Context bounds shortcut with higher kinded-types

Question

Is it possible to use the context bounds syntax shortcut with higher kinded-types?

trait One { def test[W   : ClassManifest]: Unit } // first-order ok
trait          


        

Answer 1

Note that implicitly[ClassManifest[List[_]]] is short for implicitly[ClassManifest[List[T] forSome {type T}]].

That's why it works: ClassManifest expects a proper type argument, and List[T] forSome {type T} is a proper type, but List is a type constructor. (Please see What is a higher kinded type in Scala? for a definition of "proper" etc.)

To make both ClassManifest[List[String]] and ClassManifest[List] work, we'd need to overload ClassManifest somehow with versions that take type parameters of varying kinds, something like:

class ClassManifest[T] // proper type
class ClassManifest[T[_]] // type constructor with one type parameter
class ClassManifest[T[_, _]] // type constructor with two type parameters
// ... ad nauseam

(On an academic note, the "proper" way to do this, would be to allow abstracting over kinds:

    class ClassManifest[T : K][K]

    implicitly[ClassManifest[String]] // --> compiler infers ClassManifest[String][*]
    implicitly[ClassManifest[List]] // --> compiler infers ClassManifest[List][* -> *]

)