[swift-dev] Fixit for trailing closures

[Jordan Rose]

> On Jul 5, 2016, at 17:19, Ben Langmuir via swift-dev <swift-dev at swift.org> wrote:
> 
>> 
>> On Jul 5, 2016, at 4:34 PM, Xi Ge via swift-dev <swift-dev at swift.org <mailto:swift-dev at swift.org>> wrote:
>> 
>> Hi Swift-devs,
>> I have tried to add a fixit to help developers using trailing closures more, motivated by my observation during WWDC that some developers 
>> do not even realize that we have such a feature. In my opinion, trailing closures are more concise, and once you get used to it, more readable; 
>> therefore users should adopt trailing closures whenever doing so introduces no ambiguity.
>> 
>> Fixits can enhance the discoverability of trailing closures by identifying misuses and by transforming users’ code automatically. However, adding the fixit introduces new issues:
>> 
>> Issue 1: The fixit has to be associated with a warning. Adding the warning means we declare wars against convertible non-trailing closures, which is a valid syntax choice by users.
>> 
>> Issue 2: Ambiguity checking should be exhaustive. We have several known situations when non-trailing closures cannot be convert to trailing closures, including:
>> 	
>> Trailing closures are followed by other brackets, e.g., “if foo({}) {}” cannot be converted to “if foo {} {}”.
>> Removing the label of the last closure causes ambiguous function references, e.g. “foo(v: {})” cannot be converted to “foo {}” when “foo(v1: {})” also exists.
>> 
>> So Swift-devs, is the warning worth adding? If yes, are there other situations of ambiguity that are not covered?
>> 
>> Thanks for your feedback!
>> Xi
>> 
> 
> -1, this feels like a really good tool to have in a code-linter, but not something that we should put in the compiler and prescribe to all our users.
> 
> My biggest concern is that it’s not always obvious what the closure is used for.  The argument label can help with this:
> 
> [1].lexicographicallyPrecedes([0]) { <#code#> }
> [1].lexicographicallyPrecedes([0], isOrderedBefore: { <#code#> })

I strongly agree with Ben. This is a style choice and highly context-dependent. We should not warn about it or produce a fix-it unless specifically requested, and we don’t have any place for such requests today.

(Motivation: Another case where I prefer not using a trailing closure is when there are other closure arguments in the call.)

Jordan

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