#4159: move Monad and MonadFix instances for Either from mtl to base

[Ross Paterson]

On Wed, Jul 21, 2010 at 11:05:02AM -0700, Iavor Diatchki wrote:
> PS: About (+ e) being used in papers on monads: the instance we are
> adding is actually "Either e" which would be confusing enough for
> someone who is just learning about these things.  It would be even
> more confusing when---having finally figured out the
> correspondence---they tried to define the instance from the paper and
> got an "overlapping instance" error.

Yes, Either is the Prelude version of +.  As for the overlap, the
same thing will happen if they read in those papers about (->) r
and try to declare that -- it's already there.

> PPS:  I don't mean to sound too negative here (I am told I have some
> skill for it though :-).  Examples of the utility of this instance
> would be most welcome though.

Those papers have a few.