[swift-evolution] Removing "_ in" from empty closures

[Joe Groff]

> On May 13, 2016, at 9:13 AM, Rob Napier via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Currently if a closure takes a value, it requires "_ in" to note that the value is ignored. This makes sense in many cases, but creates a bit of a mess in the case of an empty, void-returning closure:
> 
> doThing(withCompletion: { _ in })
> 
> I'd like to suggest that the compiler promote the empty closure literal {} to any void-returning closure type so that this could be written:
> 
> doThing(withCompletion: {})
> 
> This encourages the use of empty closures over optional closures, which I think is open for debate. In general I try to avoid optionals when they can be precisely replaced with a non-optional value. Furthermore, most Cocoa completion handlers are not optional.
> 
> The alternative is to not do this, but encourage that any closure that could reasonably be empty should in fact be optional. I would then want Cocoa functions with void-returning closures to be imported as optionals to avoid "{ _ in }".

+1. In general, I think we should allow implicit arguments, without requiring the closure to use all the implicit $n variables like we do today. These should all be valid:

let _: () -> () = {}
let _: (Int) -> () = {}
let _: (Int, Int) -> Int = { 5 }
let _: (Int, Int) -> Int = { $0 }
let _: (Int, Int) -> Int = { $1 }

-Joe