[swift-evolution] Should closures support inout parameters?

[Goffredo Marocchi]

Good :), will it get put with the next Swift revision then?

Sent from my iPhone

> On 19 Sep 2016, at 08:37, Chris Lattner via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
>> On Sep 18, 2016, at 8:17 AM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> 
>> on Fri Sep 16 2016, Braeden Profile <swift-evolution at swift.org> wrote:
>> 
>>> I was writing some code that would allow transformations of values as
>>> part of an expression, and I came across a strange error:
>>> 
>>> /// Returns the operand after a given transformation.
>>> ///
>>> /// Example:  `let newRect = myRect << { $0.origin.x += 3 }`
>>> func << <T> (given: T, transformation: (inout T) -> ()) -> T
>>> {
>>> 	var result = given
>>> 	transformation(&result)
>>> 	return result
>>> }
>>> 
>>> let volume = component.volume << { $0.ranges.z.width = 0 } // Error:
>>> Expression type () is ambiguous without more context.
>>> let volume = component.volume << { $0.ranges.z.width = 0; return () }
>>> // Error: Cannot assign to property: ‘$0’ is immutable.
>>> let volume = component.volume << { (x: inout SCNBoxVolume) in x.ranges.z.width = 0 } // Succeeds!
>>> 
>>> Obviously, this code could easily create a var for volume and mutate
>>> it, but it doesn’t solve my problem.  Am I misunderstanding how this
>>> could work?  This is the only overload of << that accepts a closure,
>>> and even the code completion recognizes that $0 is a SCNBoxVolume.
>>> It’s just strange that the compiler won’t recognize $0 as an inout
>>> parameter off the bat.
>> 
>> I think the subject you wanted was “should we deduce closure inout
>> parameters?”
>> 
>>> Is this a bug, or a design choice?
>> 
>> I've been wondering the same.  I hope it's a bug, and I suggest you file
>> it.
> 
> It’s a bug, it has been filed several times, it is pretty well known at this point.
> 
> -Chris
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