"Kees Bleijenberg"
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?
IO is just one of the many monads with exception support. For your case, since JSON parsing is a pure process, you would want to use a pure exception monad like `Maybe` or `Either MyError`: data MyError = InvalidDateField | {- ... -} | UnknownError There is nothing wrong with using regular exception types, if you wish, in which case you might use `Either SomeException`. Then separate concerns: decode :: String -> Either MyError Glass encode :: Glass -> String Finally the conversion function is as simple as: convert :: String -> Either MyError String convert = fmap encode . decode If `encode` can fail as well and exceptions are regular Haskell exceptions: import Control.Exception import Control.Monad decode :: String -> Either SomeException Glass encode :: Glass -> Either SomeException String convert :: String -> Either SomeException String convert = encode <=< decode I hope this helps. Greets, Ertugrul -- Not to be or to be and (not to be or to be and (not to be or to be and (not to be or to be and ... that is the list monad.