[swift-evolution] Proposal: Always flatten the single element tuple

[Gwendal Roué]

> Le 8 juin 2017 à 19:40, Brent Royal-Gordon via swift-evolution <swift-evolution at swift.org> a écrit :
> 
>> On Jun 7, 2017, at 3:03 AM, Adrian Zubarev via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> Well please no:
>> 
>> 
>>  let fn2: ((Int, Int)) -> Void = { lhs, rhs in } 
>> 
>> Instead use destructuring sugar pitched by Chris Lattner on the other thread:
>> 
>> let fn2: ((Int, Int)) -> Void = { ((lhs, rhs)) in }
>> 
> 
> I think this suggestion is better than the status quo. I'm wondering, though, if we should just drop the outer set of parentheses entirely, unless you're also putting types on the parameters. That is, a closure of type `(Int, Int) -> T` can look like this:
> 
> 	{ (x: Int, y: Int) in … }
> 
> Or it can look like this:
> 
> 	{ x, y in … }
> 
> But it *cannot* look like this:
> 
> 	{ (x, y) in … }
> 
> The `(x, y)` form can instead be a closure of a type like `((Int, Int)) -> T`, which immediately destructures the tuple parameter into separate constants.
> 
> -- 
> Brent Royal-Gordon
> Architechies

Hello,

There's a difference, in the mind of people here that try to show how bad were the recent changes, between:

1: closures defined independently
2: closures given as a parameter to a function.

I think that we all agree that the type of a closure that is defined independently should be well defined:

    // Choose between (Int, Int) -> () or ((x: Int, y: Int)) -> ()
    let a = { (x: Int, y: Int) -> Int in ... }
    let b = { ((x: Int, y: Int)) -> Int in ... }

However, when a closure is given as an argument of a function that expects a closure, we ask for the maximum possible flexibility, as Swift 3 did:

    func wantsTwoArguments(_ closure: (Int, Int) -> Int) { closure(1, 2) }
    wantsTwoArguments { a, b in a + b }
    wantsTwoArguments { (a, b) in a + b }
    wantsTwoArguments { t in t.0 + t.1 } // OK, maybe not

    func wantsATupleArgument(_ closure: ((Int, Int)) -> Int) { closure((1, 2)) }
    wantsATupleArgument { a, b in a + b }
    wantsATupleArgument { (a, b) in a + b }
    wantsATupleArgument { t in t.0 + t.1 }
    
    func wantsANamedTupleArgument(_ closure: ((lhs: Int, rhs: Int)) -> Int) { closure((lhs: 1, rhs: 2)) }
    wantsANamedTupleArgument { a, b in a + b }
    wantsANamedTupleArgument { (a, b) in a + b }
    wantsANamedTupleArgument { t in t.lhs + t.rhs }

This gives us the ability to deal with unfitted function signatures. For example, most Dictionary methods. Yes, they are usually unfitted:

    extension Dictionary {
        func forEach(_ body: ((key: Key, value: Value)) throws -> Void) rethrows
    }

Who cares about this named (key:value:) tuple? Absolutely nobody, as exemplified by this remarquable Swift 3 snippet below, where no tuple, no `key`, and no `value` is in sight:

    let scores: [String: Int] = ... // [playerName: score]
    scores.forEach { name, score in
        print("\(name): \(score)")
    }

Do you see?

Gwendal


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