<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 Jun 2, 2016, at 1:46 PM, Austin Zheng via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class="">+1.<div class=""><br class=""></div><div class="">The primary advantage is that it aligns the language semantics with how most programmers expect this common C-language-family idiom to behave and removes a potential source of silently wrong code.</div><div class=""><br class=""></div><div class="">The primary disadvantage is that it introduces special-case behavior to certain types of initializers (although, to be fair, this special-case behavior is easily recognizable: unlabeled one-argument initializer with a literal as the argument).</div></div></div></blockquote><blockquote type="cite" class=""><div class=""><div dir="ltr" class=""><div class=""><br class=""></div><div class="">I think the advantage outweighs the disadvantage.</div></div></div></blockquote><div><br class=""></div><div>Agree. This change basically means the label isn’t intended to be used by callers, but is only present to distinguish the initializer used by the protocol from any other unlabeled initializer accepting the same type. But conceptually it is treated as the *most specific* unlabelled initializer possible, thus winning the overload resolution.</div><div><br class=""></div><div>How important is it that we have the ability to distinguish between literals and non-literals with the same type? If that isn’t important, maybe the literal convertible protocols could be reworked such that the label isn’t necessary. That would eliminate the special-case elision of the label.</div><br class=""><blockquote type="cite" class=""><div class=""><div dir="ltr" class=""><div class=""><br class=""></div><div class="">This problem should be addressed one way or another. I prefer this solution, but if it is rejected for whatever reason we should at least explicitly outlaw A(literal) syntax in favor of "literal as A".</div><div class=""><br class=""></div><div class="">Austin</div></div><div class="gmail_extra"><br class=""><div class="gmail_quote">On Thu, Jun 2, 2016 at 9:08 AM, John McCall via swift-evolution <span dir="ltr" class=""><<a href="mailto:swift-evolution@swift.org" target="_blank" class="">swift-evolution@swift.org</a>></span> wrote:<br class=""><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word" 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 class=""><br class=""></div><div class="">Formally, this would be a special form of the argument conversion constraint, since the type of the expression A may not be immediately known.</div><div class=""><br class=""></div><div class="">Note that, as specified, it is possible to suppress this typing rule by wrapping the literal in parentheses. This might seem distasteful; it would be easy enough to allow the form of B to include extra parentheses. It's potentially useful to have a way to suppress this rule and get a normal construction, but there are several other ways of getting that effect, such as explicitly typing the literal argument (e.g. writing "A(Int(B))").</div><div class=""><br class=""></div><div class="">A conditional conformance counts as a declared conformance even if the generic arguments are known to not satisfy the conditional conformance. This permits the applicability of the rule to be decided without having to first decide the type arguments, which greatly simplifies the type-checking problem (and may be necessary for soundness; I didn't explore this in depth, but it certainly feels like a very nasty sort of dependence). We could potentially weaken this for cases where A is a direct type reference with bound parameters, e.g. Foo<Int>([]) or the same with a typealias, but I think there's some benefit from having a simpler specification, both for the implementation and for the explicability of the model.</div><span class="HOEnZb"><font color="#888888" class=""><div class=""><br class=""></div><div class="">John.</div></font></span></div><br class="">_______________________________________________<br class="">
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