Copyright	(C) 2013 Ross Paterson
License	BSD-style (see the file LICENSE)
Maintainer	R.Paterson@city.ac.uk
Stability	experimental
Portability	portable
Safe Haskell	Safe
Language	Haskell98

Control.Monad.Trans.Except

Contents

The Except monad
The ExceptT monad transformer
Exception operations
Lifting other operations

Description

This monad transformer extends a monad with the ability 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.

Synopsis

type Except e = ExceptT e Identity
except :: Either e a -> Except e a
runExcept :: Except e a -> Either e a
mapExcept :: (Either e a -> Either e' b) -> Except e a -> Except e' b
withExcept :: (e -> e') -> Except e a -> Except e' a
newtype ExceptT e m a = ExceptT (m (Either e a))
runExceptT :: ExceptT e m a -> m (Either e a)
mapExceptT :: (m (Either e a) -> n (Either e' b)) -> ExceptT e m a -> ExceptT e' n b
withExceptT :: Functor m => (e -> e') -> ExceptT e m a -> ExceptT e' m a
throwE :: Monad m => e -> ExceptT e m a
catchE :: Monad m => ExceptT e m a -> (e -> ExceptT e' m a) -> ExceptT e' m a
liftCallCC :: CallCC m (Either e a) (Either e b) -> CallCC (ExceptT e m) a b
liftListen :: Monad m => Listen w m (Either e a) -> Listen w (ExceptT e m) a
liftPass :: Monad m => Pass w m (Either e a) -> Pass w (ExceptT e m) a

The Except monad

type Except e = ExceptT e Identity Source #

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 :: Either e a -> Except e a Source #

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

runExcept :: Except e a -> Either e a Source #

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

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

Map the unwrapped computation using the given function.

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

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

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

The ExceptT monad transformer

newtype ExceptT e m a Source #

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

MonadTrans (ExceptT e) Source #
Instance details Methods lift :: Monad m => m a -> ExceptT e m a Source #
Monad m => Monad (ExceptT e m) Source #
Instance details Methods (>>=) :: ExceptT e m a -> (a -> ExceptT e m b) -> ExceptT e m b Source # (>>) :: ExceptT e m a -> ExceptT e m b -> ExceptT e m b Source # return :: a -> ExceptT e m a Source # fail :: String -> ExceptT e m a Source #
Functor m => Functor (ExceptT e m) Source #
Instance details Methods fmap :: (a -> b) -> ExceptT e m a -> ExceptT e m b Source # (<$) :: a -> ExceptT e m b -> ExceptT e m a Source #
MonadFix m => MonadFix (ExceptT e m) Source #
Instance details Methods mfix :: (a -> ExceptT e m a) -> ExceptT e m a Source #
MonadFail m => MonadFail (ExceptT e m) Source #
Instance details Methods fail :: String -> ExceptT e m a Source #
(Functor m, Monad m) => Applicative (ExceptT e m) Source #
Instance details Methods pure :: a -> ExceptT e m a Source # (<>) :: ExceptT e m (a -> b) -> ExceptT e m a -> ExceptT e m b Source # liftA2 :: (a -> b -> c) -> ExceptT e m a -> ExceptT e m b -> ExceptT e m c Source # (>) :: ExceptT e m a -> ExceptT e m b -> ExceptT e m b Source # (<*) :: ExceptT e m a -> ExceptT e m b -> ExceptT e m a Source #
Foldable f => Foldable (ExceptT e f) Source #
Instance details Methods fold :: Monoid m => ExceptT e f m -> m Source # foldMap :: Monoid m => (a -> m) -> ExceptT e f a -> m Source # foldr :: (a -> b -> b) -> b -> ExceptT e f a -> b Source # foldr' :: (a -> b -> b) -> b -> ExceptT e f a -> b Source # foldl :: (b -> a -> b) -> b -> ExceptT e f a -> b Source # foldl' :: (b -> a -> b) -> b -> ExceptT e f a -> b Source # foldr1 :: (a -> a -> a) -> ExceptT e f a -> a Source # foldl1 :: (a -> a -> a) -> ExceptT e f a -> a Source # toList :: ExceptT e f a -> [a] Source # null :: ExceptT e f a -> Bool Source # length :: ExceptT e f a -> Int Source # elem :: Eq a => a -> ExceptT e f a -> Bool Source # maximum :: Ord a => ExceptT e f a -> a Source # minimum :: Ord a => ExceptT e f a -> a Source # sum :: Num a => ExceptT e f a -> a Source # product :: Num a => ExceptT e f a -> a Source #
Traversable f => Traversable (ExceptT e f) Source #
Instance details Methods traverse :: Applicative f0 => (a -> f0 b) -> ExceptT e f a -> f0 (ExceptT e f b) Source # sequenceA :: Applicative f0 => ExceptT e f (f0 a) -> f0 (ExceptT e f a) Source # mapM :: Monad m => (a -> m b) -> ExceptT e f a -> m (ExceptT e f b) Source # sequence :: Monad m => ExceptT e f (m a) -> m (ExceptT e f a) Source #
(Eq e, Eq1 m) => Eq1 (ExceptT e m) Source #
Instance details Methods liftEq :: (a -> b -> Bool) -> ExceptT e m a -> ExceptT e m b -> Bool Source #
(Ord e, Ord1 m) => Ord1 (ExceptT e m) Source #
Instance details Methods liftCompare :: (a -> b -> Ordering) -> ExceptT e m a -> ExceptT e m b -> Ordering Source #
(Read e, Read1 m) => Read1 (ExceptT e m) Source #
Instance details Methods liftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (ExceptT e m a) Source # liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [ExceptT e m a] Source # liftReadPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec (ExceptT e m a) Source # liftReadListPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec [ExceptT e m a] Source #
(Show e, Show1 m) => Show1 (ExceptT e m) Source #
Instance details Methods liftShowsPrec :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> ExceptT e m a -> ShowS Source # liftShowList :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> [ExceptT e m a] -> ShowS Source #
MonadZip m => MonadZip (ExceptT e m) Source #
Instance details Methods mzip :: ExceptT e m a -> ExceptT e m b -> ExceptT e m (a, b) Source # mzipWith :: (a -> b -> c) -> ExceptT e m a -> ExceptT e m b -> ExceptT e m c Source # munzip :: ExceptT e m (a, b) -> (ExceptT e m a, ExceptT e m b) Source #
MonadIO m => MonadIO (ExceptT e m) Source #
Instance details Methods liftIO :: IO a -> ExceptT e m a Source #
(Functor m, Monad m, Monoid e) => Alternative (ExceptT e m) Source #
Instance details Methods empty :: ExceptT e m a Source # (<\|>) :: ExceptT e m a -> ExceptT e m a -> ExceptT e m a Source # some :: ExceptT e m a -> ExceptT e m [a] Source # many :: ExceptT e m a -> ExceptT e m [a] Source #
(Monad m, Monoid e) => MonadPlus (ExceptT e m) Source #
Instance details Methods mzero :: ExceptT e m a Source # mplus :: ExceptT e m a -> ExceptT e m a -> ExceptT e m a Source #
(Eq e, Eq1 m, Eq a) => Eq (ExceptT e m a) Source #
Instance details 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) Source #
Instance details 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 #
(Read e, Read1 m, Read a) => Read (ExceptT e m a) Source #
Instance details Methods readsPrec :: Int -> ReadS (ExceptT e m a) Source # readList :: ReadS [ExceptT e m a] Source # readPrec :: ReadPrec (ExceptT e m a) Source # readListPrec :: ReadPrec [ExceptT e m a] Source #
(Show e, Show1 m, Show a) => Show (ExceptT e m a) Source #
Instance details Methods showsPrec :: Int -> ExceptT e m a -> ShowS Source # show :: ExceptT e m a -> String Source # showList :: [ExceptT e m a] -> ShowS Source #

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

The inverse of ExceptT.

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

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 Source #

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

Exception operations

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

Signal an exception value e.

```
runExceptT (throwE e) = return (Left e)
```
```
throwE e >>= m = throwE e
```

catchE Source #

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 h (lift m) = lift m
```
```
catchE h (throwE e) = h e
```

Lifting other operations

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

Lift a callCC operation to the new monad.

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

Lift a listen operation to the new monad.

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

Lift a pass operation to the new monad.