[Haskell-beginners] string instead of char

Hi, I am now writing a function that replaces vowels with the letter x. eg.put in a string "help" and out comes hxlp. I tried this: f x = case x of {'a' -> 'x';'e' -> 'x';'i' -> 'x';'o' -> 'x';'u' -> 'x';_ -> x} but this wont work on strings, only on the individual letters. Any direction would be very welcome. Yours john