(* Sample ML program - Abstract Syntax Trees *)

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

fun traverse(empty) = output std_out "empty tree" |
traverse(leaf(n)) = 
let 
  val nstr = makestring(n)^" "
in
  output std_out nstr
end |
traverse(node(operator, left, right)) = 
let
  val dummy = 
    let val oper = operator^" "
    in output std_out oper
    end
  val l = traverse(left)
  val r = traverse(right)
in
  output std_out ""
end

fun prefix(tree:ast) = 
let
  val dummy = traverse(tree)
in
  output std_out "\n"
end;

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