[swift-evolution] Deprecating Trailing Closures

Erica Sadun erica at ericasadun.com
Thu Mar 24 11:59:49 CDT 2016

> On Mar 24, 2016, at 10:41 AM, Haravikk via swift-evolution <swift-evolution at swift.org> wrote:
>> On 24 Mar 2016, at 16:13, William Dillon <william at housedillon.com <mailto:william at housedillon.com>> wrote:
>> Another thing I like about trailing closures is that it allows me to make custom constructs that feel more like a part of the language.  For example, I really love this extension for NSLock that I have:
>> extension NSLock {
>>     func protect(action: (Void) -> Void) {
>>         self.lock()
>>         action()
>>         self.unlock()
>>     }
>> }
>> Now, whenever I need to use my lock, I can just do:
>> peersLock.protect {
>>     outputString += "\(self.peers.count) peers:\n"
>>     for (_, peer) in self.peers {
>>         outputString += "\(peer)\n"
>>     }
>> }
>> To me, it looks cleaner to me to not have this paren dangling around at the end.  On this one I’d definitely say that if you don’t like it, don’t use it.  I don’t *think* that you’re forced to use it anywhere.  It’s a hard sell to take it away from everyone.
>> - Will
> I’m not proposing to remove them entirely, in fact your lock example is a perfect example of when a trailing closure makes the most sense, as a form of customised language feature. But I’m wondering if perhaps cases like these should be created using an attribute that specifically enables it? e.g- your definition could become:
> 	func protect(action: @trailing (Void) -> Void) { … }
> It’s other cases like common usages of .map() and similar methods where I’ve found myself using the closure in its trailing form less and less, and am not as sure if it’s really needed, or may actually be more of a detriment to the language than a benefit.

I follow the "Rule of Kevin", which is not language enforced. Parens around functional 
closures (->T), not around procedural (->Void) ones.  This promotes "language construct"-like 
Void calls, avoids compiler parsing issues when chaining (or using "guard", "if", etc). It lets
me know instantly how the closure is used. 

While I was originally reluctant to adopt it, its advantages have become self-evident over time. 
This ends up being slightly wordier, especially in the few cases you need to use argument labels. 

I think it's worth it.

-- E
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