We currently have (in Data.Foldable)

    foldr1 :: (a -> a -> a) -> t a -> a
    foldr1 f xs = fromMaybe (error "foldr1: empty structure")
                    (foldr mf Nothing xs)
      where
        mf x Nothing = Just x
        mf x (Just y) = Just (f x y)

and something similar for foldl1. This is strict in the entire spine, unlike the list version, because it has to get all the way to the end of the list before it starts laying down Justs. I propose we change this to the obvious:

    foldr1 :: (a -> a -> a) -> t a -> a
    foldr1 f xs = fromMaybe (error "foldr1: empty structure")
                    (foldr mf Nothing xs)
      where
        mf x r = Just $ case r of
                          Nothing -> x
                          Just y  -> f x y