<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div></div><div><br></div><div><br>On 28 Oct 2017, at 10:14 pm, John McCall <<a href="mailto:rjmccall@apple.com">rjmccall@apple.com</a>> wrote:<br><br></div><blockquote type="cite"><div><meta http-equiv="Content-Type" content="text/html charset=utf-8"><br class=""><div><blockquote type="cite" class=""><div class="">On Oct 28, 2017, at 6:05 AM, Johannes Weiß via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">Hi Mike,</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><blockquote type="cite" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px;" class="">On 27 Oct 2017, at 7:05 pm, Mike Kluev <<a href="mailto:mike.kluev@gmail.com" class="">mike.kluev@gmail.com</a>> wrote:<br class=""><br class="">on Date: Fri, 27 Oct 2017 17:52:54 +0100 Johannes Weiß <<a href="mailto:johannesweiss@apple.com" class="">johannesweiss@apple.com</a>> wrote:<br class=""><br class=""><blockquote type="cite" class="">On 27 Oct 2017, at 6:27 am, Howard Lovatt via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:<br class=""><br class="">In terms of recursion you can fiddle it:<br class=""><br class="">struct RecursiveClosure<C> {<br class=""> var c: C! = nil<br class="">}<br class="">func factorial(_ n: Int) -> Int {<br class=""> var recursive = RecursiveClosure<(Int) -> Int>()<br class=""> recursive.c = { x in<br class=""> (x == 0) ? 1 : x * recursive.c(x - 1)<br class=""> }<br class=""> return recursive.c(n)<br class="">}<br class="">factorial(5) // 120<br class=""></blockquote><br class="">what a hack and a half :)<br class=""><br class="">sorry, offtopic to the thread but that you can have easier with the fixed-point combinator (<a href="https://en.wikipedia.org/wiki/Fixed-point_combinator" class="">https://en.wikipedia.org/wiki/Fixed-point_combinator</a>)<br class=""><br class="">// the fixed-point combinator<br class="">func fix<T>(_ f: @escaping ((@escaping (T) -> T) -> (T) -> T)) -> (T) -> T {<br class=""> return { (x: T) in (f(fix(f)))(x) }<br class="">}<br class=""><br class="">// demo<br class="">let fact = fix { fact_ in { n in n == 1 ? 1 : n * fact_(n-1) } }<br class="">for i in 1..<10 {<br class=""> print(fact(i))<br class="">}<br class=""><br class="">that would be a serious crime against humanity if swift allows this type of code at all :-)<br class=""></blockquote><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">the fixed-point combinator and Y combinator are pretty important in functional languages.</span></div></blockquote><div><br class=""></div><div>They're important in the <i class="">theory</i> 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.</div></div></div></blockquote><div><br></div>Definitely not arguing with that. But there are (valid?) cases when you want a recursive closure which doesn’t have a native recursion mechanism and then `fix` can be useful I’d argue. I think more straightforward than<div><br></div><div><blockquote type="cite"><blockquote type="cite" class=""><div class=""><blockquote type="cite" class=""><blockquote type="cite" class=""><font color="#000000"><span style="background-color: rgba(255, 255, 255, 0);">recursive.c = { x in<br class=""> (x == 0) ? 1 : x * recursive.c(x - 1)<br class=""> }</span></font></blockquote></blockquote></div></blockquote></blockquote><div><br></div><div>. But fortunately have local functions, I can only recall wanting a recursive closure once.</div><div><br></div><div>— Johannes<br><blockquote type="cite"><div><div><div><br class=""></div><div>John.</div><br class=""><blockquote type="cite" class=""><div class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">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.</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">-- Johannes</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><blockquote type="cite" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; 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