<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 Jul 5, 2016, at 4:34 PM, Xi Ge via swift-dev <<a href="mailto:swift-dev@swift.org" class="">swift-dev@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><meta http-equiv="Content-Type" content="text/html charset=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><span id="docs-internal-guid-d07b8252-bd62-362d-862c-7fd88323b9e5" class=""><div style="line-height: 1.38; margin-top: 0pt; margin-bottom: 0pt;" class=""><span style="font-size: 14.666666666666666px; font-family: Arial; font-variant-ligatures: normal; font-variant-position: normal; font-variant-numeric: normal; font-variant-alternates: normal; font-variant-east-asian: normal; vertical-align: baseline; white-space: pre-wrap;" class="">Hi Swift-devs,</span></div><div style="line-height: 1.38; margin-top: 0pt; margin-bottom: 0pt;" class=""><span style="font-size: 14.666666666666666px; font-family: Arial; font-variant-ligatures: normal; font-variant-position: normal; font-variant-numeric: normal; font-variant-alternates: normal; font-variant-east-asian: normal; vertical-align: baseline; white-space: pre-wrap;" class="">I have tried to add a fixit to help developers using trailing closures more, motivated by my observation during WWDC that some developers </span></div><div style="line-height: 1.38; margin-top: 0pt; margin-bottom: 0pt;" class=""><span style="font-size: 14.666666666666666px; font-family: Arial; font-variant-ligatures: normal; font-variant-position: normal; font-variant-numeric: normal; font-variant-alternates: normal; font-variant-east-asian: normal; vertical-align: baseline; white-space: pre-wrap;" class="">do not even realize that we have such a feature. In my opinion, trailing closures are more concise, and once you get used to it, more readable; </span></div><div style="line-height: 1.38; margin-top: 0pt; margin-bottom: 0pt;" class=""><span style="font-size: 14.666666666666666px; font-family: Arial; font-variant-ligatures: normal; font-variant-position: normal; font-variant-numeric: normal; font-variant-alternates: normal; font-variant-east-asian: normal; vertical-align: baseline; white-space: pre-wrap;" class="">therefore users should adopt trailing closures whenever doing so introduces no ambiguity.</span></div><br class=""><div style="line-height: 1.38; margin-top: 0pt; margin-bottom: 0pt;" class=""><span style="font-size: 14.666666666666666px; font-family: Arial; font-variant-ligatures: normal; font-variant-position: normal; font-variant-numeric: normal; font-variant-alternates: normal; font-variant-east-asian: normal; vertical-align: baseline; white-space: pre-wrap;" class="">Fixits can enhance the discoverability of trailing closures by identifying misuses and by transforming users’ code automatically. However, adding the fixit introduces new issues:</span></div><br class=""><div style="line-height: 1.38; margin-top: 0pt; margin-bottom: 0pt;" class=""><span style="font-size: 14.666666666666666px; font-family: Arial; font-variant-ligatures: normal; font-variant-position: normal; font-variant-numeric: normal; font-variant-alternates: normal; font-variant-east-asian: normal; vertical-align: baseline; white-space: pre-wrap;" class="">Issue 1: The fixit has to be associated with a warning. Adding the warning means we declare wars against convertible non-trailing closures, which is a valid syntax choice by users.</span></div></span></div></div></blockquote><blockquote type="cite" class=""><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><span id="docs-internal-guid-d07b8252-bd62-362d-862c-7fd88323b9e5" class=""><br class=""><div style="line-height: 1.38; margin-top: 0pt; margin-bottom: 0pt;" class=""><span style="font-size: 14.666666666666666px; font-family: Arial; font-variant-ligatures: normal; font-variant-position: normal; font-variant-numeric: normal; font-variant-alternates: normal; font-variant-east-asian: normal; vertical-align: baseline; white-space: pre-wrap;" class="">Issue 2: Ambiguity checking should be exhaustive. We have several known situations when non-trailing closures cannot be convert to trailing closures, including:</span></div><p dir="ltr" style="line-height:1.38;margin-top:0pt;margin-bottom:0pt;" class=""><span style="font-size: 14.666666666666666px; font-family: Arial; font-variant-ligatures: normal; font-variant-position: normal; font-variant-numeric: normal; font-variant-alternates: normal; font-variant-east-asian: normal; vertical-align: baseline; white-space: pre-wrap;" class=""><span class="Apple-tab-span" style="white-space:pre;"> </span></span></p><ol style="margin-top:0pt;margin-bottom:0pt;" class=""><li dir="ltr" style="list-style-type: lower-alpha; font-size: 14.666666666666666px; font-family: Arial; font-variant-ligatures: normal; font-variant-position: normal; font-variant-numeric: normal; font-variant-alternates: normal; font-variant-east-asian: normal; vertical-align: baseline;" class=""><div style="line-height: 1.38; margin-top: 0pt; margin-bottom: 0pt;" class=""><span style="font-size: 14.666666666666666px; font-variant-ligatures: normal; font-variant-position: normal; font-variant-numeric: normal; font-variant-alternates: normal; font-variant-east-asian: normal; vertical-align: baseline; white-space: pre-wrap;" class="">Trailing closures are followed by other brackets, e.g., “if foo({}) {}” cannot be converted to “if foo {} {}”.</span></div></li><li dir="ltr" style="list-style-type: lower-alpha; font-size: 14.666666666666666px; font-family: Arial; font-variant-ligatures: normal; font-variant-position: normal; font-variant-numeric: normal; font-variant-alternates: normal; font-variant-east-asian: normal; vertical-align: baseline;" class=""><div style="line-height: 1.38; margin-top: 0pt; margin-bottom: 0pt;" class=""><span style="font-size: 14.666666666666666px; font-variant-ligatures: normal; font-variant-position: normal; font-variant-numeric: normal; font-variant-alternates: normal; font-variant-east-asian: normal; vertical-align: baseline; white-space: pre-wrap;" class="">Removing the label of the last closure causes ambiguous function references, e.g. “foo(v: {})” cannot be converted to “foo {}” when “foo(v1: {})” also exists.</span></div></li></ol><br class=""><div style="line-height: 1.38; margin-top: 0pt; margin-bottom: 0pt;" class=""><span style="font-size: 14.666666666666666px; font-family: Arial; font-variant-ligatures: normal; font-variant-position: normal; font-variant-numeric: normal; font-variant-alternates: normal; font-variant-east-asian: normal; vertical-align: baseline; white-space: pre-wrap;" class="">So Swift-devs, is the warning worth adding? If yes, are there other situations of ambiguity that are not covered?</span></div><br class=""><div style="line-height: 1.38; margin-top: 0pt; margin-bottom: 0pt;" class=""><span style="font-size: 14.666666666666666px; font-family: Arial; font-variant-ligatures: normal; font-variant-position: normal; font-variant-numeric: normal; font-variant-alternates: normal; font-variant-east-asian: normal; vertical-align: baseline; white-space: pre-wrap;" class="">Thanks for your feedback!</span></div><div style="line-height: 1.38; margin-top: 0pt; margin-bottom: 0pt;" class=""><span style="font-size: 14.666666666666666px; font-family: Arial; font-variant-ligatures: normal; font-variant-position: normal; font-variant-numeric: normal; font-variant-alternates: normal; font-variant-east-asian: normal; vertical-align: baseline; white-space: pre-wrap;" class="">Xi</span></div><br class=""></span></div></div></blockquote><div><br class=""></div><div><div>-1, this feels like a really good tool to have in a code-linter, but not something that we should put in the compiler and prescribe to all our users.</div><div><br class=""></div><div>My biggest concern is that it’s not always obvious what the closure is used for. The argument label can help with this:</div><div><br class=""></div><div><div style="margin: 0px; font-size: 13px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">[</span><span style="font-variant-ligatures: no-common-ligatures; color: #272ad8" class="">1</span><span style="font-variant-ligatures: no-common-ligatures" class="">].lexicographicallyPrecedes([</span><span style="font-variant-ligatures: no-common-ligatures; color: #272ad8" class="">0</span><span style="font-variant-ligatures: no-common-ligatures" class="">]) { <#code#> }</span></div></div><div><div style="margin: 0px; font-size: 13px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">[</span><span style="font-variant-ligatures: no-common-ligatures; color: #272ad8" class="">1</span><span style="font-variant-ligatures: no-common-ligatures" class="">].lexicographicallyPrecedes([</span><span style="font-variant-ligatures: no-common-ligatures; color: #272ad8" class="">0</span><span style="font-variant-ligatures: no-common-ligatures" class="">], isOrderedBefore: { <#code#> })</span></div></div></div><div><br class=""></div><br class=""><blockquote type="cite" class=""><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class=""><br class="Apple-interchange-newline">
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