[Haskell-beginners] exception, not in IO

The app I'am working on, converts a jsonString to another string encoding. The function I want to write is jsonString -> (encoding, errorMsg) so String-> (String, String) For this purpose I have a typeable datastructure Glass. Because it is typeable I can do (decodeJSON jsonString) :: Glass But sometimes the jsonString is not valid (misformed or wrong fields). decodeJSON then throws a exception. I want to catch that exection and transform the result to something like ("" , theErrorMsg). Unfortunately all catch functions want IO parameters. What can I do? Kees