Moduletransformers-0.6.1.1Haskell2010

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.

2 types
15 values

Packagetransformers-0.6.1.1
Exports17
LanguageHaskell2010
LicenceBSD-3-Clause
SourceExcept.hs

The Except monad

5 declarations

typetype 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.

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

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

valuerunExcept :: Except e a -> Either e a

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

valuemapExcept :: (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)

valuewithExcept :: (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

4 declarations

newtypenewtype ExceptT e (m :: Type -> Type) 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))

Instances25Generic1, MonadTrans, Monad, Functor, MonadFix, MonadFail, …

Functor m => Generic1 (ExceptT e m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Except
MonadTrans (ExceptT e)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Except
Monad m => Monad (ExceptT e m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Except
Functor m => Functor (ExceptT e m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Except
MonadFix m => MonadFix (ExceptT e m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Except
MonadFail m => MonadFail (ExceptT e m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Except
(Functor m, Monad m) => Applicative (ExceptT e m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Except
Foldable f => Foldable (ExceptT e f)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Except
Traversable f => Traversable (ExceptT e f)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Except
(Functor m, Monad m, Monoid e) => Alternative (ExceptT e m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Except
(Monad m, Monoid e) => MonadPlus (ExceptT e m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Except
MonadIO m => MonadIO (ExceptT e m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Except
MonadZip m => MonadZip (ExceptT e m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Except
(Eq e, Eq1 m) => Eq1 (ExceptT e m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Except
(Ord e, Ord1 m) => Ord1 (ExceptT e m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Except
(Read e, Read1 m) => Read1 (ExceptT e m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Except
(Show e, Show1 m) => Show1 (ExceptT e m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Except
Contravariant m => Contravariant (ExceptT e m)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Except
(Eq e, Eq1 m, Eq a) => Eq (ExceptT e m a)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Except
(Ord e, Ord1 m, Ord a) => Ord (ExceptT e m a)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Except
(Read e, Read1 m, Read a) => Read (ExceptT e m a)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Except
(Show e, Show1 m, Show a) => Show (ExceptT e m a)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Except
Generic (ExceptT e m a)Defined in transformers-0.6.1.1 · Control.Monad.Trans.Except
type Rep (ExceptT e m a) = D1 ('MetaData "ExceptT" "Control.Monad.Trans.Except" "transformers-0.6.1.1-a11a" 'True) (C1 ('MetaCons "ExceptT" 'PrefixI 'False) (S1 ('MetaSel 'Nothing 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 (m (Either e a)))))Defined in transformers-0.6.1.1 · Control.Monad.Trans.Except
type Rep1 (ExceptT e m) = D1 ('MetaData "ExceptT" "Control.Monad.Trans.Except" "transformers-0.6.1.1-a11a" 'True) (C1 ('MetaCons "ExceptT" 'PrefixI 'False) (S1 ('MetaSel 'Nothing 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (m :.: Rec1 (Either e))))Defined in transformers-0.6.1.1 · Control.Monad.Trans.Except

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

The inverse of ExceptT.

valuemapExceptT

:: 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)

valuewithExceptT :: 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

5 declarations

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

Signal an exception value e.

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

valuecatchE

:: 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
```

valuehandleE

:: 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.

valuetryE :: 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.

valuefinallyE :: 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

3 declarations

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

Lift a callCC operation to the new monad.

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

Lift a listen operation to the new monad.

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

Lift a pass operation to the new monad.