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

[Henning Thielemann]

On Tue, 29 Jun 2010, Edward Kmett wrote:

> A reasonable case can be made for the existence of a monad that does something useful
> with fail. I don't think that that monad should be Either.
> 
> 'Either' takes on the connotation of being the 'sum type' for the category of Haskell
> types. This sum type has a very well formed and simple monad, that has a lot of useful
> theoretical properties, and is useful in a strictly larger array of scenarios than the
> monad with the error constraint, with the one notable exception that it doesn't handle
> the 'fail' property that was bolted into Monad in Haskell 98.
> 
> The obvious name for such a monad would be Error, to go with ErrorT, but the Error class
> conflicts with that name, but perhaps:
> 
> data Fail e a = Error e | OK a
> 
> would be worth adding to Control.Monad.Error, during the same general timetable as this
> change, to provide users who really want the existing mtl Either semantics an upgrade
> path.
> 
> In the interest of not killing this proposal with bikeshedding concerns over what to
> call it (Fail, Err, Error, etc), we should probably put that forward as a completely
> separate libraries proposal though.
> 
> It would address the confusion caused to new users who often on #haskell ask why 'Left'
> is the error case, and would address your use case explicitly, and even less verbosely.
> ;)


+1