In Scala, why can’t I use a context bound for type constructors? [duplicate]

Question

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Closed 8 years ago.

Possible Duplicate:
Context bounds shortcut with higher kinded-types

Why doesn't the Scala compiler let me write this?

class TypeCtor[M[_]: ClassManifest]

It complains with “error: type M takes type parameters”. If I'm only asking for the ClassManifest, the compiler should be able to insert it no matter what the parametrization of M is, no?

This works as expected:

class TypeCtor[M[_]](implicit val ev: ClassManifest[M[_]])
(new TypeCtor[Vector]).ev.erasure // => class scala.collection.immutable.Vector

Answer 1

See my answer to this question.

Answer 2

The parameterization of M is indeeed irrelevant, but the Scala compiler needs a parameter for ClassManifest in order to know which manifest to insert. Let's say we have

class TypeCtor[A,B](implicit val ev: ClassManifest)

Now the Compiler would not know whether to insert the Manifest of A or B. Also you can't write ClassManifest[M] because ClassManifest expects a Type of kind * and M has kind * -> *.