/usr/lib/hugs/packages/mtl/Control/Monad/Error.hs is in libhugs-mtl-bundled 98.200609.21-5.3ubuntu1.
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-- Needed for the same reasons as in Reader, State etc
-----------------------------------------------------------------------------
-- |
-- Module : Control.Monad.Error
-- Copyright : (c) Michael Weber <michael.weber@post.rwth-aachen.de>, 2001
-- License : BSD-style (see the file libraries/base/LICENSE)
--
-- Maintainer : libraries@haskell.org
-- Stability : experimental
-- Portability : non-portable (multi-parameter type classes)
--
-- The Error monad.
--
-- Rendered by Michael Weber <mailto:michael.weber@post.rwth-aachen.de>,
-- inspired by the Haskell Monad Template Library from
-- Andy Gill (<http://www.cse.ogi.edu/~andy/>)
--
-----------------------------------------------------------------------------
module Control.Monad.Error (
Error(..),
MonadError(..),
ErrorT(..),
mapErrorT,
module Control.Monad,
module Control.Monad.Fix,
module Control.Monad.Trans,
) where
import Prelude
import Control.Monad
import Control.Monad.Fix
import Control.Monad.Trans
import Control.Monad.Reader
import Control.Monad.Writer
import Control.Monad.State
import Control.Monad.RWS
import Control.Monad.Cont
import Control.Monad.Instances ()
import System.IO
-- ---------------------------------------------------------------------------
-- class MonadError
--
-- throws an exception inside the monad and thus interrupts
-- normal execution order, until an error handler is reached}
--
-- catches an exception inside the monad (that was previously
-- thrown by throwError
class Error a where
noMsg :: a
strMsg :: String -> a
noMsg = strMsg ""
strMsg _ = noMsg
instance Error [Char] where
noMsg = ""
strMsg = id
instance Error IOError where
strMsg = userError
class (Monad m) => MonadError e m | m -> e where
throwError :: e -> m a
catchError :: m a -> (e -> m a) -> m a
instance MonadPlus IO where
mzero = ioError (userError "mzero")
m `mplus` n = m `catch` \_ -> n
instance MonadError IOError IO where
throwError = ioError
catchError = catch
-- ---------------------------------------------------------------------------
-- Our parameterizable error monad
instance (Error e) => Monad (Either e) where
return = Right
Left l >>= _ = Left l
Right r >>= k = k r
fail msg = Left (strMsg msg)
instance (Error e) => MonadPlus (Either e) where
mzero = Left noMsg
Left _ `mplus` n = n
m `mplus` _ = m
instance (Error e) => MonadFix (Either e) where
mfix f = let
a = f $ case a of
Right r -> r
_ -> error "empty mfix argument"
in a
instance (Error e) => MonadError e (Either e) where
throwError = Left
Left l `catchError` h = h l
Right r `catchError` _ = Right r
-- ---------------------------------------------------------------------------
-- Our parameterizable error monad, with an inner monad
newtype ErrorT e m a = ErrorT { runErrorT :: m (Either e a) }
-- The ErrorT Monad structure is parameterized over two things:
-- * e - The error type.
-- * m - The inner monad.
-- Here are some examples of use:
--
-- type ErrorWithIO e a = ErrorT e IO a
-- ==> ErrorT (IO (Either e a))
--
-- type ErrorAndStateWithIO e s a = ErrorT e (StateT s IO) a
-- ==> ErrorT (StateT s IO (Either e a))
-- ==> ErrorT (StateT (s -> IO (Either e a,s)))
--
instance (Monad m) => Functor (ErrorT e m) where
fmap f m = ErrorT $ do
a <- runErrorT m
case a of
Left l -> return (Left l)
Right r -> return (Right (f r))
instance (Monad m, Error e) => Monad (ErrorT e m) where
return a = ErrorT $ return (Right a)
m >>= k = ErrorT $ do
a <- runErrorT m
case a of
Left l -> return (Left l)
Right r -> runErrorT (k r)
fail msg = ErrorT $ return (Left (strMsg msg))
instance (Monad m, Error e) => MonadPlus (ErrorT e m) where
mzero = ErrorT $ return (Left noMsg)
m `mplus` n = ErrorT $ do
a <- runErrorT m
case a of
Left _ -> runErrorT n
Right r -> return (Right r)
instance (MonadFix m, Error e) => MonadFix (ErrorT e m) where
mfix f = ErrorT $ mfix $ \a -> runErrorT $ f $ case a of
Right r -> r
_ -> error "empty mfix argument"
instance (Monad m, Error e) => MonadError e (ErrorT e m) where
throwError l = ErrorT $ return (Left l)
m `catchError` h = ErrorT $ do
a <- runErrorT m
case a of
Left l -> runErrorT (h l)
Right r -> return (Right r)
instance (Error e) => MonadTrans (ErrorT e) where
lift m = ErrorT $ do
a <- m
return (Right a)
instance (Error e, MonadIO m) => MonadIO (ErrorT e m) where
liftIO = lift . liftIO
instance (Error e, MonadReader r m) => MonadReader r (ErrorT e m) where
ask = lift ask
local f m = ErrorT $ local f (runErrorT m)
instance (Error e, MonadWriter w m) => MonadWriter w (ErrorT e m) where
tell = lift . tell
listen m = ErrorT $ do
(a, w) <- listen (runErrorT m)
return $ case a of
Left l -> Left l
Right r -> Right (r, w)
pass m = ErrorT $ pass $ do
a <- runErrorT m
return $ case a of
Left l -> (Left l, id)
Right (r, f) -> (Right r, f)
instance (Error e, MonadState s m) => MonadState s (ErrorT e m) where
get = lift get
put = lift . put
instance (Error e, MonadCont m) => MonadCont (ErrorT e m) where
callCC f = ErrorT $
callCC $ \c ->
runErrorT (f (\a -> ErrorT $ c (Right a)))
mapErrorT :: (m (Either e a) -> n (Either e' b)) -> ErrorT e m a -> ErrorT e' n b
mapErrorT f m = ErrorT $ f (runErrorT m)
-- ---------------------------------------------------------------------------
-- MonadError instances for other monad transformers
instance (MonadError e m) => MonadError e (ReaderT r m) where
throwError = lift . throwError
m `catchError` h = ReaderT $ \r -> runReaderT m r
`catchError` \e -> runReaderT (h e) r
instance (Monoid w, MonadError e m) => MonadError e (WriterT w m) where
throwError = lift . throwError
m `catchError` h = WriterT $ runWriterT m
`catchError` \e -> runWriterT (h e)
instance (MonadError e m) => MonadError e (StateT s m) where
throwError = lift . throwError
m `catchError` h = StateT $ \s -> runStateT m s
`catchError` \e -> runStateT (h e) s
instance (Monoid w, MonadError e m) => MonadError e (RWST r w s m) where
throwError = lift . throwError
m `catchError` h = RWST $ \r s -> runRWST m r s
`catchError` \e -> runRWST (h e) r s
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