[swift-evolution] Proposal: 'T(literal)' should construct T using the appropriate literal protocol if possible
rjmccall at apple.com
Fri Jun 3 13:36:44 CDT 2016
> On Jun 2, 2016, at 6:48 PM, Matthew Johnson <matthew at anandabits.com> wrote:
> On Jun 2, 2016, at 6:51 PM, John McCall via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> On Jun 2, 2016, at 4:33 PM, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> wrote:
>>> The IntegerLiteral type idea might be worth exploring. It does seem to provide some additional consistency. For example, wasn't it clarified on this list just recently that literals don't have a type and adopt one based on context? It'd be nice to point out that 42 is an IntegerLiteral when explaining that it's not an Int.
>> I think that's a very promising way of thinking about literals. Writing a literal creates a notional value whose type is the informal, infinite-precise type of all integer/FP/collection/etc. literals, which (1) can be implicitly converted to any type that implements the appropriate protocol and (2) in fact *must* be converted to some such type (possibly the default type for that literal) in order for the code to be executable. You can then think about this proposal as saying that an explicit conversion from that informal type to a literal-convertible type follows the same path as an implicit conversion would have.
> It sounds like the reason you don't want to remove the label is because it distinguishes the literal initializer and you don't want it called with a variable accidentally right?
> What if we do something like this:
> init(_ value: IntegerLiteralType, literal: IntegerLiteral = #literal)
> The idea here is that the IntegerLiteral type and friends are special types that have no members and cannot be used except by the literal convertible protocols and their conformances. There is nothing you can do with the value at all.
> The trick is that by defaulting it to #literal we are indicating the compiler synthesizes the logical value, as with #file, #line, etc. The compiler would refuse to synthesize this value when a literal is not used. This means it is never selected when the user provides a variable to a converting initializer. If an independent initializer accepting a value of the same type also exists that would be selected. However, when a literal *is* used and the type conforms to the relevant literal convertible protocol the compiler always synthesized the value making it always the most specific overload.
> Of course no runtime value would actually exist. This is just a logical value marking the fact that a literal was used to call the initializer.
> This approach solves the same problem while retaining semantic consistency with the language (no label elision or short circuit of overload resolution). The magic is arguably a lot more restrained - types for which values can only be supplied by the compiler. We could artificially restrict usage of these types if we wanted to, but we wouldn't have to. Nothing could be accomplished by using the types anywhere else so nobody do so and it wouldn't be actively harmful to allow them to be used anywhere other types can be used. Only the ability to create values of the type needs to be restricted and we can already write types like that by marking the initializers private.
> Any thoughts on this approach?
This is still a special-case type-checking rule, which means that it's still basically my proposal except, instead of just rewriting the call to use a labeled literal initializer, it rewrites the call to use a magic initializer which cannot be used by users because it requires arguments that users cannot create. I just don't understand the motivation here. It's not a simpler language model to use, explain, or implement; it purports to be conceptually simpler and less magical while actually inventing two magic new language concepts (IntegerLiteral and #literal) on top of the same special cases.
Literals are always going to involve some magic behind the scenes. The non-trivial literals, for example, all actually traffic through some sort of builtin protocol (e.g. BuiltinIntegerLiteralConvertible) which cannot be implemented by users and whose construction necessarily involves a private API contract between the standard library and compiler that we reserve the right to change at will without bringing in swift-evolution. All of the supposedly simple explanations eventually reach, "okay, but if Int isn't a special type, how does the literal turn into an Int in the first place", and that is just unanswerable, because it turns out that standard library types *are* actually special in the sense that they get to conform to BuiltinIntegerLiteralConvertible and nobody else does.
>>> On Thu, Jun 2, 2016 at 18:22 Brent Royal-Gordon via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> > So, you think that this syntax is enticing to new developers who naturally think that the feature works the way that I'm proposing it should work, and you think that the right solution is to make the syntax illegal so that you can more conveniently tell them it doesn't work that way? :)
>>> I think the difference between a cast (which merely reinterprets a value as a compatible type) and a fullwidth conversion (which creates a similar instance of an incompatible type) is very important to understanding how to write Swift, and we shouldn't muddy the waters by creating a magic syntax.
>>> > You can still tell them that it's a struct and you're calling an initializer on it; it's just that the initializer chosen is the special literal initializer because the argument is a literal.
>>> If you're planning to change `IntegerLiteralConvertible` and friends to require a fullwidth conversion initializer like `init(_ value: IntegerLiteralType)`, then this is simply an overload resolution rule. In that case, I think your proposal is fine.
>>> But if you're going to call `init(integerLiteral:)` like it's `init(_:)`, I don't think that's a good idea. Parameter labels are supposed to be significant; we don't want to lose that.
>>> Brent Royal-Gordon
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