Data.EitherR

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.

Documentation

newtype EitherR r e

If "Either e r" is the error monad, then "EitherR r e" is the corresponding success monad, where:

• return is throwE.
• (>>=) is catchE.
• Successful results abort the computation

Constructors

EitherR

Fields

runEitherR :: Either e r

Instances

Monad (EitherR r)

Functor (EitherR r)

Applicative (EitherR r)

Operations in the EitherR monad

succeed :: r -> EitherR r e

Complete error handling, returning a result

Conversions to the Either monad

throwE :: e -> Either e r

throwE in the error monad corresponds to return in the success monad

catchE :: Either a r -> (a -> Either b r) -> Either b r

catchE in the error monad corresponds to (>>=) in the success monad

handleE :: (a -> Either b r) -> Either a r -> Either b r

catchE with the arguments flipped

fmapL :: (a -> b) -> Either a r -> Either b r

Map a function over the Left value of an Either

EitherRT

newtype EitherRT r m e

EitherR converted into a monad transformer

Constructors

EitherRT

Fields

runEitherRT :: EitherT e m r

Instances

MonadTrans (EitherRT r)

Monad m => Monad (EitherRT r m)

Monad m => Functor (EitherRT r m)

Monad m => Applicative (EitherRT r m)

Operations in the EitherRT monad

succeedT :: Monad m => r -> EitherRT r m e

Complete error handling, returning a result

Conversions to the EitherT monad

throwT :: Monad m => e -> EitherT e m r

throwT in the error monad corresponds to return in the success monad

catchT :: Monad m => EitherT a m r -> (a -> EitherT b m r) -> EitherT b m r

catchT in the error monad corresponds to (>>=) in the success monad

handleT :: Monad m => (a -> EitherT b m r) -> EitherT a m r -> EitherT b m r

catchT with the arguments flipped

fmapLT :: Monad m => (a -> b) -> EitherT a m r -> EitherT b m r

Map a function over the Left value of an EitherT