[swift-evolution] Reduce with inout

[Pyry Jahkola]

> Freak Show wrote:
> 
> Am I the only one who finds this incredibly ugly and hard to read?
> 
> This is more or less solved by inject:into: idiom.  There is no reason for inout for this particular problem.

Yeah, the original signature seems more useful. If you go all `inout` like Gwendal suggested, you might as well just iterate over the sequence with `for x in xs`, updating the state as you go.

But your comment brought another idea to mind: if `mutating:` is considered a bad name for a non-`inout` argument, how about `reduce(into:combine:)`, similar to what Karl suggested earlier in this thread?

I think it reads very well at the call site, does not suggest `inout`ness of the argument too much (of course there's no `&` at the call site either), and it's still easily found with auto-completion:

    let counts = words.reduce(into: [:]) {
      $0[$1] = ($0[$1] ?? 0) + 1
    }

— Pyry
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