Staged contains method for binary trees

gistfile1.txt

class Node {
  final val;
  final left;
  final right;

  Node(this.val, this.left, this.right);

  insert(n) => 
    n.lt(this.val)
      ? new Node(this.val, this.left.insert(n), this.right)
      : new Node(this.val, this.left, this.right.insert(n));

  freeze() => new Node(((v) => <v>)(this.val), 
                       this.left.freeze(), 
                       this.right.freeze());
  contains(e) => 
    <(~e).eq(~this.val) 
       ? true
       : ((~e).lt(~this.val) 
            ? ~this.left.contains(e) 
            : ~this.right.contains(e))>;
}

class Tip {
  Tip();

  insert(n) => new Node(n, new Tip(), new Tip());

  freeze() => new Tip();
  contains(e) => <false>;
}

class Demo {
  Demo();
  
  in12345() {
    final tree = new Tip().insert(4).insert(2).insert(1).insert(3).insert(5);
    return run(<(e) => ~tree.freeze().contains(<e>)>);
  }
}
 
main() {
  print(new Demo().in12345());
  print(new Demo().in12345()(3));
}