Project Euler #20

#load "nums.cma";;

open Big_int;;

let rec factorial n =
  if eq_big_int n unit_big_int then
    unit_big_int
  else
    mult_big_int n (factorial (pred_big_int n));;

let int_list_of_string s =
  let len = String.length s in
  let rec chars_of_string s n =
    if n == 0 then
      [int_of_char s.[0] - int_of_char '0']
    else
      (chars_of_string s (n - 1)) @ [(int_of_char s.[n]) - int_of_char '0']
  in
  chars_of_string s (len - 1);;
  
Printf.printf "%d\n" 
    (List.fold_left 
       (+) 
       0 
       (int_list_of_string 
      (string_of_big_int 
         (factorial 
        (big_int_of_int 100)))));;