[swift-evolution] [Accepted] SE-0133: Rename flatten() to joined()

[Chris Lattner]

Proposal Link: https://github.com/apple/swift-evolution/blob/master/proposals/0133-rename-flatten-to-joined.md

The review of "SE-0133: Rename flatten() to joined()" ran from Active review July 24…26. The proposal has been *accepted*.

This proposal had significant positive community feedback for aligning common operation names, but raised questions about whether ‘flatten’ was term of art, and what it would mean for related operations like ‘flatMap’.  The core team discussed both sides of this debate, and decided it is best to rename ‘flatten’ to ‘joined’, but keep ‘flatMap’ as it is, to preserve its term of art.  The core team prefers that it remain a distinct overload of joined(separator:) to preserve performance.  It also requests that the returned collection types (FlattenCollection and friends) be left as-is for now, since they are not names commonly directly referenced, and we’d like to keep the change minimal.

Thank you to Jacob Bandes-Storch for driving this discussion forward!  Time is short, we’d appreciate it if someone could implement this ASAP.

-Chris Lattner
Review Manager