[swift-evolution] [Thoughts?][Phase2] default arguments and trailing closure syntax

[Douglas Gregor]

> On Jan 4, 2017, at 7:48 PM, Saagar Jha via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Check out this thread–it’s very similar to what you proposed, but it didn’t go anywhere. FWIW +1 to this as well as the ability to use multiple trailing closures like so:
> 
> animate(identifier: “”, duration: 0, update: {
> 	// update
> }, completion: {
> 	// completion
> }
> 
> Saagar Jha
> 
> 
> 
>> On Jan 4, 2017, at 6:25 PM, Jay Abbott via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> When you have a function with a closure and then another optional default = nil closure at the end, like this:
>> 
>> open static func animate(identifier: String,
>>                          duration: Double,
>>                          update: @escaping AnimationUpdate,
>>                          completion: AnimationCompletion? = nil) {
>> You can’t use trailing closure syntax for the update argument when leaving the completion argument out/default.
>> 
>> This kind of breaks one of the benefits of default arguments, which is that you can add them to existing released functions without breaking the calling code. This means you have to add a separate convenience function without the extra argument, which is annoying and inelegant.
>> 
Why not simply add the "completion" parameter before the trailing closure? That would still allow existing callers to work, without having to change the language. 

>> Another annoying thing is that you can easily miss this error if you happen to not use trailing closure syntax in your tests or other usage, because adding the extra default argument compiles fine for code that uses normal syntax.
>> 
The Swift compiler warns when a parameter written as a closure type isn't the last parameter. The warning is actually disabled in the specific case above because you've written it using a typealias... maybe we should warn on such cases (it's worth a bug report). Regardless, in the majority of instances, you'll get a warning, so it won't be silent on disabling trailing closure syntax. 

>> Are there any issues/gotchas if the trailing closure syntax were to work for the last specified argument rather than the last defined argument?
>> 
The main gotcha, and the reason the rule is the way it is, is that it means you'd lose trailing closure syntax if the caller wanted to specify an argument for the last parameter.

   - Doug


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