[swift-evolution] Parameter names in closures in Swift 3 are no longer possible

Xiaodi Wu xiaodi.wu at gmail.com
Mon Oct 10 11:20:58 CDT 2016

As mentioned previously, the core team has laid out a two-step roadmap to
restoring parameter names:


On Mon, Oct 10, 2016 at 10:55 Andrew Hart via swift-evolution <
swift-evolution at swift.org> wrote:

> I’ve been a little dismayed to see that closures in Swift 3 no longer have
> parameter names. As an example, in Swift 2, a function with a completion
> block may look like this:
> func sendMessage(completion: (success: Bool, recipientID: String?,
> senderID: String?) -> Void) {
>     //Oh no it failed
>     completion(success: false, recipientID: nil, senderID: nil)
> }
> Now, in Swift 3, it looks like this:
> func sendMessage(completion: @escaping (_ success: String, _ recipientID:
> String?, _ senderID: String?) -> Void {
>     //Oh no it failed
>     completion(false, nil, nil)
> }
> So now all parameter names, if you wish to include them, must be preceded
> by a _. Removing the _, it forces you to put it back. Or putting the
> parameter name twice, it forces you to replace the first occurrence with a
> _.
> To me, one of the great advantages of Swift over a language like Python or
> Ruby is its self-documenting nature. If I’m calling a function, I don’t
> need to look elsewhere for a reference to the parameters I’m using. When
> reading back over my code, I don’t have to look elsewhere to check whether
> I used the correct parameters, and in the correct order. In Swift 2, I can
> easily glance at my code to check its correctness, and I can clearly see
> I’m responding to the completion block correctly. In Swift 3, I need to
> look elsewhere to see what the parameters in “(false, nil, nil)" refers to,
> and that negates one of the key advantages that Swift has.
> For this kind of closure, I feel like it’s almost required that we be able
> to have parameter names. My request is that if we list the parameter names
> explicitly, like in the Swift 2 example, then they appear in the closure
> call by default.
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> swift-evolution at swift.org
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