Re: library submission: Data.Either.unzipEithers :: [Either a b] -> ([a], [b])

On Mon, Sep 17, 2007 at 01:51:34PM +0100, Conor McBride wrote:

For your general amusement, a generalisation:

...

separate :: (Traversable t, Applicative a, Monoid (a x), Monoid (a y)) => t (Either x y) -> (a x, a y) separate = foldMap (either (pure &&& mempty) (mempty &&& pure))

Or even newtype Wrap f a = Wrap { unWrap :: f a } instance Alternative f => Monoid (Wrap f a) where mempty = Wrap empty Wrap x `mappend` Wrap y = Wrap (x <|> y) separate :: (Foldable t, Alternative f) => t (Either x y) -> (f x, f y) separate = (unWrap *** unWrap) . foldMap (either ((Wrap . pure) &&& mempty) (mempty &&& (Wrap . pure)))