<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Mar 8, 2016, at 1:40 PM, Dave Abrahams <<a href="mailto:dabrahams@apple.com" class="">dabrahams@apple.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><br class="">on Mon Mar 07 2016, Jon Hoffman <<a href="http://hoffman.jon-at-gmail.com" class="">hoffman.jon-AT-gmail.com</a>> wrote:<br class=""><br class=""><blockquote type="cite" class=""><br class="">So for the last two hours or so I put myself on a task to create a<br class="">mutable linked list using value types and to do it in an elegant way<br class="">(Hey I love challenges and beating my head against the wall). <br class=""></blockquote><br class="">A linked list that conforms to MutableCollection with the right<br class="">efficiency characteristics for linked lists *and* value semantics is a<br class="">tough problem.<br class=""><br class="">I think under the proposed new indexing model you can just build it on<br class="">top of an Array, though ;-)<br class=""></div></blockquote><div><br class=""></div><div>
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<!--StartFragment--><p class="MsoNormal">But where is the fun in using an Array? The greater the challenge, the more rewarding
it is to overcome it :) </p></div></div><div><span class=""></span><span class=""></span><blockquote type="cite" class=""><div class=""><br class=""><blockquote type="cite" class="">While I have found one working solution so far it is pretty ugly<br class="">therefore I have to ask myself, that even though it is preferred that<br class="">we use value types overall, should it be preferred that we use<br class="">reference types for mutable data structures like this? <br class=""></blockquote><br class="">What do you mean by “like this?” Array is a mutable data structure; is<br class="">it “like this?”<br class=""><br class="">You're going to use reference types in the *implementation* of any type<br class="">that has arbitrary growth. Array uses a reference internally.<br class=""></div></blockquote><div><br class=""></div><div><div><p class="MsoNormal">Actually we should be able to make a decent stack out of a value type without a reference type behind it. Something like this:<o:p class=""></o:p></p></div><blockquote style="margin: 0px 0px 0px 40px; border: none; padding: 0px;" class=""><div><font size="1" class="">enum Stack<T> {</font></div><div><font size="1" class=""> case Empty</font></div><div><font size="1" class=""> indirect case Node(T, next: Stack<T>)</font></div><div><font size="1" class=""><br class=""></font></div><div><font size="1" class=""> mutating func push(value: T) {</font></div><div><font size="1" class=""> self = .Node(value, next:self)</font></div><div><font size="1" class=""> }</font></div><div><font size="1" class=""><br class=""></font></div><div><font size="1" class=""> mutating func pop() -> T? {</font></div><div><font size="1" class=""> switch self {</font></div><div><font size="1" class=""> case .Empty: return nil</font></div><div><font size="1" class=""> case let .Node(value, next: next):</font></div><div><font size="1" class=""> self = next</font></div><div><font size="1" class=""> return value</font></div><div><font size="1" class=""> }</font></div><div><font size="1" class=""> }</font></div><div><font size="1" class="">}</font></div></blockquote><div><br class=""><p class="MsoNormal">Can use it like this:</p></div><blockquote style="margin: 0px 0px 0px 40px; border: none; padding: 0px;" class=""><div><font size="1" class="">var stack = Stack<Int>.Empty</font></div><div><font size="1" class="">stack.push(1)</font></div><div><font size="1" class="">stack.push(2)</font></div><div><font size="1" class="">stack.push(3)</font></div><div><font size="1" class="">stack.pop()</font></div><div><font size="1" class="">stack.pop()</font></div></blockquote><div><br class=""></div></div><br class=""><blockquote type="cite" class=""><div class=""><br class=""><blockquote type="cite" class="">Trust me, I have not giving up finding an elegant solution yet just<br class="">wondering your thoughts on that.<br class=""></blockquote><br class="">My feeling is that linked lists are almost never the best answer in<br class="">real programs, but if you *really* need their characteristics you<br class="">might want to implement them with reference semantics.<br class=""><br class=""></div></blockquote><div><br class=""></div><div>True but linked lists, stacks and queues are good data structures to play with especially when demonstrating what data structures are and how they work. </div><br class=""><blockquote type="cite" class=""><div class="">-- <br class="">-Dave<br class=""></div></blockquote></div><br class=""></body></html>