[swift-evolution] Proposal: 'T(literal)' should construct T using the appropriate literal protocol if possible

Dave Abrahams dabrahams at apple.com
Tue Jun 7 18:25:11 CDT 2016

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 <swift-evolution at swift.org> wrote:
>> on Thu Jun 02 2016, John McCall <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> 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
>>> swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>> -- 
>> -Dave
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