generic-code.inc

instance MonadCatchIO IO where
    catch   = E.catch
    block   = E.block
    unblock = E.unblock

instance MonadCatchIO m => MonadCatchIO (ReaderT r m) where
    m `catch` f = ReaderT $ \r -> (runReaderT m r)
                                    `catch` (\e -> runReaderT (f e) r)
    block       = mapReaderT block
    unblock     = mapReaderT unblock

instance MonadCatchIO m => MonadCatchIO (StateT s m) where
    m `catch` f = StateT $ \s -> (runStateT m s)
                                   `catch` (\e -> runStateT (f e) s)
    block       = mapStateT block
    unblock     = mapStateT unblock

instance (MonadCatchIO m, Error e) => MonadCatchIO (ErrorT e m) where
    m `catch` f = mapErrorT (\m' -> m' `catch` (\e -> runErrorT $ f e)) m
    block       = mapErrorT block
    unblock     = mapErrorT unblock

instance (Monoid w, MonadCatchIO m) => MonadCatchIO (WriterT w m) where
    m `catch` f = WriterT $ runWriterT m
                              `catch` \e -> runWriterT (f e)
    block       = mapWriterT block
    unblock     = mapWriterT unblock

instance (Monoid w, MonadCatchIO m) => MonadCatchIO (RWST r w s m) where
    m `catch` f = RWST $ \r s -> runRWST m r s
                           `catch` \e -> runRWST (f e) r s
    block       = mapRWST block
    unblock     = mapRWST unblock

throw = liftIO . E.throwIO

try a = catch (a >>= \ v -> return (Right v)) (\e -> return (Left e))

tryJust p a = do
  r <- try a
  case r of
        Right v -> return (Right v)
        Left  e -> case p e of
                        Nothing -> throw e `asTypeOf` (return $ Left undefined)
                        Just b  -> return (Left b)

{-
-- | Generalized version of 'E.bracket'
bracket :: MonadCatchIO m => m a -> (a -> m b) -> (a -> m c) -> IO c
bracket before after thing =
    block (do a <- before
              r <- unblock (thing a) `onException` after a
              after a
              return r)

-- | Generalized version of 'E.onException'
onException :: MonadCatchIO m => m a -> m b -> m a
onException action onEx = action `catch` (\e -> do onEx; throw e)
-}