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

[John McCall]

> 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 <mailto:swift-evolution at swift.org>> wrote:
>> 
>>> 
>>> On Jan 19, 2016, at 20:35 , John McCall via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>>> On Jan 19, 2016, at 7:20 PM, Paul Cantrell via swift-evolution <swift-evolution at swift.org <mailto: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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