Tutorial: Using main functions

Here is a simple example of how a main method can be implemented to run when using ocamlc and ocamlrun, but not when using the interactive interpreter (ocaml). The factorial example is used here again with the main method modifications.

Factorial(w/ main) Source (download):

(* Factorial Program w/ main *)


(* Define the factorial function *)
let rec fact(n) =
  if(n=0) then 1
  else n * fact(n-1);;


(* main function checks for correct arguments
  and attempts to find the factorial of the
  supplied argument. The try...with statement
  catches the int_of_string exception if the
  argument is not numeric.
*)
let main () =
  if Array.length Sys.argv <> 2 then begin
    print_string "Usage: factmain <number>";
    print_newline ()
  end else begin
      try
        print_string("Factorial of ");
        print_int(int_of_string Sys.argv.(1));
        print_string(" is ");
        print_int(fact (int_of_string Sys.argv.(1)));
        print_newline ()
      with Failure "int_of_string" ->
        print_string "Bad integer constant";
        print_newline ()
  end
;;

(* If using ocamlc and ocamlrun call main,
  otherwise do nothing if in ocaml
  interpreter. If in interpreter, user
  can call fact function manually
*)
if !Sys.interactive then () else main ();;

Factorial (w/ main) Output:

canopus:~/ta/505/examples/fact> ocamlc -g -o factmain factmain.ml
canopus:~/ta/505/examples/fact> ./factmain
Usage: factmain <number>
canopus:~/ta/505/examples/fact> ./factmain 4
Factorial of 4 is 24
canopus:~/ta/505/examples/fact> ./factmain a
Factorial of Bad integer constant