BSD-3-Clause licensed by Edward A. Kmett
Maintained by Edward A. Kmett
This version can be pinned in stack with:either-,1973

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Contributions and bug reports are welcome!

Please feel free to contact me through github or on the #haskell IRC channel on

-Edward Kmett

Changes [2019.05.02]

  • Only incur a semigroups dependency on pre-8.0 GHCs.

5.0.1 [2018.07.03]

  • Make the Semigroup, Apply, and Applicative instances for Validation lazier.
  • Make vap lazier in its second argument.
  • Introduce vapm, an even lazier version of vap which requires a Monoid constraint. Also add apm, a counterpart for Validation.
  • Use test-framework and QuickCheck in the test suite.


  • Changed the semantics of the Validation Alt and Alternative instances to collect errors. The previous implementation did not correctly abide the laws.
  • Added vap, for when users want validation like semantics but don’t want to convert back and forth to validation all the time. Similarly, added ealt to give either’s Alt semantics to validation.
  • Dropped the deprecated Control.Monad.Trans.Either. Use Control.Monad.Trans.Except from transformers and/or transformers-compat instead.


  • Add MMonad instance for EitherT
  • Deprecate Control.Monad.Trans.Either in favor of Control.Monad.Trans.Except
  • Add firstEitherT

  • Fixed building on newer GHCs. (type synonyms require explicit foralls for unused variables these days)


  • transformers 0.5 support
  • Documentation fixes


  • Support mmorph

  • Support MonadRandom 0.4


  • Support bifunctors 5, profunctors 5, and semigroupoids 5.

  • Fixed and enhanced documentation for eitherToError.

  • Support exceptions 0.8

  • Support exceptions 0.7


  • Added eitherToError.

  • Support monad-control 1.0


  • Added Validation.

  • Updated MonadRandom support.

  • Fixed import of MonadCatch to support versions of base before 4.6


  • Inverted dependency between free and either.


  • Added instances for MonadThrow, MonadCatch.


  • Added instances for MonadBase, MonadBaseControl, and MonadTransControl.


  • Updated dependencies.


  • Added ‘Data.Either.Combinators’.


  • Trustworthy despite UndecidableInstances


  • Delegate fail to the underlying Monad, rather than error.


  • Inverted roles between Semigroup and Alt. This let us write Alternative and MonadPlus instances that are compatible.
  • Removed the Functor constraint on most instances in exchange for incurring a Monad constraint on Traversable. EitherT is after all, a Monad transformer first and foremost.


  • Changed the Semigroup to use a Semigroup to combine Left branches. Left Alt untouched, so you can mix and match.


  • Added instances for mtl classes and MonadRandom.
  • The meaning of mapEitherT has changed to match mapErrorT in the mtl. The old mapEitherT is now bimapEitherT.


  • Started CHANGELOG