Hi all,

I'll admit, the original idea for this package was something to place in ACME ;). However, it's goal is to solve a real problem: the lack of good instances on the Either type. As a brief summary, Either has no Applicative or Monad instances in the base library, has 2 reasonable definitions for Applicative, and there are conflicting orphan instances in the mtl and transformers packages. Also, the ErrorT transformer in those two packages introduces a superclass constraint many people would like to avoid.

neither supplies three datatypes: AEither, MEither and MEitherT. AEither provides the Monoid version of the Applicative instance, MEither is the monadic version, and MEitherT is a monad transformer. The package provides instances for both the transformers and mtl libraries for MonadTrans, MonadIO and MonadCatchIO.

The code is up on github[1], let me know what you think.

Michael

[1] http://github.com/snoyberg/neither