Finally tagless example

Expr.scala

trait Lit[T] {
  def lit(x: Int): T
}

trait Add[T] {
  def add(x: T, y: T): T
}

trait Mul[T] {
  def mul(x: T, y: T): T
}


trait Eval {
  val eval: Int
}

implicit val litEval = new Lit[Eval] {
  override def lit(x: Int) = new Eval {
    override val eval: Int = x
  }
}

implicit val addEval = new Add[Eval] {
  override def add(x: Eval, y: Eval) = new Eval {
    override val eval: Int = x.eval + y.eval
  }
}

implicit val mulEval = new Mul[Eval] {
  override def mul(x: Eval, y: Eval) = new Eval {
    override val eval: Int = x.eval * y.eval
  }
}


trait View {
  val view: String
}

implicit val litView = new Lit[View] {
  override def lit(x: Int): View = new View {
    override val view: String = x.toString
  }
}

implicit val addView = new Add[View] {
  override def add(x: View, y: View): View = new View {
    override val view: String = s"${x.view} + ${y.view}"
  }
}

implicit val mulView = new Mul[View] {
  override def mul(x: View, y: View): View = new View {
    override val view: String = s"(${x.view}) * (${y.view})"
  }
}

def myExpr[T](implicit lit: Lit[T], add: Add[T], mul: Mul[T]): T = {
  mul.mul(
    lit.lit(5),
    add.add(
      lit.lit(10),
      lit.lit(20)
    )
  )
}

myExpr[Eval].eval
myExpr[View].view