# New monads/MonadExit

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The Exit monad provides short-circuiting for complex program flow logic.

If you are using CPS, MonadCont, or LogicT only for this purpose, the Exit monad will likely simplify your program considerably.

**Note:** Now that a restriction on the Left type has been removed, the standard `Either`

type can be used for this purpose. No separate Exit monad is needed anymore. For a monad transformer, use the version of EitherT defined in the either package.

## The code

```
{-# OPTIONS_GHC -fglasgow-exts #-}
-- A monad that provides short-circuiting for complex program flow logic.
module Control.Monad.Exit (
MonadExit(exitWith),
Exit,
runExit,
runExitMaybe,
ExitT,
runExitT,
runExitTMaybe,
module Control.Monad,
module Control.Monad.Trans
) where
import Control.Monad
import Control.Monad.Trans
import Control.Monad.Reader
import Control.Monad.Writer
import Control.Monad.Error
import Control.Monad.State
import Control.Monad.List
import qualified System.Exit as Sys (exitWith, ExitCode)
-- The MonadExit class
class Monad m => MonadExit e m | m -> e where
exitWith :: e -> m a
instance MonadExit Sys.ExitCode IO where
exitWith = Sys.exitWith
-- The Exit monad
data Exit e a = Continue a | Exit e
runExit :: Exit e a -> e
runExit (Exit x) = x
runExit _ = error "Exit monad did not exit."
runExitMaybe :: Exit e b -> Maybe e
runExitMaybe (Exit x) = Just x
runExitMaybe _ = Nothing
instance Functor (Exit e) where
fmap f (Continue x) = Continue $ f x
fmap _ (Exit x) = Exit x
instance Monad (Exit e) where
return = Continue
(Continue x) >>= f = f x
(Exit x) >>= _ = Exit x
instance MonadExit e (Exit e) where
exitWith = Exit
-- The ExitT monad
newtype ExitT e m a = ExitT (m (Exit e a))
runExitT :: Monad m => ExitT e m a -> m e
runExitT (ExitT x) = do
y <- x
case y of
Exit z -> return z
_ -> error "ExitT monad did not exit."
runExitTMaybe :: Monad m => ExitT e m a -> m (Maybe e)
runExitTMaybe (ExitT x) = liftM runExitMaybe x
instance Monad m => Functor (ExitT e m) where
fmap f (ExitT x) = ExitT $ do
y <- x
case y of
Continue z -> return $ Continue $ f z
Exit z -> return $ Exit z
instance Monad m => Monad (ExitT e m) where
return = ExitT . return . Continue
(ExitT x) >>= f = ExitT $ do
y <- x
case y of
Continue z -> let ExitT w = f z in w
Exit z -> return $ Exit z
instance Monad m => MonadExit e (ExitT e m) where
exitWith = ExitT . return . Exit
instance MonadTrans (ExitT e) where
lift = ExitT . liftM Continue
-- Lifted instances of other monad classes from inside ExitT
-- TODO: Put a MonadFix instance here.
instance MonadIO m => MonadIO (ExitT e m) where
liftIO = lift . liftIO
instance MonadPlus m => MonadPlus (ExitT e m) where
mzero = lift mzero
(ExitT x) `mplus` (ExitT y) = ExitT (x `mplus` y)
instance MonadState s (ExitT e (State s)) where
get = lift get
put = lift . put
instance Monad m => MonadState s (ExitT e (StateT s m)) where
get = lift get
put = lift . put
instance Error err => MonadError err (ExitT e (Either err)) where
throwError = lift . throwError
catchError (ExitT x) f = ExitT $ catchError x (\e -> let ExitT y = f e in y)
instance (Error err, Monad m) => MonadError err (ExitT e (ErrorT err m)) where
throwError = lift . throwError
catchError (ExitT x) f = ExitT $ catchError x (\e -> let ExitT y = f e in y)
-- MonadExit instances for other monad transformers
instance MonadExit e (StateT s (Exit e)) where
exitWith = lift . exitWith
instance Monad m => MonadExit e (StateT s (ExitT e m)) where
exitWith = lift . exitWith
instance MonadExit e (ListT (Exit e)) where
exitWith = lift . exitWith
instance Monad m => MonadExit e (ListT (ExitT e m)) where
exitWith = lift . exitWith
instance MonadExit e (ReaderT r (Exit e)) where
exitWith = lift . exitWith
instance Monad m => MonadExit e (ReaderT r (ExitT e m)) where
exitWith = lift . exitWith
instance Monoid w => MonadExit e (WriterT w (Exit e)) where
exitWith = lift . exitWith
instance (Monoid w, Monad m) => MonadExit e (WriterT w (ExitT e m)) where
exitWith = lift . exitWith
instance Error err => MonadExit e (ErrorT err (Exit e)) where
exitWith = lift . exitWith
instance (Error err, Monad m) => MonadExit e (ErrorT err (ExitT e m)) where
exitWith = lift . exitWith
```