[swift-evolution] History and future of Swift's parentheses

[Michael Ilseman]

> On Jun 9, 2017, at 2:10 PM, Jens Persson via swift-evolution <swift-evolution at swift.org> wrote:
> 
> The point of exercise 1 is to show that it is impossible (in Swift 4) to write a generic function composition operator (or function) which works as expected for any reasonable functions.
> This was possible in Swift 3, but in Swift 4 it will only work for functions with exactly one parameter. You'd have to special-case it for every combination of parameter counts of f and g that it should be able to handle.
> 
> The following program demonstrates how it can be done in Swift 3.1 and 3.2:
> 
> func compose<T, U, V>(_ g: @escaping (U) -> V, _ f: @escaping (T) -> U) -> (T) -> V {
>     return { x in g(f(x)) }
> }
> func sum(_ a: Int, _ b: Int) -> Int { return a + b }
> func square(_ a: Int) -> Int { return a * a }
> let squaredSum = compose(square, sum)
> let result = squaredSum((3, 4)) // A bit unexepected with a tuple here but ok ...
> print(result) // 49
> // Well, it worked, not flawlessly but we did manage to write
> // a function composition function and we composed sum
> // and square, and we could call it and get a correct result.
> 
>  
> And this program demonstrates what happens if you try it in Swift 4:
> 
> func compose<T, U, V>(_ g: @escaping (U) -> V, _ f: @escaping (T) -> U) -> (T) -> V {
>     return { x in g(f(x)) }
> }
> func sum(_ a: Int, _ b: Int) -> Int { return a + b }
> func square(_ a: Int) -> Int { return a * a }
> // let squaredSum = compose(square, sum) // Error! (without the compose-variant below)
> 
> // The error message is:
> // Cannot convert value of type `(Int, Int) -> Int` to
> // expected argument type `(_) -> _`
> 
> // That's it, it is simply not possible!
> 
> // You'd have to write special variants of the compose func for every combination
> // of parameter counts! For example, in order to get this sum and square
> // example working, this specific variant must be written:
> func compose<T, U, V, W>(_ g: @escaping (V) -> W, _ f: @escaping (T, U) -> V) -> (T, U) -> W {
>     return { (x, y) in g(f(x, y)) }
> }
> // Now it will work:
> let squaredSum = compose(square, sum)
> // But only thanks to that awfully specific compose func variant ...
> // We would have to write a lot more variants for it to be practically usable on pretty much any common function.
> 
> I'm sure some will say:
> "no regular developers use function composition anyway so why ..."
> or
> "It's not very swifty to use free functions and higher order functions like that."
> 
> My answer is that this is just a simple but telling example. The issue (as I see it) exists in all situations involving generics and function types.
> 
> I'm a regular programmer and I like to be able to write basic, useful abstractions.
> It's no fun when the language forces you to write lots of specific variants of your generic code.
> 
> I would feel less worried about the parentheses situation if the language was going in a direction where you could see how this simple exercise would be a no brainer.
> 
> Can Swift's parentheses-situation be sorted out before ABI stability?
> Otherwise it would be a bit like if Swift had kept the special rule for the first parameter, only much worse.
> 

Out of curiosity, how do you think this would impact ABI? What are your concrete concerns here?

I don't think the analogy of first parameter label is relevant, as that needn't be ABI. 

> /Jens
> 
> 
> 
> 
>> On Fri, Jun 9, 2017 at 7:17 PM, Gor Gyolchanyan <gor at gyolchanyan.com> wrote:
>> Yes, except why would you need to define `((A, B)) -> C`?, If you need to pass a 2-element tuple into a function that takes two parameters - you can! If you want to pass two values into a function that  *looks* like it takes a single 2-element tuple - you can! Seems to me that the difference between `((A, B)) -> C` and `(A, B) -> C` is virtually non-existent. But keep in mind that this only works for bare tuples (the ones that can't have labels). Non-closure functions DO have labels, which is part of their signature, so this is a different story.
>> 
>>>> On Jun 9, 2017, at 6:18 PM, Gwendal Roué <gwendal.roue at gmail.com> wrote:
>>>> 
>>>> 
>>>>> Le 9 juin 2017 à 17:12, Gor Gyolchanyan via swift-evolution <swift-evolution at swift.org> a écrit :
>>>>> 
>>>>> 
>>>>> So I wonder if any of you have had any thoughts about what Swift's parentheses-related future (or evolutionary baggage) will be?
>>>>> 
>>>> 
>>>> I really wish swift used the concept of tuples **exclusively** for all purposes that involve parentheses, as well as dividing tuples into two categories:
>>>> - Bare tuples, which do not have labels.
>>>> - Rich tuples, which do.
>>>> As a consequence, here's a list of statements that would become true:
>>>> - All functions take exactly one parameter, which is a tuple.
>>>> - All closures (a.k.a. function pointers) take exactly one parameter, which is a bare tuple.
>>>> - All functions return exactly one parameter, which is a tuple.
>>>> - Pattern matching is done on a single bare tuple using a single bare tuple pattern.
>>>> 
>>>> The currently ongoing proposal to make a single-element tuple auto-flatten would work extremely well with this idea, by making all these changes completely backward-compatible.
>>> 
>>> If I have well understood, Swift has evolved away from this.
>>> 
>>> If what you describe were true, added to the fact that there is no such thing as a one-element tuple in the language, then (A,B) -> C and ((A, B)) -> C could not be distinguished, for the simple reason that ((A, B)) -> C could not be defined.
>>> 
>>> For ((A, B)) -> C to be defined, we'd need a function that takes exactly one parameter, which is a tuple (your idea), whose single element is a tuple (oops, there is no single-valued tuples).
>>> 
>>> No opinion here, just they way I have understood recent Swift history.
>>> Gwendal
>>> 
>> 
> 
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