[swift-evolution] [proposal] Allow trailing closures in 'guard' conditions

[Thorsten Seitz]

> Am 23.03.2016 um 13:11 schrieb Erica Sadun via swift-evolution <swift-evolution at swift.org>:
> 
> I don't think this issue arises if you use the rule that functional closure arguments (returning values) shouldn't trail:

I'm not very fond of that rule, so I'd prefer the solution proposed by Chris :-)

-Thorsten 

> 
> func f(arr : [Int]?) {
>    guard let x = arr?.map({ $0+1 }) else { // compiles
>        preconditionFailure()
>    }
> }
> 
> That said, this seems like a positive tweak and I have no problem with its adoption.
> 
> -- E
> 
>>> On Mar 23, 2016, at 3:52 AM, Brent Royal-Gordon via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> The solution is simple: allow trailing closures in guard bodies.  This would allow this silly example to compile correctly:
>>> 
>>> func f(arr : [Int]?) {
>>> guard let x = arr?.map {$0+1} else {
>>>   preconditionFailure()
>>> }
>>> 
>>> // ...
>>> }
>> 
>> This sounds perfectly reasonable.
>> 
>>> Alternatives considered
>>> 
>>> There are three primary alternatives: do nothing, expand the scope of ‘if' and ‘while’ conditions as well, and significantly change the syntax of guard.
>> 
>> A fourth alternative would be to change the syntaxes of `if` and `while` (and probably `for` and `switch`) to also have a keyword in this position.
>> 
>>    if expr then { code }
>>    while expr do { code }
>>    for elem in expr do { code }
>>    switch expr among { code }
>> 
>> I'm not going to say I advocate for this option, but it *would* clearly mark the end of the condition so that trailing closures could be brought to all of these statements, so it seemed worth mentioning.
>> 
>> -- 
>> Brent Royal-Gordon
>> Architechies
>> 
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> 
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