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

[Robert Bennett]

I’m personally a fan of strictness and maximum consistency. I like the idea of flattening single element tuples down to the deepest thing contained that is not a single element tuple. So (x) flattens to x and ((((x,y)))) flattens to the tuple (x,y). Past SEs have dictated that you may not flatten a tuple down into a list of arguments.

For function types, it’s a rule that you wrap the arguments in an extra set of parentheses, so (Int)->Int maps one Int to one Int; (Int, Int)->Int maps two (separate) Ints to Int; ((Int,Int))->Int and ((((Int,Int))))->Int both map a 2-tuple of Ints to Int. Int->Int is obviously not allowed.

Now, for closures, I think we should just apply the same principles. Require parentheses for the arguments, which means double parentheses (or more) are needed when unwrapping a tuple.

In my view cognitive load is a big part of ergonomics. Reducing the rule set to a single principle seems very ergonomic to me.

P.S. Maybe the compiler should also ban unnecessarily nested tuples, for clarity. Just a thought.

> On Jun 8, 2017, at 5:05 PM, Vladimir.S via swift-evolution <swift-evolution at swift.org> wrote:
> 
> On 08.06.2017 21:17, Gwendal Roué via swift-evolution wrote:
>>> Le 8 juin 2017 à 19:40, Brent Royal-Gordon via swift-evolution <swift-evolution at swift.org <mailto: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)) -> ()
>> leta = { (x: Int, y: Int) -> Intin... }
>> letb = { ((x: Int, y: Int)) -> Intin... }
>> 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:
>> funcwantsTwoArguments(_closure: (Int, Int) -> Int) { closure(1, 2) }
>> wantsTwoArguments{ a, b ina + b }
>> wantsTwoArguments{ (a, b) ina + b }
>> wantsTwoArguments{ t int.0+ t.1} // OK, maybe not
>> funcwantsATupleArgument(_closure: ((Int, Int)) -> Int) { closure((1, 2)) }
>> wantsATupleArgument{ a, b ina + b }
>> wantsATupleArgument{ (a, b) ina + b }
>> wantsATupleArgument{ t int.0+ t.1}
>> funcwantsANamedTupleArgument(_closure: ((lhs: Int, rhs: Int)) -> Int) { closure((lhs: 1, rhs: 2)) }
>> wantsANamedTupleArgument{ a, b ina + b }
>> wantsANamedTupleArgument{ (a, b) ina + b }
>> wantsANamedTupleArgument{ t int.lhs + t.rhs }
> 
> It's nice to see that we are agreed that func/closures declared separately should have clearly defined type and (at least for now) can't be interchangeable.
> 
> And personally I agree that this could be a solution to migration problem, compiler can generate closure of correct(requested) type if such closure:
> 1. declared inplace of func call as parameter of that func
> 2. has no type annotations for its arguments
> 3. (probably, can discuss) has no parenthesis for its arguments, because one pair of parenthesis in argument list declares closure of type (list of arguments)->T in other situations.
> 
> So, we can have
> 
> wantsTwoArguments{ a, b in a + b } // (Int,Int)->() will be generated
> wantsTwoArguments{ (a, b) in a + b } // syntax of (Int,Int)->() closure
> 
> wantsATupleArgument{ a, b in a + b } // ((Int,Int))->() will be generated
> wantsATupleArgument{ t in t.0+ t.1 } // syntax of ((Int,Int))->() closure
> 
> wantsANamedTupleArgument{ a, b in a + b } // ((Int,Int))->() will be generated
> wantsANamedTupleArgument{ t in t.lhs + t.rhs } // syntax of ((Int,Int))->() closure
> 
> So, { a,b in ...} will be a special syntax for closure which type will be defined by compiler by type of func's parameter.
> 
> The question is if community and core team will support this idea, if that idea is better than other ideas like {((a,b)) in ..} for tuple deconstruction, and if this could be implemented before Swift 4 release.
> 
>> *This gives us the ability to deal with unfitted function signatures.* For example, most Dictionary methods. Yes, they are usually unfitted:
>> extensionDictionary{
>> funcforEach(_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:
>> letscores: [String: Int] = ... // [playerName: score]
>>     scores.forEach { name, score in
>> print("\(name): \(score)")
>>     }
>> Do you see?
> 
>    let scores: [String: Int] = ["a":1, "b":2]
> 
>    scores.forEach { (score: (name:String, value:Int)) in
>        print("\(score.name): \(score.value)")
>    }
> 
> I'm not saying that this syntax as good as in your example, but it is not as bad/ugly as you say.
> 
>> Gwendal
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