[swift-evolution] [Pitch] Tuple Destructuring in Parameter Lists

Thorsten Seitz tseitz42 at icloud.com
Fri May 6 01:57:07 CDT 2016


I do like the syntax suggested by Dennis. Making use of Swift's ability to differentiate between external and internal parameter names is a great idea!


Am 06. Mai 2016 um 06:25 schrieb "T.J. Usiyan via swift-evolution" <swift-evolution at swift.org>:

I have wanted this since the first beta. I hadn't proposed because I haven't come up with a nice syntax to do this in functions/methods. I don't particularly like
    func takesATuple(someInt: Int, tuple (valueA, valueB): (String, String))

and the closes that I have come is to simply reuse the closure syntax with

    func takesATuple(someInt: Int, tuple: (String, String)) {  (someInt, (valueA, valueB)) in

but that gets confusing in my opinion, specifically if you choose to have different names inside and outside.

On Thu, May 5, 2016 at 11:22 AM, Dennis Weissmann via swift-evolution <swift-evolution at swift.org> wrote:

Following a short discussion with positive feedback on [swift-users](http://thread.gmane.org/gmane.comp.lang.swift.user/1812) I’d like to discuss the following:

Tuples should be destructible into their components in parameter lists.

Consider the following code:

let a = [0,1,2,3,4,5,6,7,8,9]
let b = [0,1,2,3,4,5,6,7,8,9]

let c = zip(a,b).reduce(0) { acc, tuple in
  acc + tuple.0 + tuple.1

tuple is of type (Int, Int).

The problem is that the calculation is not very comprehensible due to .0 and .1. That’s when destructuring tuples directly in the parameter list comes into play:

let c = zip(a,b).reduce(0) { acc, (valueA, valueB) in
  acc + valueA + valueB

The above is what I propose should be accepted by the compiler (but currently isn’t).

Currently tuple destructuring is possible like this:

let c = zip(a,b).reduce(0) { (acc, tuple) in
  let (valueA, valueB) = tuple
  return acc + valueA + valueB

This is not about saving one line ;-). I just find it much more intuitive to destructure the tuple in the parameter list itself.

The same thing could be done for functions:

func takesATuple(someInt: Int, tuple: (String, String))

Here we also need to destructure the tuple inside the function, but the intuitive place (at least for me) to do this would be the parameter list.

In the following example I'm making use of Swift’s feature to name parameters different from their labels (for internal use inside the function, this is not visible to consumers of the API):

func takesATuple(someInt: Int, tuple (valueA, valueB): (String, String))

Here valueA and valueB would be directly usable within the function. The tuple as a whole would not be available anymore.

Now it’s your turn!

1. What do you think?
2. Is this worth being discussed now (i.e. is it implementable in the Swift 3 timeframe) or should I delay it?


- Dennis

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