exceptions- Extensible optionally-pure exceptions

Copyright(C) Edward Kmett 2013-2015, (c) Google Inc. 2012
LicenseBSD-style (see the file LICENSE)
MaintainerEdward Kmett <ekmett@gmail.com>
Safe HaskellTrustworthy




This module supplies a 'pure' monad transformer that can be used for mock-testing code that throws exceptions, so long as those exceptions are always thrown with throwM.

Do not mix CatchT with IO. Choose one or the other for the bottom of your transformer stack!



The transformers-style monad transfomer

newtype CatchT m a Source

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)
>>> 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)




runCatchT :: m (Either SomeException a)


MonadTrans CatchT Source 
MonadRWS r w s m => MonadRWS r w s (CatchT m) Source 
MonadReader e m => MonadReader e (CatchT m) Source 
MonadState s m => MonadState s (CatchT m) Source 
MonadWriter w m => MonadWriter w (CatchT m) Source 
Monad m => Monad (CatchT m) Source 
Monad m => Functor (CatchT m) Source 
MonadFix m => MonadFix (CatchT m) Source 
Monad m => Applicative (CatchT m) Source 
Foldable m => Foldable (CatchT m) Source 
(Monad m, Traversable m) => Traversable (CatchT m) Source 
Monad m => Alternative (CatchT m) Source 
Monad m => MonadPlus (CatchT m) Source 
MonadIO m => MonadIO (CatchT m) Source 
Monad m => MonadMask (CatchT m) Source

Note: This instance is only valid if the underlying monad has a single exit point!

Monad m => MonadCatch (CatchT m) Source 
Monad m => MonadThrow (CatchT m) Source 

mapCatchT :: (m (Either SomeException a) -> n (Either SomeException b)) -> CatchT m a -> CatchT n b Source

Map the unwrapped computation using the given function.

runCatchT (mapCatchT f m) = f (runCatchT m)


The mtl style typeclass