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.