This modules provides newtypes which flip the type variables of Either and EitherT to access the symmetric monad over the opposite type variable.

This module provides the following simple benefits to the casual user:

• A type-class free alternative to MonadError
• No UndecidableInstances or any other extensions, for that matter
• A more powerful catchE statement that allows you to change the type of error value returned

More advanced users can take advantage of the fact that EitherR and EitherRT define an entirely symmetric "success monad" where error-handling computations are the default and successful results terminate the monad. This allows you to chain error-handlers and pass around values other than exceptions until you can finally recover from the error:

 runEitherRT $ do
     e2 <- ioExceptionHandler e1
     bool <- arithmeticExceptionhandler e2
     when bool $ lift $ putStrLn "DEBUG: Arithmetic handler did something"

If any of the above error handlers succeed, no other handlers are tried.