Type pattern matching in SML

Question

I'm doing a simple excercise for a Programming Languages class in SML, the exercise is to compute if a tree is balanced or not. The following code is what I thought would be a valid solution:

datatype IntTree = LEAF of int | NODE of (IntTree * IntTree);

fun balanced (LEAF l) = true 
  | balanced (NODE (LEAF l, NODE n)) = false
  | balanced (NODE (NODE n1, NODE n2)) = balanced(n1) andalso balanced(n2)
  | balanced (NODE (NODE n, LEAF l)) = false
  | balanced (NODE (LEAF l1, LEAF l2)) = true;

However when I try to run this in the interpreter I get the following error:

stdIn:98.42-98.54 Error: operator and operand don't agree [tycon mismatch]
  operator domain: IntTree
  operand:         IntTree * IntTree
  in expression:
    balanced n1
stdIn:98.63-98.75 Error: operator and operand don't agree [tycon mismatch]
  operator domain: IntTree
  operand:         IntTree * IntTree
  in expression:
    balanced n2

But NODE should be of type IntTree, why doesn't this work?

Answer 1

I don't have an SML compiler right now, so I can't test this, but try changing balanced(n1) to (balanced (NODE n1)) (same with balanced(n2)).