<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=""><div class="">I’m making extensive use of NSPredicates, which can be initialized from format strings, so I decided to extend the class to implement ExpressibleByStringLiteral. I adapted an example in the book <i class="">Advanced Swift</i> to get this:</div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(112, 61, 170);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #005493" class="">extension</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> </span><span style="font-variant-ligatures: no-common-ligatures" class="">NSPredicate</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> : </span><span style="font-variant-ligatures: no-common-ligatures" class="">ExpressibleByStringLiteral</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> {</span></div><div style="margin: 0px; font-size: 11px; 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: #005493" class="">public</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #005493" class="">convenience</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #005493" class="">init</span><span style="font-variant-ligatures: no-common-ligatures" class="">(stringLiteral value: </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">String</span><span style="font-variant-ligatures: no-common-ligatures" class="">) {</span></div><div style="margin: 0px; font-size: 11px; 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: #005493" class="">self</span><span style="font-variant-ligatures: no-common-ligatures" class="">.</span><span style="font-variant-ligatures: no-common-ligatures; color: #005493" class="">init</span><span style="font-variant-ligatures: no-common-ligatures" class="">(format: value, argumentArray: </span><span style="font-variant-ligatures: no-common-ligatures; color: #005493" class="">nil</span><span style="font-variant-ligatures: no-common-ligatures" class="">)</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> }</span></div><div style="margin: 0px; font-size: 11px; 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: #005493" class="">public</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #005493" class="">convenience</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #005493" class="">init</span><span style="font-variant-ligatures: no-common-ligatures" class="">(extendedGraphemeClusterLiteral value: </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">String</span><span style="font-variant-ligatures: no-common-ligatures" class="">) {</span></div><div style="margin: 0px; font-size: 11px; 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: #005493" class="">self</span><span style="font-variant-ligatures: no-common-ligatures" class="">.</span><span style="font-variant-ligatures: no-common-ligatures; color: #005493" class="">init</span><span style="font-variant-ligatures: no-common-ligatures" class="">(format: value, argumentArray: </span><span style="font-variant-ligatures: no-common-ligatures; color: #005493" class="">nil</span><span style="font-variant-ligatures: no-common-ligatures" class="">)</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> }</span></div><div style="margin: 0px; font-size: 11px; 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: #005493" class="">public</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #005493" class="">convenience</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #005493" class="">init</span><span style="font-variant-ligatures: no-common-ligatures" class="">(unicodeScalarLiteral value: </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">String</span><span style="font-variant-ligatures: no-common-ligatures" class="">) {</span></div><div style="margin: 0px; font-size: 11px; 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: #005493" class="">self</span><span style="font-variant-ligatures: no-common-ligatures" class="">.</span><span style="font-variant-ligatures: no-common-ligatures; color: #005493" class="">init</span><span style="font-variant-ligatures: no-common-ligatures" class="">(format: value, argumentArray: </span><span style="font-variant-ligatures: no-common-ligatures; color: #005493" class="">nil</span><span style="font-variant-ligatures: no-common-ligatures" class="">)</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> }</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">}</span></div></div><div class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><br class=""></span></div><div class=""><span style="font-variant-ligatures: no-common-ligatures" class="">Unfortunately this doesn’t compile (in Swift 3.0.2 / Xcode 8.2.1):</span></div><div class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><br class=""></span></div><div class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><div style="margin: 0px 0px 0px 12px; text-indent: -12px; font-size: 11px; line-height: normal; font-family: Menlo;" class="">error: initializer requirement 'init(stringLiteral:)' can only be satisfied by a `required` initializer in the definition of non-final class 'NSPredicate'</div><div style="margin: 0px 0px 0px 12px; text-indent: -12px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""> public convenience init(stringLiteral value: String) {</div><div style="margin: 0px 0px 0px 12px; text-indent: -12px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""> ^</div><div class="">As far as I can tell, what’s going on here is: because ExpressibleByStringLiteral doesn’t define its initializers as `convenience`, they’re required by default; thus any implementation of them has to be required; but a required initializer can only appear in the class definition not an extension.</div><div class=""><br class=""></div><div class="">So does this mean that it’s impossible to add conformance to ExpressibleByStringLiteral in a class extension?</div><div class=""><br class=""></div><div class="">—Jens</div></span></div></body></html>