[swift-evolution] [Pitch or bug?] Expanding the capability of `.` prefixed lookup

[Jarod Long]

I'd be happy to see this enhancement as well.

For what it's worth, a real case that I've wanted to use this for is modifying named colors, as in:

`view.backgroundColor = .blue.withAlphaComponent(0.5)`

Not a major inconvenience for sure, but it would be nice if this worked.

Jarod

On Jun 29, 2017, 09:54 -0700, Joe Groff via swift-evolution <swift-evolution at swift.org>, wrote:
>
> > On Jun 29, 2017, at 5:19 AM, Matt Gallagher via swift-evolution <swift-evolution at swift.org> wrote:
> >
> > Super short summary:
> >
> > I think a function argument or right-hand-side expression prefixed with `.` should allow access to *any* static member on the expected type, dropping the existing limitations of this syntax.
> >
> > Detail:
> >
> > At the moment in Swift, you can use a `.` (period or dot) prefix to perform a scoped lookup of static vars and funcs on the expected type, if those static vars or funcs return that type.
> >
> > For example:
> >
> > // If we have a type `SomeNontrivialTypeName`
> > struct SomeNontrivialTypeName {
> > // With a static function returning `SomeNontrivialTypeName`
> > static func a() -> SomeNontrivialTypeName
> > }
> >
> > // And a function that requires a `SomeNontrivialTypeName` parameter
> > func f(a: SomeNontrivialTypeName)
> >
> > // We can call the function like this:
> > f(a: .a())
> >
> > The `.` prefix allows us to omit the typename `SomeNontrivialTypeName`; since the parameter already expects `SomeNontrivialTypeName`, the `.` already implies lookup in the list of static func/vars for `SomeNontrivialTypeName`.
> >
> > The purpose is syntactic efficiency and it's used to great extent across a wide range of Swift/AppKit/Foundation interfaces for enum-like value lookups. It lets us have very simple names that won't clash with names in the global namespace because they're not in the global namespace – and yet, they're only a single `.` more syntactic overhead.
> >
> > Unfortunately, there is no extendability. This approach will look up only static vars or funcs that immediately return the expected type and you can't transform the result – it's one function and done. For example, if `SomeNontrivialTypeName` has a fluent-style interface (i.e. an interface where instance methods return mutated `self` or new instances of `SomeNontrivialTypeName`):
> >
> > extension SomeNontrivialTypeName {
> > func addThings() -> SomeNontrivialTypeName
> > }
> >
> > trying to append this function won't work, even though the return type remains correct:
> >
> > f(a: .a().addThings())
> >
> > This fails and claims that we've forgotten to provide a parameter (!).
> >
> > A completely different kind of transformation might go via a different type
> >
> > extension SomeNontrivialTypeName {
> > static func another() -> AnotherType
> > }
> >
> > struct AnotherType {
> > func back() -> SomeNontrivialTypeName
> > }
> >
> > It would be nice to be able to use this "there-and-back-again" transformation:
> >
> > f(a: .another().back())
> >
> > But it also won't work.
> >
> > I realize that this is a point about minor syntactic efficiency. Yes, you could simply write:
> >
> > f(a: SomeNontrivialTypeName.another().back())
> >
> > but it's clunky and the type name gets in the way.
> >
> > There's also an element of consistency. Swift already lets us look up static functions in this way – but:
> >
> > * only functions that return the expected type
> > * and we can't *use* the function result ourselves, it must be immediately yielded to the parameter or left-hand-side
> >
> > Seems more than a little strange.
> >
> > Anyone else care or have thoughts on this point?
>
> I've also wanted this. It seems like a straightforward extension of the existing feature, which already has to set up a constraint system dependent on the contextual type, but which happens to be artificially constrained to adding only one step of member lookup or of member lookup followed by a call of that member to the system.
>
> -Joe
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