Lyndon, You wrote: You can turn anything into an IO action with return, or you could catch the exception at a level where you are performing IO. Would this be what you're after? Wait until you perform IO, seems quit uneasy to me and why? Converting a string to a typeable is not a IO action. Problem is that this function lives in a dll. The IO is done by the calling program (not a Haskell program). The function is not in a monad, it is a pure function. So I think I can’t do a return. Kees On Sun, Jul 14, 2013 at 7:47 PM, Kees Bleijenberg wrote: 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 _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners