Portability  nonportable 

Stability  experimental 
Maintainer  Edward Kmett <ekmett@gmail.com> 
Safe Haskell  Trustworthy 
 newtype CatchT m a = CatchT {
 runCatchT :: m (Either SomeException a)
 type Catch = CatchT Identity
 runCatch :: Catch a > Either SomeException a
 mapCatchT :: (m (Either SomeException a) > n (Either SomeException b)) > CatchT m a > CatchT n b
 module Control.Monad.Catch
Transformer
The transformers
style monad transfomer
Add Exception
handling abilities to a Monad
.
This should never be used in combination with IO
. Think of CatchT
as an alternative base monad for use with mocking code that solely throws
exceptions via throwM
.
Note: that IO
monad has these abilities already, so stacking CatchT
on top
of it does not add any value and can possibly be confusing:
>>>
(error "Hello!" :: IO ()) `catch` (\(e :: ErrorCall) > liftIO $ print e)
Hello!
>>>
runCatchT $ (error "Hello!" :: CatchT IO ()) `catch` (\(e :: ErrorCall) > liftIO $ print e)
*** Exception: Hello!
>>>
runCatchT $ (throwM (ErrorCall "Hello!") :: CatchT IO ()) `catch` (\(e :: ErrorCall) > liftIO $ print e)
Hello!
CatchT  

MonadTrans CatchT  
MonadRWS r w s m => MonadRWS r w s (CatchT m)  
MonadReader e m => MonadReader e (CatchT m)  
MonadState s m => MonadState s (CatchT m)  
MonadWriter w m => MonadWriter w (CatchT m)  
Monad m => Monad (CatchT m)  
Monad m => Functor (CatchT m)  
MonadFix m => MonadFix (CatchT m)  
Monad m => MonadPlus (CatchT m)  
Monad m => Applicative (CatchT m)  
Foldable m => Foldable (CatchT m)  
(Monad m, Traversable m) => Traversable (CatchT m)  
Monad m => Alternative (CatchT m)  
MonadIO m => MonadIO (CatchT m)  
Monad m => MonadMask (CatchT m)  Note: This instance is only valid if the underlying monad has a single exit point! 
Monad m => MonadCatch (CatchT m)  
Monad m => MonadThrow (CatchT m) 
runCatch :: Catch a > Either SomeException aSource
mapCatchT :: (m (Either SomeException a) > n (Either SomeException b)) > CatchT m a > CatchT n bSource
Typeclass
The mtl style typeclass
module Control.Monad.Catch