Proposal to solve the `EitherT` problem.

[Ross Paterson]

On Wed, Aug 14, 2013 at 10:51:01AM -0400, Edward Kmett wrote:
> I was trying to fire off one last shot across the bow that in the big 2.0
> switch there was a move to make "State s" be "StateT s Identity" that was
> mostly argued for code reuse and simplification reasons, that it cut code
> duplication by a factor of 2 in the body of transformers and the mtl and
> reduced the chance for human error. 
> 
> The fact that State s = StateT s Identity rather than merely being isomorphic
> seems to me to be an emergent property of this change, not its purpose.

I think there's still mileage in that argument.  For example, the module
Control.Error.Safe in the errors package has 13 functions of the form

	fooErr :: e -> args -> Either e r

and another 13 of the form

	tryFoo :: Monad m => e -> args -> EitherT e m r

If the recommended base exception monad were a specialization of the
recommended transformer, only one set would be needed.