Safe Haskell  SafeInfered 

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 typeclass 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
errorhandling computations are the default and successful results terminate
the monad. This allows you to chain errorhandlers 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.
 newtype EitherR r e = EitherR {
 runEitherR :: Either e r
 succeed :: r > EitherR r e
 throwE :: e > Either e r
 catchE :: Either a r > (a > Either b r) > Either b r
 handleE :: (a > Either b r) > Either a r > Either b r
 fmapL :: (a > b) > Either a r > Either b r
 newtype EitherRT r m e = EitherRT {
 runEitherRT :: EitherT e m r
 right :: Monad m => r > EitherRT r m e
 succeedT :: Monad m => r > EitherRT r m e
 throwT :: Monad m => e > EitherT e m r
 catchT :: Monad m => EitherT a m r > (a > EitherT b m r) > EitherT b m r
 handleT :: Monad m => (a > EitherT b m r) > EitherT a m r > EitherT b m r
 fmapLT :: Monad m => (a > b) > EitherT a m r > EitherT b m r
Documentation
If "Either e r
" is the error monad, then "EitherR r e
" is the
corresponding success monad, where:
EitherR  

Operations in the EitherR monad
Conversions to the Either monad
EitherRT
EitherR
converted into a monad transformer
EitherRT  
