defun interp in Java (sketch)

interp.java

// This is how I would structure this file:
// https://github.com/luisggpina/defunctionalized-CPS-interpreter/blob/master/src/c/lambda/Interpreter.java

// It ends up looking very much like the ML code you'd write
// using the visitor pattern to achieve pattern matching.

// This is just typed on the fly so it's probably got some
// trivial bugs in it.


interface Exp {
  <A> A visit(ExpVisitor<A> v);
}

interface ExpVisitor<A> {
  A var(String x);
  A num(Integer n);
  A app(Exp e1, Exp e2);
  A lam(String x, Exp e);
}

interface ValVisitor<A> {
  A clos(String x, Exp e, Env r);
  A num(Integer n);
}

// The eval "function"
class Eval implements ExpVisitor<Value> {
  Env r;
  Eval(Env r) {
    this.r = r;
  }

  Value var(String x) { return r.lookup(x); }
  Value num(Integer n) { return new IntValue(n); }
  Value app(Exp e1, Exp e2) { 
    return e1.visit(this).visit(new App(e2.visit(this)));
  }
  Value lam(String x, Exp e) {
    return new Clos(x, e, this.r);
  }
}

// The apply "function"
class App implements ValVisitor<Value> {
  Value a;
  App(Value a) {
    this.a = a;
  }

  Value num(Integer n) { throw new TypeErrorException(); }
  Value clos(String x, Exp e, Env r) {
    return e.visit(new Eval(r.extend(x, this.a)));
  }
}