| 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
Description
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.
Synopsis
- type Except e = ExceptT e Identity
- except :: Monad m => Either e a -> ExceptT e m 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
except :: Monad m => Either e a -> ExceptT e m 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).
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.
Instances
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(mapExceptTf m) = f (runExceptTm)
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
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.