(* Sample ML program - Abstract Syntax Trees *)

(* Declare the ast datatype *)
datatype ast = empty
	| leaf of int
	| node of string*ast*ast;

fun traverse(empty) = print "empty tree" |
traverse(leaf(n)) = (print (Int.toString(n)); print " ") |
traverse(node(operator, left, right)) = (
  traverse(left);
  print operator;
  traverse(right));

fun prefix(tree:ast) = (traverse(tree); print "\n");

prefix(empty);
prefix(leaf(4));
prefix(node("*",node("+",leaf(5),leaf(3)),node("-",leaf(10),leaf(4))));