Control.Monad.Trans.Except

This monad transformer extends a monad with the ability to throw exceptions.

A sequence of actions terminates normally, producing a value, only if none of the actions in the sequence throws an exception. If one throws an exception, the rest of the sequence is skipped and the composite action exits with that exception.

If the value of the exception is not required, the variant in Control.Monad.Trans.Maybe may be used instead.

The Except monad

type Except e = ExceptT e Identity

The parameterizable exception monad.

Computations are either exceptions or normal values.

The return function returns a normal value, while >>= exits on the first exception. For a variant that continues after an error and collects all the errors, see Errors.

except :: Monad m => Either e a -> ExceptT e m a

Constructor for computations in the exception monad. (The inverse of runExcept).

runExcept :: Except e a -> Either e a

Extractor for computations in the exception monad. (The inverse of except).

mapExcept :: (Either e a -> Either e' b) -> Except e a -> Except e' b

Map the unwrapped computation using the given function.

• runExcept (mapExcept f m) = f (runExcept m)

withExcept :: (e -> e') -> Except e a -> Except e' a

Transform any exceptions thrown by the computation using the given function (a specialization of withExceptT).

The ExceptT monad transformer

newtype ExceptT e m a

A monad transformer that adds exceptions to other monads.

ExceptT constructs a monad parameterized over two things:

• e - The exception type.
• m - The inner monad.

The return function yields a computation that produces the given value, while >>= sequences two subcomputations, exiting on the first exception.

Constructors

ExceptT (m (Either e a))

Instances

Functor m => Generic1 (ExceptT e m :: Type -> Type)

Defined in Control.Monad.Trans.Except

Associated Types

type Rep1 (ExceptT e m) :: k -> Type

Methods

from1 :: forall (a :: k). ExceptT e m a -> Rep1 (ExceptT e m) a

to1 :: forall (a :: k). Rep1 (ExceptT e m) a -> ExceptT e m a

MonadTrans (ExceptT e)

Defined in Control.Monad.Trans.Except

Methods

lift :: Monad m => m a -> ExceptT e m a

MonadFail m => MonadFail (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

fail :: String -> ExceptT e m a

MonadFix m => MonadFix (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

mfix :: (a -> ExceptT e m a) -> ExceptT e m a

MonadIO m => MonadIO (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

liftIO :: IO a -> ExceptT e m a

MonadZip m => MonadZip (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

mzip :: ExceptT e m a -> ExceptT e m b -> ExceptT e m (a, b)

mzipWith :: (a -> b -> c) -> ExceptT e m a -> ExceptT e m b -> ExceptT e m c

munzip :: ExceptT e m (a, b) -> (ExceptT e m a, ExceptT e m b)

Foldable f => Foldable (ExceptT e f)

Defined in Control.Monad.Trans.Except

Methods

fold :: Monoid m => ExceptT e f m -> m

foldMap :: Monoid m => (a -> m) -> ExceptT e f a -> m

foldMap' :: Monoid m => (a -> m) -> ExceptT e f a -> m

foldr :: (a -> b -> b) -> b -> ExceptT e f a -> b

foldr' :: (a -> b -> b) -> b -> ExceptT e f a -> b

foldl :: (b -> a -> b) -> b -> ExceptT e f a -> b

foldl' :: (b -> a -> b) -> b -> ExceptT e f a -> b

foldr1 :: (a -> a -> a) -> ExceptT e f a -> a

foldl1 :: (a -> a -> a) -> ExceptT e f a -> a

toList :: ExceptT e f a -> [a]

null :: ExceptT e f a -> Bool

length :: ExceptT e f a -> Int

elem :: Eq a => a -> ExceptT e f a -> Bool

maximum :: Ord a => ExceptT e f a -> a

minimum :: Ord a => ExceptT e f a -> a

sum :: Num a => ExceptT e f a -> a

product :: Num a => ExceptT e f a -> a

(Eq e, Eq1 m) => Eq1 (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

liftEq :: (a -> b -> Bool) -> ExceptT e m a -> ExceptT e m b -> Bool

(Ord e, Ord1 m) => Ord1 (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

liftCompare :: (a -> b -> Ordering) -> ExceptT e m a -> ExceptT e m b -> Ordering

(Read e, Read1 m) => Read1 (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

liftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (ExceptT e m a)

liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [ExceptT e m a]

liftReadPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec (ExceptT e m a)

liftReadListPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec [ExceptT e m a]

(Show e, Show1 m) => Show1 (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

liftShowsPrec :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> ExceptT e m a -> ShowS

liftShowList :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> [ExceptT e m a] -> ShowS

Contravariant m => Contravariant (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

contramap :: (a' -> a) -> ExceptT e m a -> ExceptT e m a'

(>$) :: b -> ExceptT e m b -> ExceptT e m a

Traversable f => Traversable (ExceptT e f)

Defined in Control.Monad.Trans.Except

Methods

traverse :: Applicative f0 => (a -> f0 b) -> ExceptT e f a -> f0 (ExceptT e f b)

sequenceA :: Applicative f0 => ExceptT e f (f0 a) -> f0 (ExceptT e f a)

mapM :: Monad m => (a -> m b) -> ExceptT e f a -> m (ExceptT e f b)

sequence :: Monad m => ExceptT e f (m a) -> m (ExceptT e f a)

(Functor m, Monad m, Monoid e) => Alternative (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

empty :: ExceptT e m a

(<|>) :: ExceptT e m a -> ExceptT e m a -> ExceptT e m a

some :: ExceptT e m a -> ExceptT e m [a]

many :: ExceptT e m a -> ExceptT e m [a]

(Functor m, Monad m) => Applicative (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

pure :: a -> ExceptT e m a

(<*>) :: ExceptT e m (a -> b) -> ExceptT e m a -> ExceptT e m b

liftA2 :: (a -> b -> c) -> ExceptT e m a -> ExceptT e m b -> ExceptT e m c

(*>) :: ExceptT e m a -> ExceptT e m b -> ExceptT e m b

(<*) :: ExceptT e m a -> ExceptT e m b -> ExceptT e m a

Functor m => Functor (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

fmap :: (a -> b) -> ExceptT e m a -> ExceptT e m b

(<$) :: a -> ExceptT e m b -> ExceptT e m a

Monad m => Monad (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

(>>=) :: ExceptT e m a -> (a -> ExceptT e m b) -> ExceptT e m b

(>>) :: ExceptT e m a -> ExceptT e m b -> ExceptT e m b

return :: a -> ExceptT e m a

(Monad m, Monoid e) => MonadPlus (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

mzero :: ExceptT e m a

mplus :: ExceptT e m a -> ExceptT e m a -> ExceptT e m a

Generic (ExceptT e m a)

Defined in Control.Monad.Trans.Except

Associated Types

type Rep (ExceptT e m a) :: Type -> Type

Methods

from :: ExceptT e m a -> Rep (ExceptT e m a) x

to :: Rep (ExceptT e m a) x -> ExceptT e m a

(Read e, Read1 m, Read a) => Read (ExceptT e m a)

Defined in Control.Monad.Trans.Except

Methods

readsPrec :: Int -> ReadS (ExceptT e m a)

readList :: ReadS [ExceptT e m a]

readPrec :: ReadPrec (ExceptT e m a)

readListPrec :: ReadPrec [ExceptT e m a]

(Show e, Show1 m, Show a) => Show (ExceptT e m a)

Defined in Control.Monad.Trans.Except

Methods

showsPrec :: Int -> ExceptT e m a -> ShowS

show :: ExceptT e m a -> String

showList :: [ExceptT e m a] -> ShowS

(Eq e, Eq1 m, Eq a) => Eq (ExceptT e m a)

Defined in Control.Monad.Trans.Except

Methods

(==) :: ExceptT e m a -> ExceptT e m a -> Bool

(/=) :: ExceptT e m a -> ExceptT e m a -> Bool

(Ord e, Ord1 m, Ord a) => Ord (ExceptT e m a)

Defined in Control.Monad.Trans.Except

Methods

compare :: ExceptT e m a -> ExceptT e m a -> Ordering

(<) :: ExceptT e m a -> ExceptT e m a -> Bool

(<=) :: ExceptT e m a -> ExceptT e m a -> Bool

(>) :: ExceptT e m a -> ExceptT e m a -> Bool

(>=) :: ExceptT e m a -> ExceptT e m a -> Bool

max :: ExceptT e m a -> ExceptT e m a -> ExceptT e m a

min :: ExceptT e m a -> ExceptT e m a -> ExceptT e m a

type Rep1 (ExceptT e m :: Type -> Type)

Defined in Control.Monad.Trans.Except

type Rep1 (ExceptT e m :: Type -> Type) = D1 ('MetaData "ExceptT" "Control.Monad.Trans.Except" "transformers-0.6.1.0-I7oGgfKsYBJJXzcRYqYaGl" 'True) (C1 ('MetaCons "ExceptT" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (m :.: Rec1 (Either e))))

type Rep (ExceptT e m a)

Defined in Control.Monad.Trans.Except

type Rep (ExceptT e m a) = D1 ('MetaData "ExceptT" "Control.Monad.Trans.Except" "transformers-0.6.1.0-I7oGgfKsYBJJXzcRYqYaGl" 'True) (C1 ('MetaCons "ExceptT" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 (m (Either e a)))))

runExceptT :: ExceptT e m a -> m (Either e a)

The inverse of ExceptT.

mapExceptT :: (m (Either e a) -> n (Either e' b)) -> ExceptT e m a -> ExceptT e' n b

Map the unwrapped computation using the given function.

• runExceptT (mapExceptT f m) = f (runExceptT m)

withExceptT :: Functor m => (e -> e') -> ExceptT e m a -> ExceptT e' m a

Transform any exceptions thrown by the computation using the given function.

Exception operations

throwE :: Monad m => e -> ExceptT e m a

Signal an exception value e.

• runExceptT (throwE e) = return (Left e)

• throwE e >>= m = throwE e

catchE

Arguments

:: Monad m

=> ExceptT e m a

the inner computation

-> (e -> ExceptT e' m a)

a handler for exceptions in the inner computation

-> ExceptT e' m a

Handle an exception.

• catchE (lift m) h = lift m

• catchE (throwE e) h = h e

handleE :: Monad m => (e -> ExceptT e' m a) -> ExceptT e m a -> ExceptT e' m a

The same as flip catchE, which is useful in situations where the code for the handler is shorter.

tryE :: Monad m => ExceptT e m a -> ExceptT e m (Either e a)

Similar to catchE, but returns an Either result which is (Right a) if no exception was thown, or (Left ex) if an exception ex was thrown.

finallyE :: Monad m => ExceptT e m a -> ExceptT e m () -> ExceptT e m a

finallyE a b executes computation a followed by computation b, even if a exits early by throwing an exception. In the latter case, the exception is re-thrown after b has been executed.

Lifting other operations

liftCallCC :: CallCC m (Either e a) (Either e b) -> CallCC (ExceptT e m) a b

Lift a callCC operation to the new monad.

liftListen :: Monad m => Listen w m (Either e a) -> Listen w (ExceptT e m) a

Lift a listen operation to the new monad.

liftPass :: Monad m => Pass w m (Either e a) -> Pass w (ExceptT e m) a

Lift a pass operation to the new monad.