{-# LANGUAGE CPP #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE UndecidableInstances #-}
module Control.Monad.Error.Class (
Error(..),
MonadError(..),
liftEither,
) where
import Control.Monad.Trans.Except (Except, ExceptT)
import Control.Monad.Trans.Error (Error(..), ErrorT)
import qualified Control.Monad.Trans.Except as ExceptT (throwE, catchE)
import qualified Control.Monad.Trans.Error as ErrorT (throwError, catchError)
import Control.Monad.Trans.Identity as Identity
import Control.Monad.Trans.List as List
import Control.Monad.Trans.Maybe as Maybe
import Control.Monad.Trans.Reader as Reader
import Control.Monad.Trans.RWS.Lazy as LazyRWS
import Control.Monad.Trans.RWS.Strict as StrictRWS
import Control.Monad.Trans.State.Lazy as LazyState
import Control.Monad.Trans.State.Strict as StrictState
import Control.Monad.Trans.Writer.Lazy as LazyWriter
import Control.Monad.Trans.Writer.Strict as StrictWriter
import Control.Monad.Trans.Class (lift)
import Control.Exception (IOException, catch, ioError)
import Control.Monad
#if defined(__GLASGOW_HASKELL__) && __GLASGOW_HASKELL__ < 707
import Control.Monad.Instances ()
#endif
import Data.Monoid
import Prelude (Either(..), Maybe(..), either, (.), IO)
class (Monad m) => MonadError e m | m -> e where
throwError :: e -> m a
catchError :: m a -> (e -> m a) -> m a
#if __GLASGOW_HASKELL__ >= 707
{-# MINIMAL throwError, catchError #-}
#endif
liftEither :: MonadError e m => Either e a -> m a
liftEither = either throwError return
instance MonadError IOException IO where
throwError = ioError
catchError = catch
instance MonadError () Maybe where
throwError () = Nothing
catchError Nothing f = f ()
catchError x _ = x
instance MonadError e (Either e) where
throwError = Left
Left l `catchError` h = h l
Right r `catchError` _ = Right r
instance (Monad m, Error e) => MonadError e (ErrorT e m) where
throwError = ErrorT.throwError
catchError = ErrorT.catchError
instance Monad m => MonadError e (ExceptT e m) where
throwError = ExceptT.throwE
catchError = ExceptT.catchE
instance MonadError e m => MonadError e (IdentityT m) where
throwError = lift . throwError
catchError = Identity.liftCatch catchError
instance MonadError e m => MonadError e (ListT m) where
throwError = lift . throwError
catchError = List.liftCatch catchError
instance MonadError e m => MonadError e (MaybeT m) where
throwError = lift . throwError
catchError = Maybe.liftCatch catchError
instance MonadError e m => MonadError e (ReaderT r m) where
throwError = lift . throwError
catchError = Reader.liftCatch catchError
instance (Monoid w, MonadError e m) => MonadError e (LazyRWS.RWST r w s m) where
throwError = lift . throwError
catchError = LazyRWS.liftCatch catchError
instance (Monoid w, MonadError e m) => MonadError e (StrictRWS.RWST r w s m) where
throwError = lift . throwError
catchError = StrictRWS.liftCatch catchError
instance MonadError e m => MonadError e (LazyState.StateT s m) where
throwError = lift . throwError
catchError = LazyState.liftCatch catchError
instance MonadError e m => MonadError e (StrictState.StateT s m) where
throwError = lift . throwError
catchError = StrictState.liftCatch catchError
instance (Monoid w, MonadError e m) => MonadError e (LazyWriter.WriterT w m) where
throwError = lift . throwError
catchError = LazyWriter.liftCatch catchError
instance (Monoid w, MonadError e m) => MonadError e (StrictWriter.WriterT w m) where
throwError = lift . throwError
catchError = StrictWriter.liftCatch catchError