Error.hs

{- |
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 libraries/base/LICENSE)

Maintainer  :  libraries@haskell.org
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.
-}

module Control.Monad.Trans.Error (
    -- * 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.Trans.Class

import Control.Applicative
import Control.Exception (IOException)
import Control.Monad
import Control.Monad.Fix
import Control.Monad.Instances ()
import System.IO

instance MonadPlus IO where
    mzero       = ioError (userError "mzero")
    m `mplus` n = m `catch` \_ -> n

-- | 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

-- ---------------------------------------------------------------------------
-- Our parameterizable error monad

instance Applicative (Either e) where
    pure          = Right
    Left  e <*> _ = Left e
    Right f <*> r = fmap f r

instance (Error e) => Alternative (Either e) where
    empty        = Left noMsg
    Left _ <|> n = n
    m      <|> _ = m

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

-- | 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) }

-- | 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 (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
    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 (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

-- | Signal an error value @e@.
--
-- * @'runErrorT' ('throwErrorT' e) = 'return' ('Left' e)@
throwError :: (Monad m, Error e) => e -> ErrorT e m a
throwError l = ErrorT $ return (Left l)

-- | Handle an error.
catchError :: (Monad m, Error e) =>
    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 :: (((Either e a -> m (Either e b)) -> m (Either e a)) ->
    m (Either e a)) -> ((a -> ErrorT e m b) -> ErrorT e m a) -> ErrorT e m a
liftCallCC callCC f = ErrorT $
    callCC $ \c ->
    runErrorT (f (\a -> ErrorT $ c (Right a)))

-- | Lift a @listen@ operation to the new monad.
liftListen :: Monad m =>
    (m (Either e a) -> m (Either e a,w)) -> ErrorT e m a -> ErrorT e m (a,w)
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 => (m (Either e a,w -> w) -> m (Either e a)) ->
    ErrorT e m (a,w -> 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

> -- wraps IO action that can throw an error e
> type ErrorWithIO e a = ErrorT e IO a
> ==> ErrorT (IO (Either e a))
>
> -- 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)))
-}