequenceType {
func scan<T>(initial: T, @noescape combine: (T, Self.Generator.Element)
throws -> T) rethrows -> [T]
func dropWhile(@noescape dropElement: (Self.Generator.Element) throws ->
Bool) rethrows -> [Self.Generator.Element]
func takeWhile(@noescape takeElement: (Self.Generator.Element) throws ->
Bool) rethrows -> [Self.Generator.Element]
}
These all take functions, so to follow convention they're @noescape and return
arrays. We also provide an extension of CollectionType that overrides a couple
of these methods:
extension CollectionType {
func dropWhile(@noescape dropElement: (Self.Generator.Element) throws ->
Bool) rethrows -> Self.SubSequence
func takeWhile(@noescape takeElement: (Self.Generator.Element) throws ->
Bool) rethrows -> Self.SubSequence
}
We also provide lazy versions:
extension LazySequenceType {
func scan<T>(initial: T, combine: (T, Self.Generator.Element) -> T) ->
LazyScanSequence<Self.Elements, T>
func dropWhile(dropElement: (Self.Generator.Element) -> Bool) ->
LazyDropWhileSequence<Self.Elements>
func takeWhile(takeElement: (Self.Generator.Element) -> Bool) ->
LazyTakeWhileSequence<Self.Elements>
}
extension LazyCollectionType {
func dropWhile(dropElement: (Self.Generator.Element) -> Bool) ->
LazyDropWhileCollection<Self.Elements>
func takeWhile(takeElement: (Self.Generator.Element) -> Bool) ->
LazyTakeWhileCollection<Self.Elements>
}
No collection variant of scan is provided because that would require storing
the last value in the index itself, which would cause problems if the combine
function isn't pure.
LazyDropWhileCollection would behave similarly to LazyFilterCollection in that
it runs the predicate against the elements to drop when accessing startIndex;
unlike LazyFilterCollection, because there's nothing else to skip after that
point, the index itself can actually be Self.Elements.Index (just like a
slice). LazyTakeWhileCollection also runs the predicate against the first
element when accessing startIndex, but it does need a unique index type
(because endIndex has to be some sentinel value, as it doesn't know where the
end is until you reach that point; this index type would therefore only conform
to ForwardIndexType).
And finally, we provide a global function
func iterate<T>(initial: T, _ f: T -> T) -> IterateSequence<T>
This function is inherently lazy and yields an infinite list of nested
applications of the function, so iterate(x, f) yields a sequence like [x, f(x),
f(f(x)), ...].
-Kevin Ballard
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