Documentation

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 MonadError class. (If you are using the deprecated Control.Monad.Error or Control.Monad.Trans.Error, you may also have to define an Error instance.)

Lifts an Either e into any MonadError e.

do { val <- liftEither =<< action1; action2 }

where action1 returns an Either to represent errors.

Since: 2.2.2

MonadError analogue to the try function.

MonadError analogue to the withExceptT function. Modify the value (but not the type) of an error. The type is fixed because of the functional dependency m -> e. If you need to change the type of e use mapError.

As handle is flipped catch, handleError is flipped catchError.

MonadError analogue of the mapExceptT function. The computation is unwrapped, a function is applied to the Either, and the result is lifted into the second MonadError instance.