Re: Why is there no Monoid instance for Either?

27 Nov 2014

      Hi Andreas,

Well, we still made the decision to have a similarly arbitrary
instance for `Maybe`, so I thought that instance might be justified
too.

My use case: I'm traversing terms and I want to either collect
meta-variables, or fail.  So I'd like to use `Either Error
MetaVariables` to automatically handle the merging of results for
subterms.  In the end I defined my own type:
https://github.com/bitonic/tog/blob/master/src/Data/Collect.hs.

Francesco

On 27 November 2014 at 18:45, Andreas Abel  wrote:
...
Hi Francesco,
maybe you should add the use cases you have in mind, to strengthen your
case.
A priori, it is not clear why an error-propagating monoid should be
preferred over an error-correcting one.  (Both seem useful.)
Your proposal has the flavor of "All" whereas the Semigroup one corresponds
to "Any".
Personally, I think neither of the alternatives is sufficiently canonical
(both versions throw away information), thus, there shouldn't be instances
(also not for Semigroup).
Cheers,
Andreas
On 27.11.2014 18:29, Francesco Mazzoli wrote:
...
Hi,
Lately, I found myself wanting to use the following Monoid instance:
````
instance (Monoid b) => Monoid (Either a b) where
   mempty = Right mempty
Left e  `mappend` _       = Left e
   _       `mappend` Left e  = Left e
   Right x `mappend` Right y = Right (x `mappend` y)
````
It would seem quite justified to me, given the left-bias of Either
elsewhere in base.  Is there any particular reason why it shouldn't be
defined?
It would break the instance for Either in `semigroups`, which is
````
instance Semigroup (Either a b) where
   Left _ <> b = b
   a      <> _ = a
````
But I think mine is more useful, and has the additional advantage of
having `mappend` to behave very similar to the `mappend` for `Maybe`.
Francesco
_______________________________________________
Libraries mailing list
Libraries@haskell.org
http://www.haskell.org/mailman/listinfo/libraries
--
Andreas Abel  <><      Du bist der geliebte Mensch.
Department of Computer Science and Engineering
Chalmers and Gothenburg University, Sweden
andreas.abel@gu.se
http://www2.tcs.ifi.lmu.de/~abel/