Description:

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.

This package provides the shared library.