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object errorInterpreter {
trait M[A] {
def show: String
def bind[B](k: A => M[B]): M[B]
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] = new Suc(a)
def errorM[A](msg: String): M[A] = new Err(msg)
def showM(m: M[Value]): String = m.show
class Suc[A](x: A) extends M[A] {
def bind[B](k: A => M[B]): M[B] = k(x)
def show: String = "Success: " + x
}
class Err[A](msg: String) extends M[A] {
def bind[B](k: A => M[B]): M[B] = new Err(msg)
def show: String = "Error: " + msg
}
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[Pair[Name, Value]]
def lookup(x: Name, e: Environment): M[Value] = e match {
case List() => errorM("unbound variable: " + x);
case Pair(y, b) :: e1 => if (x == y) unitM(b) else lookup(x, e1)
}
def add(a: Value, b: Value): M[Value] = Pair(a, b) match {
case Pair(Num(m), Num(n)) => unitM(Num(m + n))
case _ => errorM("should be numbers: " + a + "," + b)
}
def apply(a: Value, b: Value): M[Value] = a match {
case Fun(k) => k(b)
case _ => errorM("should be function: " + a)
}
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, Pair(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))
def main(args: Array[String]) = {
System.out.println(test(term0))
System.out.println(test(term1))
}
}
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