on Tue Jun 07 2016, John McCall <rjmccall-AT-apple.com> wrote: >> 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.
I know. I'm saying, fixing this for literals without giving value types copy initializers leaves us with only a partial realization of a larger mental model to which I believe people are programming. > >> >>> 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> -- Dave _______________________________________________ swift-evolution mailing list [email protected] https://lists.swift.org/mailman/listinfo/swift-evolution
