Type parameter does not extend given type

Question

I would like to define a generic such that its type parameter does NOT extend a given type.

For example,

trait myTrait[T <: Throwable] {
  // ....
}

would define a trait where its type parameter extends Throwable. I want something like (not real Scala code):

trait myTrait[T Not(<:) Throwable] {
  // ....
}

Where the type type parameter does NOT extend Throwable. Is there a way to construct such a notion in Scala?

Funny you should ask; Miles Sabin just posted an answer to this on the scala-language list under the thread "Enforcing that the function returns something (other than Unit)": groups.google.com/group/scala-language/browse_thread/thread/… — Rex Kerr

Daniel C. Sobral Daniel C. Sobral · Accepted Answer · 2011-10-16T05:17:38

You can do such a thing using implicits. Here's a trick from Miles Sabin on scala-language:

// Encoding for "A is not a subtype of B"
trait <:!<[A, B]

// Uses ambiguity to rule out the cases we're trying to exclude
implicit def nsub[A, B] : A <:!< B = null
implicit def nsubAmbig1[A, B >: A] : A <:!< B = null
implicit def nsubAmbig2[A, B >: A] : A <:!< B = null

// Type alias for context bound
type NOT[T] = {
 type Lambda[U] = U <:!< T
}

// foo does not accept T of type Unit
def foo[T : NOT[Unit]#Lambda](t : T) = t

Type parameter does not extend given type

1 Answers