My personal Scala lesson of the day - Unions

Unions.scala

describe("Unions") {
  trait TermUnion[T]
  trait Expression { /* ... */ }

  object TermUnion {
    implicit object DoubleWitness extends Union[Double]
    implicit object ExpressionWitness extends Union[Expression]
  }

  /** Only accepts union types Double and Expression. */
  def read[T: TermUnion](value: T) = value match {
    case _: Double => "Double"
    case _: Expression => "Expression"
  }

  read(42d) should equal ("Double")
  read(new Expression{}) should equal ("Expression")

  // read("Hallo") on the other hand won't even compile, yay!
}