First OCaml program

straight.ml

type id = string

type binop = Plus | Minus | Times | Div

type stm = CompoundStm of stm * stm
         | AssignStm of id * exp
         | PrintStm of exp list
 and exp = IdExp of id
         | NumExp of int
         | OpExp of exp * binop * exp
         | EseqExp of stm * exp

let prog1 =
  CompoundStm(
    AssignStm("a", OpExp(NumExp 5, Plus, NumExp 3)),
    CompoundStm(
      AssignStm(
        "b",
        EseqExp(
          PrintStm[
            IdExp "a";
            OpExp(IdExp "a", Minus, NumExp 1)
          ],
          OpExp(NumExp 10, Times, IdExp "a")
        )
      ),
      PrintStm[IdExp "b"]
    )
  )

(* Print maximum number of arguments to a print statement *)
let rec maxargs prog =
  let rec maxargs_exp e = 
    match e with
    | IdExp name -> 0
    | NumExp n -> 0
    | OpExp (x, op, y) -> max (maxargs_exp x) (maxargs_exp y)
    | EseqExp (s, sub_expression) -> max (maxargs s) (maxargs_exp sub_expression)
    in
  match prog with
  | CompoundStm (first, second) -> max (maxargs first) (maxargs second)
  | AssignStm (name, value) -> maxargs_exp value
  | PrintStm (expressions) -> List.length expressions

let () =
  print_int (maxargs prog1);
  print_newline ()