Control.Monad.Error.Hoist

HoistError extends MonadError with hoistError, which enables lifting of partiality types such as Maybe and Either e into the monad.

For example, consider the following App monad that may throw BadPacket errors:

data AppError = BadPacket String

newtype App a = App (EitherT AppError IO) a
 deriving (Functor, Applicative, Monad, MonadError AppError, MonadIO)

We may have an existing function that parses a String into a Maybe Packet

parsePacket :: String -> Maybe Packet

which can be lifted into the App monad with hoistError

appParsePacket :: String -> App Packet
appParsePacket s = hoistError (\() -> BadPacket "no parse") (parsePacket s)

Similar instances exist for Either e and EitherT e m.

Documentation

class Monad m => HoistError m t e e' | t -> e where

A tricky class for easily hoisting errors out of partiality types (e.g. Maybe, Either e) into a monad. The parameter e represents the error information carried by the partiality type t, and e' represents the type of error expected in the monad m.

Minimal complete definition

hoistError

Methods

hoistError :: (e -> e') -> t α -> m α

Given a conversion from the error in t α to e', we can hoist the computation into m.

hoistError :: MonadError e m -> (() -> e) -> Maybe       a -> m a
hoistError :: MonadError e m -> (a  -> e) -> Either  a   b -> m b
hoistError :: MonadError e m -> (a  -> e) -> ExceptT a m b -> m b

Instances

MonadError e m => HoistError m Maybe () e

Methods

hoistError :: (() -> e) -> Maybe α -> m α

MonadError e' m => HoistError m (Either e) e e'

Methods

hoistError :: (e -> e') -> Either e α -> m α

MonadError e' m => HoistError m (ErrorT e m) e e'

Methods

hoistError :: (e -> e') -> ErrorT e m α -> m α

(<%?>) :: HoistError m t e e' => t α -> (e -> e') -> m α infixl 8

A flipped synonym for hoistError.

<%?> :: MonadError e m => Maybe       a -> (() -> e) ->             m a
<%?> :: MonadError e m => Either  a   b -> (a  -> e) ->             m b
<%?> :: MonadError e m => ExceptT a m b -> (a  -> e) -> ExceptT a m b

(<%!?>) :: HoistError m t e e' => m (t α) -> (e -> e') -> m α infixl 8

A version of <%?> that operates on values already in the monad.

<%!?> :: MonadError e m => m (Maybe       a) -> (() -> e) ->             m a
<%!?> :: MonadError e m => m (Either  a   b) -> (a  -> e) ->             m b
<%!?> :: MonadError e m =>    ExceptT a m b  -> (a  -> e) -> ExceptT a m b

(<?>) :: HoistError m t e e' => t α -> e' -> m α infixl 8

A version of hoistError that ignores the error in t α and replaces it with a new one in e'.

<?> :: MonadError e m => Maybe       a -> e ->             m a
<?> :: MonadError e m => Either  a   b -> e ->             m b
<?> :: MonadError e m => ExceptT a m b -> e -> ExceptT a m b

(<!?>) :: HoistError m t e e' => m (t α) -> e' -> m α infixl 8

A version of <?> that operates on values already in the monad.

<!?> :: MonadError e m => m (Maybe       a) -> e ->             m a
<!?> :: MonadError e m => m (Either  a   b) -> e ->             m b
<!?> :: MonadError e m =>    ExceptT a m b  -> e -> ExceptT a m b