> On Jun 5, 2016, at 5:18 PM, Dave Abrahams via swift-evolution > <[email protected]> wrote: > on Thu Jun 02 2016, John McCall <[email protected] > <mailto:[email protected]>> wrote: > >> The official way to build a literal of a specific type is to write the >> literal in an explicitly-typed context, like so: >> let x: UInt16 = 7 >> or >> let x = 7 as UInt16 >> >> Nonetheless, programmers often try the following: >> UInt16(7) >> >> Unfortunately, this does not 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. >> >> 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. >> >> Therefore, I propose that we adopt the following typing rule: >> >> Given a function call expression of the form A(B) (that is, an >> expr-call with a single, unlabelled argument) where B is an >> expr-literal or expr-collection, 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. >> >> Formally, this would be a special form of the argument conversion >> constraint, since the type of the expression A may not be immediately >> known. > > I realize this is somewhat tangential, but... IMO this may not be entirely > about literals. > > We have a standard that full-width type conversions are written as a > label-free initializer > <https://swift.org/documentation/api-design-guidelines/#type-conversion > <https://swift.org/documentation/api-design-guidelines/#type-conversion>>. > I believe that is partly responsible for setting up the expectation that > Int(42) works as one would expect. It gets ultra-weird when you can > convert from type A to type B using B(someA) but you can't write > B(someB). We should automatically generate a label-free “copy > initializer” for value types, to complete implementation of the expected > mental model.
That may also be a good idea, but it won't magically be preferred for literal construction if the type has any other constructors of literal-convertible type. John. > >> 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))"). >> >> 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. >> >> John. >> _______________________________________________ >> swift-evolution mailing list >> [email protected] >> https://lists.swift.org/mailman/listinfo/swift-evolution >> > > -- > -Dave > > _______________________________________________ > swift-evolution mailing list > [email protected] <mailto:[email protected]> > https://lists.swift.org/mailman/listinfo/swift-evolution > <https://lists.swift.org/mailman/listinfo/swift-evolution>
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