FT1.sc

abstract class Exp
case class Lit(n: Int) extends Exp
case class Neg(e: Exp) extends Exp
case class Add(e1: Exp, e2: Exp) extends Exp

object Exp {
  implicit def toLit(n: Int): Lit = Lit(n)
  def eval(e: Exp): Int = e match {
    case Lit(n) => n
    case Neg(e) => -eval(e)
    case Add(e1, e2) => eval(e1) + eval(e2)
  }

  def view(e: Exp): String = e match {
    case Lit(n) => n.toString
    case Neg(e) => s"(-${view(e)})"
    case Add(e1, e2) => s"(${view(e1)} + ${view(e2)})"
  }
}

val ti1 = Add(Lit(8), Neg(Add(Lit(1), Lit(2))))

import Exp._

val ti2 = Add(8, Neg(Add(1, 2)))

eval(ti2)

object Repr {
  type Repr = Int

  def lit(x: Int): Repr = x
  def neg(e: Repr): Repr = -e
  def add(e1: Repr, e2: Repr) = e1 + e2
}

import Repr._

val tf1 = add(lit(8), neg(add(lit(1), lit(2))))

view(ti1)