either

An either monad transformer

http://github.com/ekmett/either/

Version on this page:4.3.2.1
LTS Haskell 23.19:5.0.2@rev:2
Stackage Nightly 2025-04-30:5.0.2@rev:2
Latest on Hackage:5.0.2@rev:2

See all snapshots either appears in

BSD-3-Clause licensed by Edward A. Kmett
Maintained by Edward A. Kmett
This version can be pinned in stack with:either-4.3.2.1@sha256:a400cc38079a6f654eae3d4ff4514e43b8e7afc0ce2758d4b356dfd04eacf953,1427

Module documentation for 4.3.2.1

  • Control
    • Control.Monad
      • Control.Monad.Trans
        • Control.Monad.Trans.Either
  • Data
    • Data.Either
      • Data.Either.Combinators
      • Data.Either.Validation

either

Build Status

This provides an Either monad transformer that unlike ErrorT is unencumbered by a constraint on its Left hand argument. This is needed for a number of applications of this monad transformer, notably in recursion-schemes.

Contact Information

Contributions and bug reports are welcome!

Please feel free to contact me through github or on the #haskell IRC channel on irc.freenode.net.

-Edward Kmett

Changes

4.3.2.1

  • Support monad-control 1.0

4.3.2

  • Added Validation.

4.3.0.2

  • Updated MonadRandom support.

4.3.0.1

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

4.3

  • Inverted dependency between free and either.

4.2

  • Added instances for MonadThrow, MonadCatch.

4.1

  • Added instances for MonadBase, MonadBaseControl, and MonadTransControl.

4.0

  • Updated dependencies.

3.4.2

  • Added ‘Data.Either.Combinators’.

3.4.1

  • Trustworthy despite UndecidableInstances

3.4

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

3.3

  • 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.

3.2

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

3.1

  • 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.

3.0.3

  • Started CHANGELOG