This is my p.ml: ("p" for process)
--begin
(* We're getting a string. *)
let process x =
        let r = "" in
                for i = 0 to (String.length(x)) do
                        r.[i] <- x.[i];
                done;
                r;;

open Printf;;
printf (process "abc$or");;
--end

This is the error I get:

gustav@gustav-P15xEMx:~/public_html$ ocaml str.cma p.ml
File "p.ml", line 11, characters 7-25:
Error: This expression has type string but an expression was expected of type
         ('a, out_channel, unit) format =
           ('a, out_channel, unit, unit, unit, unit) format6

What the deus?