let freq str =

  let len = String.length str in
  let freqs = ref [ref ('a', 0)] in

  for i=0 to (len-1) do
    let tempchar = String.get str i in
    try
      let ch = (List.find (fun x-> (fst !x) == tempchar) !freqs) in
      ch := ((fst !ch), (snd !ch)+1)
    with
    Not_found -> freqs:= !freqs@[ref (tempchar, 1)]
  done;

  !freqs
;;
(*
# freq "i hope this shit works!";;
- : (char * int) ref list =
[{contents = ('i', 3)}; {contents = (' ', 4)}; {contents = ('h', 3)};
 {contents = ('o', 2)}; {contents = ('p', 1)}; {contents = ('e', 1)};
 {contents = ('t', 2)}; {contents = ('s', 3)}; {contents = ('w', 1)};
 {contents = ('r', 1)}; {contents = ('k', 1)}; {contents = ('!', 1)}]
#
*)