My first ocaml program

cat.ml

let usage_msg = "cat <file1> [<file2>] ... -o <output>"

let input_files = ref []
let output_file = ref ""

let anon_fun filename = input_files := !input_files @ [filename]

let speclist = [
  ("-o", Arg.Set_string output_file, "Set output file name");
]

let () = 
  Arg.parse speclist anon_fun usage_msg;
  try
    let oc =
      match !output_file with
        | "" -> Stdlib.stdout
        | _ -> open_out !output_file
    in
      let f filename =
        try 
          let ic = open_in filename in
            let rec loop () =
              let line = input_line ic in
                Printf.fprintf oc "%s\n" line;
              loop ()
            in
              try
                loop ()
              with 
               | End_of_file -> close_in ic
               | e -> raise e
        with e ->
          Printf.fprintf Stdlib.stderr "Failed to open %s\n" filename
      in
        List.iter f !input_files
    with e ->
      Printf.fprintf Stdlib.stderr "Failed to create output %s\n" !output_file