[swift-evolution] Making capturing semantics of local

[John McCall]

> On Oct 28, 2017, at 6:05 AM, Johannes Weiß via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Hi Mike,
> 
>> On 27 Oct 2017, at 7:05 pm, Mike Kluev <mike.kluev at gmail.com> wrote:
>> 
>> on Date: Fri, 27 Oct 2017 17:52:54 +0100 Johannes Weiß <johannesweiss at apple.com> wrote:
>> 
>>> On 27 Oct 2017, at 6:27 am, Howard Lovatt via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> In terms of recursion you can fiddle it:
>>> 
>>> struct RecursiveClosure<C> {
>>>    var c: C! = nil
>>> }
>>> func factorial(_ n: Int) -> Int {
>>>    var recursive = RecursiveClosure<(Int) -> Int>()
>>>    recursive.c = { x in
>>>        (x == 0) ? 1 : x * recursive.c(x - 1)
>>>    }
>>>    return recursive.c(n)
>>> }
>>> factorial(5) // 120
>> 
>> what a hack and a half :)
>> 
>> sorry, offtopic to the thread but that you can have easier with the fixed-point combinator (https://en.wikipedia.org/wiki/Fixed-point_combinator)
>> 
>> // the fixed-point combinator
>> func fix<T>(_ f: @escaping ((@escaping (T) -> T) -> (T) -> T)) -> (T) -> T {
>>    return { (x: T) in (f(fix(f)))(x) }
>> }
>> 
>> // demo
>> let fact = fix { fact_ in { n in n == 1 ? 1 : n * fact_(n-1) } }
>> for i in 1..<10 {
>>    print(fact(i))
>> }
>> 
>> that would be a serious crime against humanity if swift allows this type of code at all :-)
> 
> the fixed-point combinator and Y combinator are pretty important in functional languages.

They're important in the theory of functional languages.  Anyone seriously promoting using the Y combinator in actual programming instead of using the language's native recursive-binding features is, frankly, someone you should not being taking advice from.

John.

> And the above code works in Swift. The good thing is that you need to write `fix` only once and you can then use it for all closures that need to be recursive.
> 
> 
> -- Johannes
> 
>> 
>> Mike
>> 
> 
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