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

Excerpts from Ross Paterson's message of Tue Jun 29 05:26:49 -0400 2010:

The proposal is to move the Monad and MonadFix instances for Either (currently in the mtl package) to Control.Monad.Instances and Control.Monad.Fix respectively (both in the base package). The Monad instance is still an orphan, to retain Haskell 98 compatibility, but the MonadFix instance is together with its class. The Error constraint is removed from both instances, and the default definition of fail is used.

+1, and long overdue, I think. Cheers, Edward