Proposal: add fromRight and fromLeft to Data.Either

David Feuer david.feuer at
Sat May 21 12:58:01 UTC 2016

No one wants to remove fromMaybe. In fact, it could serve as a template for
total fromLeft and fromRight:

fromLeft :: b -> (a -> b) -> Either a x -> b
fromRight :: b -> (a -> b) -> Either x a -> b
On May 21, 2016 8:55 AM, "Edward Kmett" <ekmett at> wrote:

I'm personally -1 on this.

A large portion of the community is vehemently against adding new partial
functions, and they have to share base with the rest of us. I'd say more
people want to remove fromMaybe, head, tail, etc. than want to double down
on the pattern at this time.


On Fri, May 20, 2016 at 4:08 PM, Anton Felix Lorenzen <anfelor at>

> When working with Either,
> I am often missing two simple functions:
> fromRight :: Either a b -> b
> fromLeft  :: Either a b -> a
> It has been implemented a couple of times:
> But I don't want to depend on yet another library
> for such a basic function.
> Could it be added?
> Anton
> _______________________________________________
> Libraries mailing list
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