| Copyright | (c) Michael Weber <michael.weber@post.rwth-aachen.de> 2001 (c) Jeff Newbern 2003-2006 (c) Andriy Palamarchuk 2006 |
|---|---|
| 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.Error
Description
Deprecated: Use Control.Monad.Trans.Except instead
This monad transformer adds the ability to fail or throw exceptions to a monad.
A sequence of actions succeeds, producing a value, only if all the actions in the sequence are successful. If one fails with an error, the rest of the sequence is skipped and the composite action fails with that error.
If the value of the error is not required, the variant in Control.Monad.Trans.Maybe may be used instead.
Note: This module will be removed in a future release. Instead, use Control.Monad.Trans.Except, which does not restrict the exception type, and also includes a base exception monad.
Synopsis
- class Error a where
- class ErrorList a where
- newtype ErrorT e m a = ErrorT {}
- mapErrorT :: (m (Either e a) -> n (Either e' b)) -> ErrorT e m a -> ErrorT e' n b
- throwError :: Monad m => e -> ErrorT e m a
- catchError :: Monad m => ErrorT e m a -> (e -> ErrorT e m a) -> ErrorT e m a
- liftCallCC :: CallCC m (Either e a) (Either e b) -> CallCC (ErrorT e m) a b
- liftListen :: Monad m => Listen w m (Either e a) -> Listen w (ErrorT e m) a
- liftPass :: Monad m => Pass w m (Either e a) -> Pass w (ErrorT e m) a
The ErrorT monad transformer
Minimal complete definition
Nothing
Methods
Creates an exception without a message.
The default implementation is strMsg ""
Instances
| Error IOException Source # | |
Defined in Control.Monad.Trans.Error | |
| ErrorList a => Error [a] Source # | A string can be thrown as an error. |
The error monad transformer. It can be used to add error handling to other monads.
The ErrorT Monad structure is parameterized over two things:
- e - The error type.
- m - The inner monad.
The return function yields a successful computation, while >>=
sequences two subcomputations, failing on the first error.
Instances
Error operations
throwError :: Monad m => e -> ErrorT e m a Source #
Signal an error value e.
runErrorT(throwErrore) =return(Lefte)throwErrore >>= m =throwErrore
Arguments
| :: Monad m | |
| => ErrorT e m a | the inner computation |
| -> (e -> ErrorT e m a) | a handler for errors in the inner computation |
| -> ErrorT e m a |
Handle an error.
catchErrorh (liftm) =liftmcatchErrorh (throwErrore) = h e
Lifting other operations
liftCallCC :: CallCC m (Either e a) (Either e b) -> CallCC (ErrorT e m) a b Source #
Lift a callCC operation to the new monad.
liftListen :: Monad m => Listen w m (Either e a) -> Listen w (ErrorT e m) a Source #
Lift a listen operation to the new monad.
liftPass :: Monad m => Pass w m (Either e a) -> Pass w (ErrorT e m) a Source #
Lift a pass operation to the new monad.
Examples
Wrapping an IO action that can throw an error e:
type ErrorWithIO e a = ErrorT e IO a ==> ErrorT (IO (Either e a))
An IO monad wrapped in StateT inside of ErrorT:
type ErrorAndStateWithIO e s a = ErrorT e (StateT s IO) a ==> ErrorT (StateT s IO (Either e a)) ==> ErrorT (StateT (s -> IO (Either e a,s)))