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