CODEWITHSUNDEEP
scala
user997206
user997206

Reputation: 289

Type parameter does not extend given type

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?

Upvotes: 16

Views: 1617

Answers (1)

Daniel C. Sobral
Daniel C. Sobral

Reputation: 297155

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

Upvotes: 23

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