[Haskell-cafe] Call for comments: neither package

Antoine Latter aslatter at gmail.com
Mon Jun 28 22:11:32 EDT 2010

It looks like good work, but I would be hesitent about depending on a
package which pulled in both mtl and tranformers.

Maybe that's just superstition - I haven't tried it.


On Jun 28, 2010 5:51 PM, "Michael Snoyman" <michael at snoyman.com> wrote:

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.


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

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