https://en.wikibooks.org/wiki/Haskell/Lists_III "Scans" exercise

scan_exercise.hs

scanr_rec :: (a -> b -> b) -> b -> [a] -> [b]
scanr_rec _ acc [] = [acc]
scanr_rec f acc (x:xs) = let next = scanr_rec f acc xs in f x (head next) : next

scanr_fold :: (a -> b -> b) -> b -> [a] -> [b]
scanr_fold f acc = foldr scanFold [acc]
  where
    scanFold x acc = f x (head acc) : acc

scanl_rec :: (b -> a -> b) -> b -> [a] -> [b]
scanl_rec f acc [] = [acc]
scanl_rec f acc (x:xs) = acc : scanl_rec f (f acc x) xs

scanl_fold :: (b -> a -> b) -> b -> [a] -> [b]
scanl_fold f acc = reverse . foldl scanFold [acc]
  where
    scanFold acc x = f (head acc) x : acc