CODEWITHSUNDEEP
scalamanifestimplicit
monkey_p
monkey_p

Reputation: 2879

Is it possible to get access to the Manifest when initialising a val inside a trait that uses generics?

What I'm trying to do (this is a simplified example, but contains all I need)

trait MyTrait[T] {

  val name = Somefunction(simpleName)

  def simpleName(implicit m:Manifest[T]) = m.erasure.getSimpleName

}

But I get a "No Manifest available for T" compiler error on the val initialisation at simpleName.

doing

val name = Somefunction(implicitly[Manifest[T]].erasure.getSimpleName)

does the same

Somefunction returns a object that is expensive to create, so I only want to create it once.

The only way I got this to work is by using a function that checks if the object has already been created and if so then just return it else create it first and then return it.

EDIT -- added example of how I can get it to work

example

trait MyTrait[T] {
  var n:MyClass = null
  def name(implicit m:Manifest[T]) = {
    if(n == null) n = Somefunction(implicitly[Manifest[T]].erasure.getSimpleName)
    n
  }
}

Upvotes: 0

Views: 291

Answers (1)

oxbow_lakes
oxbow_lakes

Reputation: 134270

You cannot declare the type parameter of a trait as having a Manifest context-bound because it is equivalent to requiring a constructor parameter (which a trait cannot have). I understand that allowing this is planned for some future version of scala.

Furthermore, a val cannot have parameters, so it is simply not possible to instantiate a val dependent on an instance of Manifest[T] that I can see. But you have said you have got this to work: how?

Upvotes: 1

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