Copyright	(C) Edward Kmett 2013-2014, (c) Google Inc. 2012
License	BSD-style (see the file LICENSE)
Maintainer	Edward Kmett <ekmett@gmail.com>
Stability	experimental
Portability	non-portable
Safe Haskell	Trustworthy
Language	Haskell98

Control.Monad.Catch.Pure

Contents

Transformer
Typeclass

Description

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!

Synopsis

Transformer

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)
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!

Constructors

CatchT
Fields runCatchT :: m (Either SomeException a)

Instances

MonadTrans CatchT
MonadRWS r w s m => MonadRWS r w s (CatchT m)
MonadReader e m => MonadReader e (CatchT m)
MonadWriter w m => MonadWriter w (CatchT m)
MonadState s m => MonadState s (CatchT m)
Monad m => Alternative (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)
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)

type Catch = CatchT Identity Source

runCatch :: Catch a -> Either SomeException a 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)

Typeclass

The mtl style typeclass

module Control.Monad.Catch