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

[Ross Paterson]

On Fri, Jul 16, 2010 at 12:58:39AM -0700, Iavor Diatchki wrote:
> I am against adding the instances for Monad Either to the base package
> for the same reasons as the ones in the thread "proposal #4095: add
> Applicative instance for Either".
> http://www.haskell.org/pipermail/libraries/2010-June/013715.html

namely:
> The benefit of omitting some instances is that when programmers find them
> lacking, they might consider an alternative way to achieve their goal.
> In the case of Monad and Either, my feeling is that an alternative would
> almost always lead to code which is easier to understand.

I don't believe that witholding valid canonical instances to force a
different style is viable in general.  In this particular case there
is already an instance in mtl, and (+ e) is used as an example is just
about every paper on monads.