Control.Monad.Catch.Pure

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!

Transformer

The transformers-style monad transfomer

newtype CatchT m a

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

Defined in Control.Monad.Catch.Pure

Methods

lift :: Monad m => m a -> CatchT m a

MonadRWS r w s m => MonadRWS r w s (CatchT m)

Defined in Control.Monad.Catch.Pure

MonadWriter w m => MonadWriter w (CatchT m)

Defined in Control.Monad.Catch.Pure

Methods

writer :: (a, w) -> CatchT m a

tell :: w -> CatchT m ()

listen :: CatchT m a -> CatchT m (a, w)

pass :: CatchT m (a, w -> w) -> CatchT m a

MonadState s m => MonadState s (CatchT m)

Defined in Control.Monad.Catch.Pure

Methods

get :: CatchT m s

put :: s -> CatchT m ()

state :: (s -> (a, s)) -> CatchT m a

MonadReader e m => MonadReader e (CatchT m)

Defined in Control.Monad.Catch.Pure

Methods

ask :: CatchT m e

local :: (e -> e) -> CatchT m a -> CatchT m a

reader :: (e -> a) -> CatchT m a

Monad m => Monad (CatchT m)

Defined in Control.Monad.Catch.Pure

Methods

(>>=) :: CatchT m a -> (a -> CatchT m b) -> CatchT m b

(>>) :: CatchT m a -> CatchT m b -> CatchT m b

return :: a -> CatchT m a

fail :: String -> CatchT m a

Monad m => Functor (CatchT m)

Defined in Control.Monad.Catch.Pure

Methods

fmap :: (a -> b) -> CatchT m a -> CatchT m b

(<$) :: a -> CatchT m b -> CatchT m a

MonadFix m => MonadFix (CatchT m)

Defined in Control.Monad.Catch.Pure

Methods

mfix :: (a -> CatchT m a) -> CatchT m a

Monad m => MonadFail (CatchT m)

Defined in Control.Monad.Catch.Pure

Methods

fail :: String -> CatchT m a

Monad m => Applicative (CatchT m)

Defined in Control.Monad.Catch.Pure

Methods

pure :: a -> CatchT m a

(<*>) :: CatchT m (a -> b) -> CatchT m a -> CatchT m b

liftA2 :: (a -> b -> c) -> CatchT m a -> CatchT m b -> CatchT m c

(*>) :: CatchT m a -> CatchT m b -> CatchT m b

(<*) :: CatchT m a -> CatchT m b -> CatchT m a

Foldable m => Foldable (CatchT m)

Defined in Control.Monad.Catch.Pure

Methods

fold :: Monoid m0 => CatchT m m0 -> m0

foldMap :: Monoid m0 => (a -> m0) -> CatchT m a -> m0

foldr :: (a -> b -> b) -> b -> CatchT m a -> b

foldr' :: (a -> b -> b) -> b -> CatchT m a -> b

foldl :: (b -> a -> b) -> b -> CatchT m a -> b

foldl' :: (b -> a -> b) -> b -> CatchT m a -> b

foldr1 :: (a -> a -> a) -> CatchT m a -> a

foldl1 :: (a -> a -> a) -> CatchT m a -> a

toList :: CatchT m a -> [a]

null :: CatchT m a -> Bool

length :: CatchT m a -> Int

elem :: Eq a => a -> CatchT m a -> Bool

maximum :: Ord a => CatchT m a -> a

minimum :: Ord a => CatchT m a -> a

sum :: Num a => CatchT m a -> a

product :: Num a => CatchT m a -> a

(Monad m, Traversable m) => Traversable (CatchT m)

Defined in Control.Monad.Catch.Pure

Methods

traverse :: Applicative f => (a -> f b) -> CatchT m a -> f (CatchT m b)

sequenceA :: Applicative f => CatchT m (f a) -> f (CatchT m a)

mapM :: Monad m0 => (a -> m0 b) -> CatchT m a -> m0 (CatchT m b)

sequence :: Monad m0 => CatchT m (m0 a) -> m0 (CatchT m a)

MonadIO m => MonadIO (CatchT m)

Defined in Control.Monad.Catch.Pure

Methods

liftIO :: IO a -> CatchT m a

Monad m => Alternative (CatchT m)

Defined in Control.Monad.Catch.Pure

Methods

empty :: CatchT m a

(<|>) :: CatchT m a -> CatchT m a -> CatchT m a

some :: CatchT m a -> CatchT m [a]

many :: CatchT m a -> CatchT m [a]

Monad m => MonadPlus (CatchT m)

Defined in Control.Monad.Catch.Pure

Methods

mzero :: CatchT m a

mplus :: CatchT m a -> CatchT m a -> CatchT m a

Monad m => MonadMask (CatchT m)

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

For example, IO or Either would be invalid base monads, but Reader or State would be acceptable.

Defined in Control.Monad.Catch.Pure

Methods

mask :: ((forall a. CatchT m a -> CatchT m a) -> CatchT m b) -> CatchT m b

uninterruptibleMask :: ((forall a. CatchT m a -> CatchT m a) -> CatchT m b) -> CatchT m b

generalBracket :: CatchT m a -> (a -> ExitCase b -> CatchT m c) -> (a -> CatchT m b) -> CatchT m (b, c)

Monad m => MonadCatch (CatchT m)

Defined in Control.Monad.Catch.Pure

Methods

catch :: Exception e => CatchT m a -> (e -> CatchT m a) -> CatchT m a

Monad m => MonadThrow (CatchT m)

Defined in Control.Monad.Catch.Pure

Methods

throwM :: Exception e => e -> CatchT m 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

Map the unwrapped computation using the given function.

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

Typeclass

The mtl style typeclass

module Control.Monad.Catch