[swift-evolution] Proposal: Make $0 always refer to a closure’s first argument

[T.J. Usiyan]

`$$` Doesn't take another sigil away and shouldn't be ambiguous since it
isn't used in current syntax.


On Wed, Jan 20, 2016 at 2:39 PM, John McCall via swift-evolution <
swift-evolution at swift.org> wrote:

> On Jan 20, 2016, at 9:49 AM, Joe Groff <jgroff at apple.com> wrote:
>
> On Jan 20, 2016, at 8:57 AM, Jordan Rose via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>
> On Jan 19, 2016, at 20:35 , John McCall via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> On Jan 19, 2016, at 7:20 PM, Paul Cantrell via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> Surprisingly, this code does not compile:
>
>     func foo(val: Int) { }
>
>     func bar(closure: (Int,Int) -> Void) {
>         closure(0, 1)
>     }
>
>     bar { foo($0) }       // compiler error
>     bar { foo($1) }       // just dandy
>     bar { foo($0 + $1) }  // also works
>
> The compiler error is:
>
>     Cannot convert value of type (Int, Int) to expected argument type Int
>
> It appears that the meaning of $0 is overloaded: it can refer either to
> the tuple of all arguments, or to just the first argument. The presence of
> another placeholder variable ($1 in the third example) seems to trigger
> the latter behavior.
>
>
> It’s dumber than that.  The type-checker assumes that the closure has a
> tuple of arguments ($0, $1, …, $N), where $N is the largest N seen in the
> closure.  Thus, a two-argument closure falls down if you ignore the second
> argument.  It’s dumb, and we’ve known about it for a long time; and yet
> it’s been remarkably annoying to fix, and so we haven’t yet.
>
> Anyway, it’s a bug and doesn’t need to go through evolution.
>
>
> I wouldn't go as far as to say it's a bug. It's known and occasionally
> useful for forwarding arguments. (For a while I had it as a fix-it for
> doing function representation conversions.)
>
> I agree that having it always be the first argument is less surprising and
> probably more generally useful, though.
>
>
> In the spirit of the Great Argument Simplification to distinguish
> arguments from tuples, we probably ought to introduce a separate '$*'-like
> sigil to bind "all arguments" distinct from $0.
>
>
> Yeah, that makes sense to me.  $_, clearly. :)
>
> John.
>
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