Why doesn't this work

(* Counts the number of instances of num in x::xs
 * for instance, (countNum ([1;1;2;3], 0, 1)) should return 2*)

let rec countNum (x::xs, soFar, num) =
   match x with
      | [] -> soFar
      | num -> countX (xs, (soFar+1), x)
      | _ -> countX (xs, soFar, x);;