[swift-evolution] [Pitch] Unify joined(separator:) and flatten()

[Jacob Bandes-Storch]

Here's a proposal draft. Comments welcome:

https://gist.github.com/jtbandes/7978dc1848f7c37eeaa8e9aba27c7325

On Fri, Jul 22, 2016 at 2:51 PM, Ben Rimmington <me at benrimmington.com>
wrote:

>
> > On 22 Jul 2016, at 20:43, Jacob Bandes-Storch <jtbandes at gmail.com>
> wrote:
> >
> >> On Fri, Jul 22, 2016 at 8:35 AM, Ben Rimmington <me at benrimmington.com>
> wrote:
> >>
> >>> On 22 Jul 2016, at 02:46, Jacob Bandes-Storch wrote:
> >>>
> >>> In the swift-lang Slack channel, a few of us were discussing
> joined(separator:) and realized that flatten() does almost exactly the same
> thing.
> >>>
> >>> Is there interest in renaming flatten() to joined()?  Since joined
> takes a separator that's any Sequence, we can't have a default value for
> the separator parameter, but we can have a variant of joined() with no
> arguments.
> >>
> >> I'd like default separators for the joined() methods.
> >>
> >> <https://bugs.swift.org/browse/SR-1428>
> >>
> >> But renaming flatten() to joined() seems complicated.
> >>
> >> <
> https://github.com/apple/swift/blob/master/stdlib/public/core/Flatten.swift.gyb
> >
> >> <
> https://github.com/apple/swift/blob/master/stdlib/public/core/Join.swift>
> >
> > What makes it seem complicated? At the very least, one could just rename
> the flatten() function. There might also be an opportunity to combine the
> two files and delete some code from stdlib.
>
> There's only one joined() method (for a sequence of sequences):
>
>         extension Sequence {
>           func joined<Separator: Sequence>(separator: Separator) ->
> JoinedSequence<Self>
>         }
>
> There are many flatten() methods (`where` clauses omitted for brevity):
>
>         extension Sequence {
>           func flatten() -> FlattenSequence<Self>
>         }
>
>         extension LazySequenceProtocol {
>           func flatten() -> LazySequence<FlattenSequence<Self.Elements>>
>         }
>
>         extension LazyCollectionProtocol {
>           func flatten() ->
> LazyCollection<FlattenCollection<Self.Elements>>
>         }
>
>         extension Collection {
>           func flatten() -> FlattenCollection<Self>
>         }
>
>         extension BidirectionalCollection {
>           func flatten() -> FlattenBidirectionalCollection<Self>
>         }
>
> So it's not a simple one-to-one rename.
>
> When there's no `separator` argument, will FlattenIterator perform better
> than JoinedIterator?
>
> >> And what would happen to the flatMap() methods? Is flatten() a term of
> art?
> >>
> >> <
> https://github.com/apple/swift/blob/master/stdlib/public/core/FlatMap.swift
> >
> >
> > I'd say flatMap is more a "term of art" than flatten. "flatten" just
> describes literally what is being done. Frankly I'm surprised it was never
> named flattened(). Anyway, flatMap should stay.
>
> ## Future directions
>
> Will the flatMap(_:) methods also have flatMap(separator:_:) variants?
>

That's an interesting idea. It seems to be purely additive, however, so I
imagine it wouldn't happen until after Swift 3.


> -- Ben
>
>
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