<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="">On Jan 29, 2016, at 12:23 AM, Jacob Bandes-Storch via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:<div><blockquote type="cite" class=""><div class=""><div dir="ltr" class="">I've wanted something like this as well. I think it would be harder than it seems, because "x = 1" might need to perform initialization, or assignment, depending how it's used.<div class=""><br class=""></div><div class="">It could make sense to have something like "@noescape(executed_exactly_once)" but this might be so limited it's not worth it. And I'm not sure how it should interact with throws.</div></div></div></blockquote><div><br class=""></div><div>I think that something like this is implementable, and making it a modifier to @noescape is sensible.</div><div><br class=""></div><div>The semantics we could support is that the function is guaranteed to call the closure exactly once on any path that could lead to a return or throw.</div><div><br class=""></div><div>This approach allows you to pass the closure down the stack, and composes with error handling. It is obviously limited what you can do with the closure, but that is necessary to validate correctness.</div><div><br class=""></div><div>-Chris</div><div><br class=""></div><div><br class=""></div><br class=""><blockquote type="cite" class=""><div class=""><div dir="ltr" class=""><div class="gmail_extra"><br clear="all" class=""><div class=""><div class="gmail_signature"><div dir="ltr" class=""><div class="">Jacob<br class=""></div></div></div></div>
<br class=""><div class="gmail_quote">On Thu, Jan 28, 2016 at 11:38 PM, Gwendal Roué <span dir="ltr" class=""><<a href="mailto:swift-evolution@swift.org" target="_blank" class="">swift-evolution@swift.org</a>></span> wrote:<br class=""><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word" class="">Hello,<div class=""><br class=""></div><div class="">I’d like to discuss the opportunity to let functions declare that a closure argument is guaranteed to have been executed when the function has returned.</div><div class=""><br class=""></div><div class="">For example:</div><div class=""><br class=""></div><div class=""><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""><span style="color:#bb2ca2" class=""><span style="white-space:pre-wrap" class=""> </span>func</span> f(<span style="color:#bb2ca2" class="">@noescape</span>(<span style="color:#703daa" class="">executed</span>) closure: () -> ()) {</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""><span style="white-space:pre-wrap" class=""> </span> closure()</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""><span style="white-space:pre-wrap" class=""> </span>}</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;min-height:13px" class=""><br class=""></div></div><div class="">The expected advantage is that the compiler would know that a variable set inside the closure is guaranteed to be initialized, and that it can be used after the execution of the function, as below:</div><div class=""><br class=""></div><div class=""><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""><div style="margin:0px;line-height:normal;color:rgb(0,132,0)" class=""><span style="color:#bb2ca2" class=""><span style="white-space:pre-wrap" class=""> </span>let</span><span style="" class=""> x: </span><span style="color:#703daa" class="">Int</span><span style="" class=""> </span>// Not initialized</div><div style="margin:0px;line-height:normal" class=""><span style="color:#31595d" class=""><span style="white-space:pre-wrap" class=""> </span>f</span> { <span style="color:#4f8187" class="">x</span> = <span style="color:#272ad8" class="">1</span> }</div><div style="margin:0px;line-height:normal;color:rgb(0,132,0)" class=""><span style="color:#3d1d81" class=""><span style="white-space:pre-wrap" class=""> </span>print</span><span style="" class="">(</span><span style="color:#4f8187" class="">x</span><span style="" class="">) </span>// Guaranteed to be initialized</div><div class=""><br class=""></div></div></div><div class="">Today developers have to write pessimistic code like below:</div><div class=""><br class=""></div><div class=""><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(0,132,0)" class=""><span style="color:#bb2ca2" class=""><span style="white-space:pre-wrap" class=""> </span>var</span><span style="" class=""> x: </span><span style="color:#703daa" class="">Int</span><span style="" class=""> = </span><span style="color:#272ad8" class="">0</span><span style="" class=""> </span>// `var` declaration, with some irrelevant value</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""><span style="color:#31595d" class=""><span style="white-space:pre-wrap" class=""> </span>f</span> { <span style="color:#4f8187" class="">x</span> = <span style="color:#272ad8" class="">1</span> }</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(61,29,129)" class=""><span style="white-space:pre-wrap" class=""> </span>print<span style="" class="">(</span><span style="color:#4f8187" class="">x</span><span style="" class="">)</span></div></div><div class=""><span style="" class=""><br class=""></span></div><div class="">As for a real world usage, I’d like to access a database in a safe (queued) way, and fetch values out of it:</div><div class=""><br class=""></div><div class=""><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""><span style="color:#bb2ca2" class=""><span style="white-space:pre-wrap" class=""> </span>let</span> items: [<span style="color:#4f8187" class="">Item</span>]</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""><span style="color:#bb2ca2" class=""><span style="white-space:pre-wrap" class=""> </span>let</span> users: [<span style="color:#4f8187" class="">User</span>]</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(49,89,93)" class=""><span style="color:#4f8187" class=""><span style="white-space:pre-wrap" class=""> </span>dbQueue</span><span style="" class="">.</span>inDatabase<span style="" class=""> { db </span><span style="color:#bb2ca2" class="">in</span></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""><span style="white-space:pre-wrap" class=""> </span> <span style="color:#4f8187" class="">items</span> = <span style="color:#4f8187" class="">Item</span>.all().fetchAll(db)</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""><span style="white-space:pre-wrap" class=""> </span> <span style="color:#4f8187" class="">users</span> = <span style="color:#4f8187" class="">Item</span>.all().fetchAll(db)</div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo" class=""><span style="white-space:pre-wrap" class=""> </span>}</div></div><span class="HOEnZb"><font color="#888888" class=""><div class=""><br class=""></div><div class="">Gwendal Roué</div><div class=""><br class=""></div></font></span></div><br class="">_______________________________________________<br class="">
swift-evolution mailing list<br class="">
<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a><br class="">
<a href="https://lists.swift.org/mailman/listinfo/swift-evolution" rel="noreferrer" target="_blank" class="">https://lists.swift.org/mailman/listinfo/swift-evolution</a><br class="">
<br class=""></blockquote></div><br class=""></div></div>
_______________________________________________<br class="">swift-evolution mailing list<br class=""><a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a><br class="">https://lists.swift.org/mailman/listinfo/swift-evolution<br class=""></div></blockquote></div><br class=""></body></html>