[Haskell-beginners] Re: Either Monadic Trouble

[Nicolas Pouillard]

Excerpts from Ertugrul Soeylemez's message of Mon Nov 09 21:44:44 +0100 2009:
> Nicolas Pouillard <nicolas.pouillard at gmail.com> wrote:
> 
> > > Either is not a monad, you can check this by typing
> > >    :i Either
> > > in GHCi; you will not see a line like
> > >    instance Monad Either
> > > in the result. Compare this to
> > >    :i Maybe
> >
> > In fact the Either Monad instance is defined in the 'transformers' (or
> > 'mtl') packages.
> 
> Either is still not a monad.  Have a look at its kind.

OK, right Either is not but (Either e), where e must be in
the Error type class.

-- 
Nicolas Pouillard
http://nicolaspouillard.fr