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

[Ben Rimmington]

> 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>

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>

-- Ben