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

[Karl Wagner]

Do you mean you want a default closure?

func forEach<U,T>(transform: (U)->T = { return $0 }, do: (T)->Void)

doSomething { 
    if let g = $0 as? Giraffe { g.eatTheTallLeaves() } 
}

doSomething(transform: { $0 as! Giraffe }) { 
    $0.eatTheTallLeaves() 
}



> On 6 Jan 2017, at 15:28, Jay Abbott via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Some good arguments regarding warnings and multiple closures. But I can't help that feel when designing an API, if you have to add a convenience method that takes just one closure simply to call the real method, this is exactly the sort of boilerplate code that default arguments are supposed to replace.
> 
> On Thu, 5 Jan 2017 at 05:48 Douglas Gregor <dgregor at apple.com <mailto:dgregor at apple.com>> wrote:
>> On Jan 4, 2017, at 9:44 PM, Saagar Jha <saagar at saagarjha.com <mailto:saagar at saagarjha.com>> wrote:
>> 
>> So, then this should have a warning? I’m still not getting one.
>> 
>> func foo(a: () -> (), b: (() -> ())? = nil, c: Int) {
>> 	a()
>> 	b?()
>> }
>> 
>> foo(a: {
>> 	print(“Bar”)
>> }, c: 0)
>> 
> 
> If you give “c” a default value (e.g., “= 0”), it will give a warning.
> 
> I’m happy for the warning to get more eager, so long as it also gets a Fix-It at the same time.
> 
> 	- Doug
> 
>> Saagar Jha
>> 
>> 
>> 
>>> On Jan 4, 2017, at 9:34 PM, Douglas Gregor <dgregor at apple.com <mailto:dgregor at apple.com>> wrote:
>>> 
>>> 
>>>> On Jan 4, 2017, at 9:32 PM, Saagar Jha <saagar at saagarjha.com <mailto:saagar at saagarjha.com>> wrote:
>>>> 
>>>> 
>>>> 
>>>> Saagar Jha
>>>> 
>>>> 
>>>> 
>>>>> On Jan 4, 2017, at 8:35 PM, Douglas Gregor <dgregor at apple.com <mailto:dgregor at apple.com>> wrote:
>>>>> 
>>>>> 
>>>>> On Jan 4, 2017, at 7:48 PM, Saagar Jha via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>>> 
>>>>>> Check out this thread <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160606/020470.html>–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 <mailto: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. 
>>>> 
>>>> Tried this out in the playground:
>>>> 
>>>> func foo(a: () -> (), b: (() -> ())? = nil) {
>>>> 	a()
>>>> 	b?()
>>>> }
>>>> 
>>>> foo(a: {
>>>> 	print(“Bar”)
>>>> })
>>>> 
>>>> and didn’t receive a warning for it, either.
>>> 
>>> We don’t warn here because ‘foo’ does have a trailing closure… it’s for the second parameter. I guess we could still warn about ‘a’ (maybe lump it into the same bug about the typealias case).
>>> 
>>> 	- Doug
>>> 
>> 
> 
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