(* 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)) = 
let 
  val nstr = Int.toString(n)^" "
in
  print(nstr)
end |
traverse(node(operator, left, right)) = 
let
  val dummy = 
    let val oper = operator^" "
    in print(oper)
    end
  val l = traverse(left)
  val r = traverse(right)
in
  print("")
end

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

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

The output from SML is as follows:

- use "pre.sml";
[opening pre.sml]
datatype ast = empty | leaf of int | node of string * ast * ast
val traverse = fn : ast -> unit
val prefix = fn : ast -> unit
4
val it = () : unit
* + 5 3 - 10 4
val it = () : unit
val it = () : unit
-