It may make things more terrible. The solution introduces two separated type
system and one is Swift 3 mode for asterisk syntax.
> Robert Bennett via swift-evolution <swift-evolution@swift.org> 於 2017年6月10日
> 上午6:02 寫道：
>
> I see. My ideal solution to this would be an explicit tuple packing and
> unpacking API, as opposed to implicitly flexible functions. Something akin to
> (borrowing Python’s asterisk syntax):
>
> func compose<T, U, V>(_ g: (*U)->*V, _ f: (*T)->*U) -> (*T)->*V {...}
>
> The asterisks indicate that the arguments to the function will be packed into
> a tuple (or not, in the case of a single-argument function) whose type is
> denoted by T, U, V, or conversely that the functions take/return argument
> lists that look like *T, *U, *V where T, U, V are tuple types. Then the
> function definition would look like
>
> {
> return { x: *T in g(f(x)) }
> }
>
> This would in essence create a new non nominal type: function argument lists.
> These would be distinct from tuples, although the two may freely be converted
> into one another using the asterisk. This type would be structurally distinct
> from tuples by virtue of the fact that single-element argument lists exist
> while single-element tuples do not. (An asterisk will convert a
> single-element argument list into a single (non-tuple) element and vice
> versa.)
>
> To users, argument lists would be accessed solely through tuples with the
> asterisk syntax. The identity **T == T would hold whenever T is a tuple or
> argument list. Their usefulness would lie in the ability to call f(*x) when f
> takes multiple arguments and x is a tuple.
>
> I acknowledge that this is probably not going to happen for Swift 4 but I
> would sure like it if it did.
>
>> On Jun 9, 2017, at 5:10 PM, Jens Persson via swift-evolution
>> <swift-evolution@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.
>>
>> /Jens
>>
>>
>>
>>
>>> On Fri, Jun 9, 2017 at 7:17 PM, Gor Gyolchanyan <g...@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.r...@gmail.com> wrote:
>>>>>
>>>>>
>>>>>> Le 9 juin 2017 à 17:12, Gor Gyolchanyan via swift-evolution
>>>>>> <swift-evolution@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
>>>>
>>>
>>
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution@swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
> _______________________________________________
> swift-evolution mailing list
> swift-evolution@swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
_______________________________________________
swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution
