The neither package

[Tags:bsd3, deprecated, library]
Deprecated. in favor of either, monad-control

The standard Either datatype suffers from a lack of monad and applicative instances. To make matters worse, the mtl and transformers packages provide orphan instances which conflict with each other, as well as defining a transformer version which has an usually unnecessary superclass constraint.

Besides these annoyances, there is another issue: there exist two reasonable definitions of the Applicative instance for Either: one the holds onto only the first Left value, or one that appends all Left values together via a Monoid instance. The former is compatible with the monad instance, while the latter is not.

This package defines three datatypes, some helpers functions and instances. The data types are AEither, MEither and MEitherT. AEither provides an Applicative instance which appends Left values, MEither provides the monadic definition, and MEitherT is a monad transformer.


Versions 0.0.0, 0.0.1, 0.0.2, 0.1.0, 0.2.0, 0.3.0,, 0.3.1,
Dependencies base (==4.*), failure (>=0.1.0 && <0.2), transformers (>=0.2.1 && <0.3) [details]
License BSD3
Author Michael Snoyman <>
Maintainer Michael Snoyman <>
Category Data
Home page
Source repository head: git clone git://
Uploaded Wed Jan 18 17:25:15 UTC 2012 by MichaelSnoyman
Distributions NixOS:
Downloads 1932 total (5 in the last 30 days)
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Status Docs uploaded by user
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