[Haskell-cafe] Call for comments: neither package

Michael Snoyman michael at snoyman.com
Tue Jun 29 01:20:19 EDT 2010

As far as I know, the only issue with depending on both is the conflicting
orphan Monad instance for Either. Can anyone either confirm or deny this?

On Tue, Jun 29, 2010 at 5:11 AM, Antoine Latter <aslatter at gmail.com> wrote:

> 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.
> Antoine
> 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.
> Michael
> [1] http://github.com/snoyberg/neither
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