[swift-evolution] Reduce with inout

[Xiaodi Wu]

Hmm, it reads well, but IMO it avoids being misleading only because it
doesn't mean anything. In what way are you reducing "into" the first
argument any more so than the classic reduce function?
On Tue, Jan 24, 2017 at 12:44 Matthew Johnson via swift-evolution <
swift-evolution at swift.org> wrote:

> On Jan 24, 2017, at 12:36 PM, Pyry Jahkola via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>
> 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
>     }
>
>
> +1.  This is concise and I think it captures the essence of what is
> happening pretty well!
>
> The third variation where the seed argument actually *is* `inout` might
> also be interesting in some cases where you *already* have a `var` that you
> want to accumulate into.  I believe I have done this in the past in my own
> code but don’t have an example handy.
>
>
> — Pyry
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