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{-# LANGUAGE CPP #-}
#if __GLASGOW_HASKELL__ >= 702
{-# LANGUAGE Safe #-}
#endif
#if __GLASGOW_HASKELL__ >= 710
{-# LANGUAGE AutoDeriveTypeable #-}
#endif
#if !(MIN_VERSION_base(4,9,0))
{-# OPTIONS_GHC -fno-warn-orphans #-}
#endif
-----------------------------------------------------------------------------
-- |
-- Module : Control.Monad.Trans.Error
-- Copyright : (c) Michael Weber <michael.weber@post.rwth-aachen.de> 2001,
-- (c) Jeff Newbern 2003-2006,
-- (c) Andriy Palamarchuk 2006
-- License : BSD-style (see the file LICENSE)
--
-- Maintainer : R.Paterson@city.ac.uk
-- Stability : experimental
-- Portability : portable
--
-- This monad transformer adds the ability to fail or throw exceptions
-- to a monad.
--
-- A sequence of actions succeeds, producing a value, only if all the
-- actions in the sequence are successful. If one fails with an error,
-- the rest of the sequence is skipped and the composite action fails
-- with that error.
--
-- If the value of the error is not required, the variant in
-- "Control.Monad.Trans.Maybe" may be used instead.
--
-- /Note:/ This module will be removed in a future release.
-- Instead, use "Control.Monad.Trans.Except", which does not restrict
-- the exception type, and also includes a base exception monad.
-----------------------------------------------------------------------------
module Control.Monad.Trans.Error
{-# DEPRECATED "Use Control.Monad.Trans.Except instead" #-} (
-- * The ErrorT monad transformer
Error(..),
ErrorList(..),
ErrorT(..),
mapErrorT,
-- * Error operations
throwError,
catchError,
-- * Lifting other operations
liftCallCC,
liftListen,
liftPass,
-- * Examples
-- $examples
) where
import Control.Monad.IO.Class
import Control.Monad.Signatures
import Control.Monad.Trans.Class
import Data.Functor.Classes
#if MIN_VERSION_base(4,12,0)
import Data.Functor.Contravariant
#endif
import Control.Applicative
import Control.Exception (IOException)
import Control.Monad
#if MIN_VERSION_base(4,9,0)
import qualified Control.Monad.Fail as Fail
#endif
import Control.Monad.Fix
#if !(MIN_VERSION_base(4,6,0))
import Control.Monad.Instances () -- deprecated from base-4.6
#endif
import Data.Foldable (Foldable(foldMap))
import Data.Monoid (mempty)
import Data.Traversable (Traversable(traverse))
import System.IO.Error
#if !(MIN_VERSION_base(4,9,0))
-- These instances are in base-4.9.0
instance MonadPlus IO where
mzero = ioError (userError "mzero")
m `mplus` n = m `catchIOError` \ _ -> n
instance Alternative IO where
empty = mzero
(<|>) = mplus
# if !(MIN_VERSION_base(4,4,0))
-- exported by System.IO.Error from base-4.4
catchIOError :: IO a -> (IOError -> IO a) -> IO a
catchIOError = catch
# endif
#endif
instance (Error e) => Alternative (Either e) where
empty = Left noMsg
Left _ <|> n = n
m <|> _ = m
instance (Error e) => MonadPlus (Either e) where
mzero = Left noMsg
Left _ `mplus` n = n
m `mplus` _ = m
#if !(MIN_VERSION_base(4,3,0))
-- These instances are in base-4.3
instance Applicative (Either e) where
pure = Right
Left e <*> _ = Left e
Right f <*> r = fmap f r
instance Monad (Either e) where
return = Right
Left l >>= _ = Left l
Right r >>= k = k r
instance MonadFix (Either e) where
mfix f = let
a = f $ case a of
Right r -> r
_ -> error "empty mfix argument"
in a
#endif /* base to 4.2.0.x */
-- | An exception to be thrown.
--
-- Minimal complete definition: 'noMsg' or 'strMsg'.
class Error a where
-- | Creates an exception without a message.
-- The default implementation is @'strMsg' \"\"@.
noMsg :: a
-- | Creates an exception with a message.
-- The default implementation of @'strMsg' s@ is 'noMsg'.
strMsg :: String -> a
noMsg = strMsg ""
strMsg _ = noMsg
instance Error IOException where
strMsg = userError
-- | A string can be thrown as an error.
instance (ErrorList a) => Error [a] where
strMsg = listMsg
-- | Workaround so that we can have a Haskell 98 instance @'Error' 'String'@.
class ErrorList a where
listMsg :: String -> [a]
instance ErrorList Char where
listMsg = id
-- | The error monad transformer. It can be used to add error handling
-- to other monads.
--
-- The @ErrorT@ Monad structure is parameterized over two things:
--
-- * e - The error type.
--
-- * m - The inner monad.
--
-- The 'return' function yields a successful computation, while @>>=@
-- sequences two subcomputations, failing on the first error.
newtype ErrorT e m a = ErrorT { runErrorT :: m (Either e a) }
instance (Eq e, Eq1 m) => Eq1 (ErrorT e m) where
liftEq eq (ErrorT x) (ErrorT y) = liftEq (liftEq eq) x y
instance (Ord e, Ord1 m) => Ord1 (ErrorT e m) where
liftCompare comp (ErrorT x) (ErrorT y) = liftCompare (liftCompare comp) x y
instance (Read e, Read1 m) => Read1 (ErrorT e m) where
liftReadsPrec rp rl = readsData $
readsUnaryWith (liftReadsPrec rp' rl') "ErrorT" ErrorT
where
rp' = liftReadsPrec rp rl
rl' = liftReadList rp rl
instance (Show e, Show1 m) => Show1 (ErrorT e m) where
liftShowsPrec sp sl d (ErrorT m) =
showsUnaryWith (liftShowsPrec sp' sl') "ErrorT" d m
where
sp' = liftShowsPrec sp sl
sl' = liftShowList sp sl
instance (Eq e, Eq1 m, Eq a) => Eq (ErrorT e m a) where (==) = eq1
instance (Ord e, Ord1 m, Ord a) => Ord (ErrorT e m a) where compare = compare1
instance (Read e, Read1 m, Read a) => Read (ErrorT e m a) where
readsPrec = readsPrec1
instance (Show e, Show1 m, Show a) => Show (ErrorT e m a) where
showsPrec = showsPrec1
-- | Map the unwrapped computation using the given function.
--
-- * @'runErrorT' ('mapErrorT' f m) = f ('runErrorT' m)@
mapErrorT :: (m (Either e a) -> n (Either e' b))
-> ErrorT e m a
-> ErrorT e' n b
mapErrorT f m = ErrorT $ f (runErrorT m)
instance (Functor m) => Functor (ErrorT e m) where
fmap f = ErrorT . fmap (fmap f) . runErrorT
instance (Foldable f) => Foldable (ErrorT e f) where
foldMap f (ErrorT a) = foldMap (either (const mempty) f) a
instance (Traversable f) => Traversable (ErrorT e f) where
traverse f (ErrorT a) =
ErrorT <$> traverse (either (pure . Left) (fmap Right . f)) a
instance (Functor m, Monad m) => Applicative (ErrorT e m) where
pure a = ErrorT $ return (Right a)
f <*> v = ErrorT $ do
mf <- runErrorT f
case mf of
Left e -> return (Left e)
Right k -> do
mv <- runErrorT v
case mv of
Left e -> return (Left e)
Right x -> return (Right (k x))
instance (Functor m, Monad m, Error e) => Alternative (ErrorT e m) where
empty = mzero
(<|>) = mplus
instance (Monad m, Error e) => Monad (ErrorT e m) where
#if !(MIN_VERSION_base(4,8,0))
return a = ErrorT $ return (Right a)
#endif
m >>= k = ErrorT $ do
a <- runErrorT m
case a of
Left l -> return (Left l)
Right r -> runErrorT (k r)
#if !(MIN_VERSION_base(4,13,0))
fail msg = ErrorT $ return (Left (strMsg msg))
#endif
#if MIN_VERSION_base(4,9,0)
instance (Monad m, Error e) => Fail.MonadFail (ErrorT e m) where
fail msg = ErrorT $ return (Left (strMsg msg))
#endif
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 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
#if MIN_VERSION_base(4,12,0)
instance Contravariant m => Contravariant (ErrorT e m) where
contramap f = ErrorT . contramap (fmap f) . runErrorT
#endif
-- | Signal an error value @e@.
--
-- * @'runErrorT' ('throwError' e) = 'return' ('Left' e)@
--
-- * @'throwError' e >>= m = 'throwError' e@
throwError :: (Monad m) => e -> ErrorT e m a
throwError l = ErrorT $ return (Left l)
-- | Handle an error.
--
-- * @'catchError' h ('lift' m) = 'lift' m@
--
-- * @'catchError' h ('throwError' e) = h e@
catchError :: (Monad m) =>
ErrorT e m a -- ^ the inner computation
-> (e -> ErrorT e m a) -- ^ a handler for errors in the inner
-- computation
-> ErrorT e m a
m `catchError` h = ErrorT $ do
a <- runErrorT m
case a of
Left l -> runErrorT (h l)
Right r -> return (Right r)
-- | Lift a @callCC@ operation to the new monad.
liftCallCC :: CallCC m (Either e a) (Either e b) -> CallCC (ErrorT e m) a b
liftCallCC callCC f = ErrorT $
callCC $ \ c ->
runErrorT (f (\ a -> ErrorT $ c (Right a)))
-- | Lift a @listen@ operation to the new monad.
liftListen :: (Monad m) => Listen w m (Either e a) -> Listen w (ErrorT e m) a
liftListen listen = mapErrorT $ \ m -> do
(a, w) <- listen m
return $! fmap (\ r -> (r, w)) a
-- | Lift a @pass@ operation to the new monad.
liftPass :: (Monad m) => Pass w m (Either e a) -> Pass w (ErrorT e m) a
liftPass pass = mapErrorT $ \ m -> pass $ do
a <- m
return $! case a of
Left l -> (Left l, id)
Right (r, f) -> (Right r, f)
{- $examples
Wrapping an IO action that can throw an error @e@:
> type ErrorWithIO e a = ErrorT e IO a
> ==> ErrorT (IO (Either e a))
An IO monad wrapped in @StateT@ inside of @ErrorT@:
> 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)))
-}
|