<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 7 Nov 2016, at 11:58, Charlie Monroe <<a href="mailto:charlie@charliemonroe.net" class="">charlie@charliemonroe.net</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><meta http-equiv="Content-Type" content="text/html charset=us-ascii" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">I'd personally not make this automatic, but require explicit action from the developer.<div class=""><br class=""></div><div class="">In case of nullability, I have previously suggested "nonnil" keyword:</div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 9px; line-height: normal; font-family: Menlo; color: rgb(209, 47, 27);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">let</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> foo: </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">String</span><span style="font-variant-ligatures: no-common-ligatures;" class="">? = </span><span style="font-variant-ligatures: no-common-ligatures" class="">"Hello World"</span></div><div style="margin: 0px; font-size: 9px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">guard</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="color: rgb(186, 45, 162);" class="">nonnil</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(79, 129, 135);" class="">foo</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: rgb(186, 45, 162);" class="">else</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> {</span></div><div style="margin: 0px; font-size: 9px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> <font color="#ba2da2" class="">return</font></span></div><div style="margin: 0px; font-size: 9px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">}</span></div></div><div class=""><br class=""></div><div class="">In which way you explicitly request the type narrowing. Or:</div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 9px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">let</span><span style="font-variant-ligatures: no-common-ligatures" class=""> foo: </span><span style="font-variant-ligatures: no-common-ligatures; color: #ba2da2" class="">Any</span></div><div style="margin: 0px; font-size: 9px; line-height: normal; font-family: Menlo; color: rgb(186, 45, 162);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">guard</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">foo</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures" class="">as</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">String</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures" class="">else</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> {</span></div><div style="margin: 0px; font-size: 9px; line-height: normal; font-family: Menlo;" class=""> <span style="color: rgb(186, 45, 162);" class="">return</span></div><div style="margin: 0px; font-size: 9px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">}</span></div><div class=""><br class=""></div><div class="">I.e. not using "is" which returns a boolean, but using the cast operator, which IMHO makes more sense and prevents from unintentional type narrowing…</div></div></div></div></blockquote><br class=""></div><div>Normally I'm a proponent of being more rather than less explicit, but the biggest draw of type-narrowing to me is that you're *already* telling the type-checker, it's really just confirming what you know automatically.</div><div><br class=""></div><div>So if I do:</div><div><br class=""></div><div><font face="Monaco" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>if foo is String {</font></div><div><font face="Monaco" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>// Do lots of non-string stuff</font></div><div><font face="Monaco" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>(foo as String).somethingStringSpecific</font></div><div><font face="Monaco" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>}</font></div><br class=""><div class="">On the last line of the block the type-checker is able to remind me that I already know that foo is a String, so I don't need to cast it.</div><div class=""><br class=""></div><div class="">Really when it comes down to it the type-narrowing never takes anything away from you; you can always handle the value as a less specific (wider) type if you want to, but if you want to treat it like a String because you know it's one, then you can do that too.</div><div class=""><br class=""></div><div class="">I'm concerned that if the feature had to be explicit, it would lack discoverability, and really the point is almost to get rid of the need to do things explicitly when you don't need to. It's like type inference on overdrive in a way.</div><div class=""><br class=""></div><div class=""><br class=""></div><div class="">I guess I just don't see why you'd think that "guard nonnil foo" is really more explicit than "guard foo != nil", in both cases you know that foo can't be nil past that point, so is a new keyword really justified?</div></body></html>