expression-problem3.scala

trait Value[T] {
	def eval(e: T): Int
}

def value[T: Value](e: T) = implicitly[Value[T]].eval(e)

sealed trait Expr

case class Num(n: Int) extends Expr

object Num {
	implicit val numValue = 
		new Value[Num] {
			def eval(e: Num) = e.n
		}
}

case class Add[T <: Expr, U: Value](e1: T, e2: U) extends Expr

object Add {
	implicit def addValue[T <: Expr : Value, U <: Expr : Value] = 
		new Value[Add[T, U]] {
			def eval(e: Add[T, U]) = value(e.e1) + value(e.e2)
		}
}

case class Sub[T <: Expr, U <: Expr](e1: T, e2: U) extends Expr

object Sub {
	implicit def subValue[T <: Expr : Value, U <: Expr : Value] = 
		new Value[Sub[T, U]] {
			def eval(e: Sub[T, U]) = value(e.e1) - value(e.e2)
		}
}