Unboxed union types with a context bound

gistfile1.scala

/**
 * this is an experiment to create unboxed union types with a phantom type and a context bound.
 * All the good ideas come from @milessabin post, comments and links: http://www.chuusai.com/2011/06/09/scala-union-types-curry-howard/#comment-22
 */

/** trait for anything that can be A or B */
trait Or[A, B] {
  // a phantom type, there will be no instance of this type that we'll use
  type l[T]
  // an alias for l[t]
  type anyOf[T] = l[T]
}

// implicit definitions for various combinations of (A Or B)[T], (A Or B Or C)[T], (A Or B Or C Or D)[T]
implicit def OrInstance[A, B]: Or[A, B]#l[A] = 
  null.asInstanceOf[Or[A, B]#l[A]]

implicit def OrInstance2[A, B]: Or[A, B]#l[B] = 
  null.asInstanceOf[Or[A, B]#l[B]]

implicit def OrInstance3[A, B, C]: Or[Or[A, B], C]#l[A] = 
  null.asInstanceOf[Or[Or[A, B], C]#l[A]]

implicit def OrInstance4[A, B, C]: Or[Or[A, B], C]#l[B] = 
  null.asInstanceOf[Or[Or[A, B], C]#l[B]]

implicit def OrInstance5[A, B, C]: Or[Or[A, B], C]#l[C] = 
  null.asInstanceOf[Or[Or[A, B], C]#l[C]]

implicit def OrInstance6[A, B, C, D]: Or[Or[Or[A, B], C], D]#l[A] = 
  null.asInstanceOf[Or[Or[Or[A, B], C], D]#l[A]]

implicit def OrInstance7[A, B, C, D]: Or[Or[Or[A, B], C], D]#l[B] = 
  null.asInstanceOf[Or[Or[Or[A, B], C], D]#l[B]]

implicit def OrInstance8[A, B, C, D]: Or[Or[Or[A, B], C], D]#l[C] = 
  null.asInstanceOf[Or[Or[Or[A, B], C], D]#l[C]]

implicit def OrInstance9[A, B, C, D]: Or[Or[Or[A, B], C], D]#l[D] = 
  null.asInstanceOf[Or[Or[Or[A, B], C], D]#l[D]]

// a type alias to play with a different syntax
type /[A, B] = Or[A, B]

case class A()

/** 
 * a size method which only accepts Int or String or A or Double
 */
def size[T : (Int / String / A / Double)#anyOf](t: T) = t match {
  case i: Int    => i
  case d: Double => d.toInt
  case s: String => s.length
  case other     => other.toString.length
}

size(3)
size("1234")
size(A())
size(5.0)

// compile-error here because no implicit definition can be found
size(1f)