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

John McCall rjmccall at apple.com
Thu Jun 2 16:25:52 CDT 2016

> On Jun 2, 2016, at 1:56 PM, Vladimir.S <svabox at gmail.com> wrote:
> > 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.
> Seems like I'm very slow today.. Could you present a couple of examples where such initialization(like UInt16(7)) can produce some unexpected behavior / error at runtime?

UIntN has unlabeled initializers taking all of the standard integer types, including itself.  The literal type will therefore get defaulted to Int.  The legal range of values for Int may not be a superset of the legal range of values for UIntN.  If the literal is in the legal range for an Int but not for the target type, this might trap at runtime.  Now, for a built-in integer type like UInt16, we will recognize that the coercion always traps and emit an error at compile-time, but this generally won't apply to other types.


> On 02.06.2016 19:08, John McCall via swift-evolution 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.
>> 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.
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