CODEWITHSUNDEEP
scalaplayframework-2.0
Ali Salehi
Ali Salehi

Reputation: 6999

Scala type error - takes no type parameters, expected: one

Hi fellow Scala developers,

Could anyone please explain to me what is wrong with type inference in the following code and how it can be fixed.

The following code is a custom action for Play 2.2 using Scala 2.10.2

class Test {

  trait Entity

  class NodeRequest[A,K <:Entity](val entity: K,
                                  val request: Request[A])
    extends WrappedRequest[A](request)

  def LocateResource[A,K](itemId: Int, v: List[K],
                          forceOwners:Boolean = true) =
    new ActionBuilder[NodeRequest[A,K]]() {
      def invokeBlock[A](request: Request[A],
                         block: (NodeRequest[A,K]) => Future[SimpleResult]) = {
        Future.successful(Ok)
      }
    }

[error]   Test.this.NodeRequest[A,K] takes no type parameters, expected: one
[error]   def LocateResource[A,K](itemId: Int, v: List[K] , forceOwners:Boolean = true) = new ActionBuilder[NodeRequest[A,K]]() {
[error]                                                                                                     ^

Upvotes: 4

Views: 2347

Answers (1)

Robin Green
Robin Green

Reputation: 33033

The error message is a bit confusing - it actually refers to the type parameter of ActionBuilder. What you need is a type function (or more particularly, a partial type application). This is a bit tricky in Scala. The Scala 2.8 language reference actually says you can't do it, but that is not true any more. Try this:

def LocateResource[A,K](itemId: Int, v: List[K],
                        forceOwners:Boolean = true) =
  new ActionBuilder[({type λ[B] = NodeRequest[B,K]})#λ]() {

Upvotes: 6

Related Questions