res such
-- that the 'Lens' views at least one element of the structure.
--
-- >>> foldl1Of each (+) (1,2,3,4)
-- 10
--
-- @
-- 'foldl1Of' l f ≡ 'Prelude.foldl1' f '.' 'toListOf' l
-- 'Data.Foldable.foldl1' ≡ 'foldl1Of' 'folded'
-- @
--
-- @
-- 'foldl1Of' :: 'Getter' s a     -> (a -> a -> a) -> s -> a
-- 'foldl1Of' :: 'Fold' s a       -> (a -> a -> a) -> s -> a
-- 'foldl1Of' :: 'Iso'' s a       -> (a -> a -> a) -> s -> a
-- 'foldl1Of' :: 'Lens'' s a      -> (a -> a -> a) -> s -> a
-- 'foldl1Of' :: 'Traversal'' s a -> (a -> a -> a) -> s -> a
-- @
foldl1Of :: Getting (Dual (Endo (Maybe a))) s a -> (a -> a -> a) -> s -> a
foldl1Of l f xs = fromMaybe (error "foldl1Of: empty structure") (foldlOf l mf Nothing xs) where
  mf mx y = Just $ case mx of
    Nothing -> y
    Just x  -> f x y
{-# INLINE foldl1Of #-}

-- | Strictly fold right over the elements of a structure.
--
-- @
-- 'Data.Fol
foldl1Of :: Getting (Dual (Endo (Maybe a))) s a -> (a -> a -> a) -> s -> a
foldl1Of l f xs = fromMaybe (error "foldl1Of: empty structure") (foldlOf l mf Nothing xs) where
  mf mx y = Just $ case mx of
    Nothing -> y
    Just x  -> f x y

foldl1Of l f xs = fromMaybe (error "foldl1Of: empty structure") (foldlOf l mf Nothing xs) where
  mf mx y = Just $ case mx of
    Nothing -> y
    Just x  -> f x y
