Personally, I think it would be a shame to lose foldMap on EIther. I frequently foldMap over Maybe values (where mempty is suitable in case of "failure"), and I can certainly see myself doing the same thing with Either.

- ocharles

On Thu, Mar 2, 2017 at 5:08 PM Francesco Ariis <fa-ml@ariis.it> wrote:

On Thu, Mar 02, 2017 at 05:59:18PM +0100, Andreas Abel wrote:
> Ok, Foldable is a formal condition for Traversable, but not actually used in
> the implementation of Traversable Either. This still leaves room to
> implement Foldable for Either by
>
> instance Foldable (Either a) where
> foldMap _ _ = error "Folding Either? Naah, I don't think this is a good
> idea."

I don't fancy Foldable for Either either, but runtime failures are
ugly too.

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