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object directInterpreter {
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;
case class Num(n: Int) extends Value;
case class Fun(f: Value => Value)extends Value;
def showval(v: Value): String = v match {
case Wrong => "<wrong>"
case Num(n) => n.toString()
case Fun(f) => "<function>"
}
type Environment = List[Pair[Name, Value]];
def lookup(x: Name, e: Environment): Value = e match {
case List() => Wrong
case Pair(y, b) :: e1 => if (x == y) b else lookup(x, e1)
}
def add(a: Value, b: Value): Value = Pair(a, b) match {
case Pair(Num(m), Num(n)) => Num(m + n)
case _ => Wrong
}
def apply(a: Value, b: Value) = a match {
case Fun(k) => k(b)
case _ => Wrong
}
def interp(t: Term, e: Environment): Value = t match {
case Var(x) => lookup(x, e)
case Con(n) => Num(n)
case Add(l, r) => add(interp(l, e), interp(r, e))
case Lam(x, t) => Fun(a => interp(t, Pair(x, a) :: e))
case App(f, t) => apply(interp(f, e), interp(t, e))
}
def test(t: Term): String =
showval(interp(t, List()));
val term0 = App(Lam("x", Add(Var("x"), Var("x"))), Add(Con(10), Con(11)));
def main(args: Array[String]) =
System.out.println(test(term0));
}
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