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

Matt Gallagher mattxg at gmail.com
Thu Jun 29 07:19:39 CDT 2017

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.


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?

Matt Gallagher.

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