open Buffer
(* We're getting a string. *)
let process x =
        let r = Buffer.create 100 in
                for i = 0 to (String.length(x)-1) do
                        add_char r, x.[i];
                done;
                contents r;;

open Printf;;
printf "%s" (process "abc$or");;



----
File "p.ml", line 6, characters 3-21:
Warning 10: this expression should have type unit.