<html><head><meta http-equiv="Content-Type" content="text/html charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div><blockquote type="cite" class=""><div class="">On Jun 2, 2016, at 10:31 PM, L. Mihalkovic <<a href="mailto:laurent.mihalkovic@gmail.com" class="">laurent.mihalkovic@gmail.com</a>> wrote:</div><div class=""><div dir="auto" class=""><div class="">On Jun 2, 2016, at 6:08 PM, John McCall via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:<br class=""><br class=""></div><blockquote type="cite" class=""><div class=""><meta http-equiv="Content-Type" content="text/html charset=us-ascii" class="">The official way to build a literal of a specific type is to write the literal in an explicitly-typed context, like so:<div class=""><font face="Andale Mono" class=""> let x: UInt16 = 7</font></div><div class="">or</div><div class=""><font face="Andale Mono" class=""> let x = 7 as UInt16</font></div><div class=""><br class=""></div><div class="">Nonetheless, programmers often try the following:</div><div class=""><font face="Andale Mono" class=""> UInt16(7)</font></div><div class=""><br class=""></div><div class="">Unfortunately, this does <i class="">not</i> attempt to construct the value using the appropriate literal protocol; it instead performs overload resolution using the standard rules, i.e. considering only single-argument unlabelled initializers of a type which conforms to IntegerLiteralConvertible. Often this leads to static ambiguities or, worse, causes the literal to be built using a default type (such as Int); this may have semantically very different results which are only caught at runtime.</div><div class=""><br class=""></div><div class="">In my opinion, using this initializer-call syntax to build an explicitly-typed literal is an obvious and natural choice with several advantages over the "as" syntax. However, even if you disagree, it's clear that programmers are going to continue to independently try to use it, so it's really unfortunate for it to be subtly wrong.</div><div class=""><br class=""></div><div class="">Therefore, I propose that we adopt the following typing rule:</div><div class=""><br class=""></div><div class=""> Given a function call expression of the form A(B) (that is, an <i class="">expr-call</i> with a single, unlabelled argument) where B is an <i class="">expr-literal</i> or <i class="">expr-collection</i>, if A has type T.Type for some type T and there is a declared conformance of T to an appropriate literal protocol for B, then the expression is always resolves as a literal construction of type T (as if the expression were written "B as A") rather than as a general initializer call.</div></div></blockquote><div class=""><br class=""></div><div class="">Looking transversally at all literal protocols as this proposes to operates reminds me that the knowledge that a protocol has the right semantic is based on a convention, rather than on conformance. Would it be conceibable to look into something like the following, that all others would specialize.</div><div class=""><br class=""></div><div class="">protocol LiteralConvertible {}</div><div class=""><br class=""></div><div class="">This might offer a stronger identification than the name. It might also be interesting to define an associated type, but that would exclude NilLiteralConvertible.</div><div class=""><br class=""></div><div class="">Note: as compiler expert, I would appreciate your thinking on the notion of formally expressing what might otherwise be a known strong semantic relationship. Is there any incentive to pursue, known disavantages, ...</div></div></div></blockquote><div><br class=""></div>I don't know what you're saying here. Literal types already do explicitly conform to a protocol that's specific to the literal; it's not just convention. There's nothing linking those protocols because there's no useful operation in common: there is no useful generic code that you can write that works for an arbitrary type that allows some unknown kind of literal. We intentionally do not add protocols that do not serve some useful purpose in generic programming.</div><div><br class=""></div><div>John.</div></body></html>