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

[Jay Abbott]

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> wrote:

> On Jan 4, 2017, at 9:44 PM, Saagar Jha <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> wrote:
>
>
> On Jan 4, 2017, at 9:32 PM, Saagar Jha <saagar at saagarjha.com> wrote:
>
>
>
> Saagar Jha
>
>
>
> On Jan 4, 2017, at 8:35 PM, Douglas Gregor <dgregor at apple.com> wrote:
>
>
> On Jan 4, 2017, at 7:48 PM, Saagar Jha via swift-evolution <
> 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> 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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