error +mtl

module Control.Monad.Error
mtl Control.Monad.Error
* Computation type: Computations which may fail or throw exceptions. * Binding strategy: Failure records information about the cause/location of the failure. Failure values bypass the bound function, other values are used as inputs to the bound function. * Useful for: Building computations from sequences of functions that may fail or using exception handling to structure error handling. * Zero and plus: Zero is represented by an empty error and the plus operation executes its second argument if the first fails. * Example type: Either String a The Error monad (also called the Exception monad).
class Error a
mtl Control.Monad.Error.Class, mtl Control.Monad.Error
An exception to be thrown. Minimal complete definition: noMsg or strMsg.
ErrorT :: m (Either e a) -> ErrorT e a
mtl Control.Monad.Error
catchError :: MonadError e m => m a -> (e -> m a) -> m a
mtl Control.Monad.Error.Class, mtl Control.Monad.Error
mapErrorT :: (m (Either e a) -> n (Either e' b)) -> ErrorT e m a -> ErrorT e' n b
mtl Control.Monad.Error
Map the unwrapped computation using the given function. *  (mapErrorT f m) = f (runErrorT >  
class Monad m => MonadError e m | m -> e
mtl Control.Monad.Error.Class, mtl Control.Monad.Error
The strategy of combining computations that can throw exceptions by bypassing bound functions from the point an exception is thrown to the point that it is handled. Is parameterized over the type of error information and the monad type constructor. It is common to use Either String as the monad type constructor for an error monad in which error descriptions take the form of strings. In that case and many other common cases the resulting monad is already defined as an instance of the MonadError class. You can also define your own error type and/or use a monad type constructor other than Either String or Either IOError. In these cases you will have to explicitly define instances of the Error and/or MonadError classes.
runErrorT :: ErrorT e a -> m (Either e a)
mtl Control.Monad.Error
throwError :: MonadError e m => e -> m a
mtl Control.Monad.Error.Class, mtl Control.Monad.Error