* Vincent Hanquez
On 2012-12-07 09:44, 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).
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).
The patch is attached. [*]
[*] against transformers-0.3.0.0, because the darcs version is not buildable (Control/Monad/Signatures.hs is not in the repository).
[Sorry to revive this 1 yr 1/2 thread, but I was looking at the reason why we don't have eitherT is a more canonical place like transformers, and ended up here ...]
Is there a reason why this thread ran out of steam ? It not totally obvious from the answers what was missing to move forward. It seems like there's general agreement that EitherT is a good addition, did this proposal got forgotten ? Is there a way to revive this proposal ?
IIRC, last time this was brought up, Ross said that he'd take action after GHC 7.8 is released. So your reminder is well timed, in fact ;) Roman