[swift-evolution] Code blocks and trailing closures

[Rien]

If I wrote this:

if serverCert.write(to: certificateUrl) { showErrorInKeyWindow(message); return }

then the meaning of return would have been different.

Imo it is a problem that two pieces of code impact the understanding of each other and that those two pieces of code could potentially be very far apart.

I do agree that the parenthesis are not a perfect solution, it was just the first thing that I could think of and that has some level of similarity in meaning.

Regards,
Rien

Site: http://balancingrock.nl
Blog: http://swiftrien.blogspot.com
Github: http://github.com/Balancingrock
Project: http://swiftfire.nl





> On 15 Mar 2017, at 12:00, Adrian Zubarev <adrian.zubarev at devandartist.com> wrote:
> 
> I’m slightly confused by this. How is a trailing closure different from a code block in Swift? It’s basically the same thing with some extra syntax sugar because it happens to be the last parameter of your function.
> 
> You can simply write this if you wanted to:
> 
> myFucntion(someLabel: abc, closureLabel: { …; return })
> 
> Parentheses are indicating that your closure is immediately invoked.
> 
> let someInt = { return 42 }()  
> print(someInt)
> 
> let someClosureWhichReturnsAnInt = { return 42 } // You can reuse the closure
> print(someClosureWhichReturnsAnInt()) // Invocation happens now here
> 
> return is scope based and it’s totally clear (to me) that in your case return will return from your closure with a value of Void.
> 
> 
> 
> 
> -- 
> Adrian Zubarev
> Sent with Airmail
> 
> Am 15. März 2017 um 11:35:39, Rien via swift-evolution (swift-evolution at swift.org) schrieb:
> 
>> What does the following code fragment do?
>> 
>> serverCert.write(to: certificateUrl) { showErrorInKeyWindow(message); return }
>> 
>> The only possible answer is: I don’t know.
>> 
>> The problem is finding out what the “return” statement will do.
>> 
>> Without knowing if the {...} is a code block or a trailing closure it is impossible to know what the return statement will do. It will either end the closure or it will end the function that contains this code block.
>> 
>> This could be disambiguated by using the same syntax as for lazy variables:
>> 
>> serverCert.write(to: serverCertificateUrl) { showErrorInKeyWindow(message: message); return }()
>> 
>> Now it is clear that the return statement will only terminate the (trailing) closure.
>> 
>> A question to the educators on the list: Is this a real problem?
>> 
>> Personally, I dislike this situation, but I am also ambivalent towards the solution I just described.
>> 
>> Regards,
>> Rien
>> 
>> Site: http://balancingrock.nl
>> Blog: http://swiftrien.blogspot.com
>> Github: http://github.com/Balancingrock
>> Project: http://swiftfire.nl
>> 
>> 
>> 
>> 
>> 
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