import scala.reflect.runtime.universe._

object Test extends App {
  reify {
    case class M[A](value: A) {
      def bind[B](k: A => M[B]): M[B] =  k(value)
      def map[B](f: A => B): M[B] =  bind(x => unitM(f(x)))
      def flatMap[B](f: A => M[B]): M[B] = bind(f)
    }

    def unitM[A](a: A): M[A] = M(a)

    def showM(m: M[Value]): String = m.value.toString();

    type Name = String

    trait Term;
    case class Var(x: Name) extends Term
    case class Con(n: Int) extends Term
    case class Add(l: Term, r: Term) extends Term
    case class Lam(x: Name, body: Term) extends Term
    case class App(fun: Term, arg: Term) extends Term

    trait Value
    case object Wrong extends Value {
     override def toString() = "wrong"
    }
    case class Num(n: Int) extends Value {
      override def toString() = n.toString()
    }
    case class Fun(f: Value => M[Value]) extends Value {
      override def toString() = "<function>"
    }

    type Environment = List[Tuple2[Name, Value]]

    def lookup(x: Name, e: Environment): M[Value] = e match {
      case List() => unitM(Wrong)
      case (y, b) :: e1 => if (x == y) unitM(b) else lookup(x, e1)
    }

    def add(a: Value, b: Value): M[Value] = (a, b) match {
      case (Num(m), Num(n)) => unitM(Num(m + n))
      case _ => unitM(Wrong)
    }

    def apply(a: Value, b: Value): M[Value] = a match {
      case Fun(k) => k(b)
      case _ => unitM(Wrong)
    }

    def interp(t: Term, e: Environment): M[Value] = t match {
      case Var(x) => lookup(x, e)
      case Con(n) => unitM(Num(n))
      case Add(l, r) => for (a <- interp(l, e);
           b <- interp(r, e);
           c <- add(a, b))
                        yield c
      case Lam(x, t) => unitM(Fun(a => interp(t, (x, a) :: e)))
      case App(f, t) => for (a <- interp(f, e);
           b <- interp(t, e);
           c <- apply(a, b))
            yield c
    }

    def test(t: Term): String =
      showM(interp(t, List()))

    val term0 = App(Lam("x", Add(Var("x"), Var("x"))), Add(Con(10), Con(11)))
    val term1 = App(Con(1), Con(2))

    println(test(term0))
    println(test(term1))
  }.eval
}