Proposal: merge either into transformers

Michael Snoyman michael at
Fri Dec 7 10:56:45 CET 2012

On Fri, Dec 7, 2012 at 11:44 AM, Roman Cheplyaka <roma at> 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-, because the darcs version is not
> buildable (Control/Monad/Signatures.hs is not in the repository).
> Roman
> _______________________________________________
> Libraries mailing list
> Libraries at

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <>

More information about the Libraries mailing list