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

[Matthew Johnson]

Sent from my iPad

> On May 13, 2016, at 11:16 AM, Joe Groff via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
>> 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 }

+1.  Having to explicitly discard unnecessary arguments bugs me every time I have to do it.

> 
> -Joe
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