liboleg-2010.1.10.0: An evolving collection of Oleg Kiselyov's Haskell modules



The ability to use functions catch, bracket, catchDyn, etc. in MonadIO other than IO itself has been a fairly frequently requested feature:

The reason it is not implemented is because these functions cannot be defined for a general MonadIO. However, these functions can be easily defined for a large and interesting subset of MonadIO. The following code demonstrates that. It uses no extensions (other than those needed for the Monad Transformer Library itself), patches no compilers, and proposes no extensions. The generic catch has been useful in a database library (Takusen), where many operations work in a monad (ReaderT Session IO): IO with the environment containing the database session data. Many other foreign libraries have a pattern of passing around various handles, which are better hidden in a monad. Still, we should be able to handle IO errors and user exceptions that arise in these computations.



class MonadIO m => CaughtMonadIO m whereSource

The implementation is quite trivial.


gcatch :: m a -> (Exception -> m a) -> m aSource


CaughtMonadIO IO 
(Monoid w, CaughtMonadIO m) => CaughtMonadIO (WriterT w m)

The following instances presume that an exception that occurs in m discard the state accumulated since the beginning of ms execution. If that is not desired -- don't use StateT. Rather, allocate IORef and carry that _immutable_ value in a ReaderT. The accumulated state will thus persist. One can always use IORefs within any MonadIO.

CaughtMonadIO m => CaughtMonadIO (StateT s m) 
(CaughtMonadIO m, Error e) => CaughtMonadIO (ErrorT e m) 
CaughtMonadIO m => CaughtMonadIO (ReaderT r m)

The following is almost verbatim from Control.Monad.Error Section MonadError instances for other monad transformers

(Monoid w, CaughtMonadIO m) => CaughtMonadIO (RWST r w s m) 

catchDyn :: (Typeable e, CaughtMonadIO m) => m a -> (e -> m a) -> m aSource