This module provides throwE and catchE for Either. These two functions reside here because throwE and catchE correspond to return and (>>=) for the flipped Either monad: EitherR. Similarly, this module defines throwT and catchT for EitherT, which correspond to the Monad operations for EitherRT.

These throw and catch functions improve upon MonadError because:

• catch is more general and allows you to change the left value's type
• They are Haskell98

More advanced users can use EitherR and EitherRT to program in an entirely symmetric "success monad" where exceptional results are the norm and successful results terminate the computation. This allows you to chain error-handlers using do notation and pass around exceptional values of varying types 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.

If you choose not to typefully distinguish between the error and sucess monad, then use flipE and flipET, which swap the type variables without changing the type.