decide-maybes.hs

-- Decide if applying f over a collection gives either Nothing or Just for every element.
decideMaybes :: Traversable f => (a -> Maybe b) -> f a -> Maybe (Either (f a) (f (a, b)))
decideMaybes f xs =
  asum [Left <$> nothingsOf pairs, Right <$> justsOf pairs, Nothing]
  where
    pairs      = fmap (\x -> (x, f x)) xs
    nothingsOf = traverse (\(x,y) -> x <$ guard (isNothing y))
    justsOf    = traverse (\(x,y) -> (x, fromJust y) <$ guard (isJust y))