<html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Having ExpressibleByStringInterpolation refine ExpressibleByStringLiteral makes sense. I think there's a more powerful alternative design you should also consider. If the protocol looked like this:<div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class="">protocol ExpressibleByStringInterpolation: ExpressibleByStringLiteral {</div><div class=""> associatedtype LiteralSegment: ExpressibleByStringLiteral</div><div class=""> associatedtype InterpolatedSegment</div><div class=""> init(forStringInterpolation: Void)</div><div class=""><br class=""></div><div class=""> mutating func append(literalSegment: LiteralSegment)</div><div class=""> mutating func append(interpolatedSegment: InterpolatedSegment)</div><div class="">}</div></blockquote><div class=""><br class="">Then an interpolation expression like this in `Thingy` type context:<br class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class="">"foo \(bar) bas \(zim: 1, zang: 2)\n"</div></blockquote><div class=""><br class=""></div><div class="">could desugar to something like:<br class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class="">{</div><div class=""> var x = Thingy(forStringInterpolation: ())</div><div class=""> // Literal segments get appended using append(literalSegment: "literal")</div><div class=""> x.append(literalSegment: "foo ")</div><div class=""> // \(...) segments are arguments to a InterpolatedSegment constructor</div><div class=""> x.append(interpolatedSegment: Thingy.InterpolatedSegment(bar))</div><div class=""> x.append(literalSegment: " bas ")</div><div class=""> x.append(interpolatedSegment: Thingy.InterpolatedSegment(zim: 1, zang: 2))</div><div class=""><br class=""></div><div class=""> return x</div><div class="">}()</div></blockquote><div class=""><br class=""></div><div class="">This design should be more efficient, since there's no temporary array of segments that needs to be formed for a variadic argument, you don't need to homogenize everything to Self type up front, and the string can be built up in-place. It also provides means to address problems 3 and 4, since the InterpolatedSegment associated type can control what types it's initializable from, and can provide initializers with additional arguments for formatting or other purposes.</div><div class=""><br class=""></div><div class="">-Joe</div></body></html>