Standard ML NJ

datatype 'a tree = empty | leaf of 'a | node of 'a * 'a tree * 'a tree; Max question... I need help with writing a function using Standard ML NJ. It's using a polymorphic binary tree using this definition for the data type... datatype 'a tree = empty | leaf of 'a | node of 'a * 'a tree * 'a tree; And look for the max/min label within that tree. It should take into account an empty tree as well raising an exception but the leaves can be empty... val myTree = node(1,node(2,leaf 3,empty), node(12,node(4,leaf 5,leaf 11),node(4,leaf 10, leaf 5))) So... maxFunc (op < ) tree should return the value 12 maxFunc mytree empty should return "EmptyTree" which is a raised exception maxFunc (op > ) tree should return the value 1 The function must be polymorphic Traversal question For the traversal, I need a binary tree with this definition... datatype 'a tree = empty | leaf of 'a | node of 'a * 'a tree * 'a tree; With a traversal helper function that only takes a list and returns the order it needs to be in like this... fun preorder (x, y, z) = [x] @ y @ z; But I can't get this to work as a generic function as I'm getting confused with the two functions for the generic function. getLabels takes a traversal function and a tree, returns a list in the right order. For example... val tree = node(3,node(2,leaf 4,empty), node(11,node(8,leaf 7,leaf 1) leaf 10); getLabels postorder tree; would return [4, 2, 3, 7, 8, 1, 11, 10] getLabels preorder tree; would return [3, 2, 4, 11, 8, 7, 1, 10] My attempt... fun getLabels func empty = [] | getLabels func (leaf(x)) = x | getLabels func (node(internal, left, right)) = let val (node(first, second, third)) = func internal left right in getLabels func first @ getLabels func second @ getLabels func third end; Functions working separately with expected output but I cannot use this... fun preorder empty = [] | preorder (leaf(x)) = [x] | preorder (node(internal, left, right)) = [internal] @ preorder left @ preorder right; fun postorder empty = [] | postorder (leaf(x)) = [x] | postorder (node(internal, left, right)) = postorder left @ [internal] @ postorder right; fun inorder empty = [] | inorder (leaf(x)) = [x] | inorder (node(internal, left, right)) = inorder left @ inorder right @ [internal];