string to list of char

Try this:

let explode s = List.init (String.length s) (String.get s)

Nice and simple:

let rec list_car ch = match ch with
    | "" -> []
    | ch -> (String.get ch 0 ) :: (list_car (String.sub ch 1 ( (String.length ch)-1) ) )  ;;

Aside, but very important:

Your code is obviously wrong because you have a recursive call for which all the parameters are the exact same one you got in. It is going to induce an infinite sequence of calls with the same values in, thus looping forever (a stack overflow won't happen in tail-rec position).

---

The code that does what you want would be:

let explode s =
  let rec exp i l =
    if i < 0 then l else exp (i - 1) (s.[i] :: l) in
  exp (String.length s - 1) []

Source: http://caml.inria.fr/pub/old_caml_site/FAQ/FAQ_EXPERT-eng.html#strings

---

Alternatively, you can choose to use a library: batteries String.to_list or extlib String.explode