On Fri, Dec 07, 2012 at 09:44:27AM +0000, Roman Cheplyaka wrote:
I propose to add the sole module of the 'either' package[1], Control.Monad.Trans.Either, to the transformers package.
It provides EitherT, a very basic and fundamental data type. The difference between EitherT and ErrorT is that the latter has an Error constraint, which is used to imlement 'fail'.
Note that 'either' depends on the 'semigroupoids' and 'semigroup' packages to provide appropriate instances. The proposal is not to add those instances to 'transformers' to avoid additional dependencies. The instances can then be left in the 'either' package or moved to the 'semigroupoids' and 'semigroup' packages respectively. ('semigroupoids' already depends on 'transformers', while 'semigroups' does not).
Orphan instances are to be avoided, so moving the instances to those packages seems the only option.
Compared to the 'either' package, Show, Read, Eq and Ord instances will be dropped to keep the code Haskell2010 (those instances require FlexibleInstances, FlexibleContexts, and UndecidableInstances).
That's true. Some other points: The either package has mapEitherT as the binary map mapEitherT :: Functor m => (e -> f) -> (a -> b) -> EitherT e m a -> EitherT f m b but consistency with the rest of transformers would apply this name to mapEitherT :: (m (Either e a) -> n (Either e' b)) -> EitherT e m a -> EitherT e' n b mapEitherT f m = EitherT $ f (runEitherT m) (The binary map can't be recovered using Bifunctor because of the argument order.) either has hoistEither :: Monad m => Either e a -> EitherT e m a Maybe transformers should have similar functions for all the other monad transformers. left and right are used with different meanings in Control.Arrow (surmountable with qualification, of course). I see that the idea is to be symmetrical, but the monad structure is asymmetric. Would we want a catch function?