scanl :: (b -> a -> b) -> b -> [a] -> [b]
scanl f b = g [b] b
  where g xs x []  = reverse xs
        g xs x (y:ys) = g (x':xs) x' ys
          where x' = x `f` y

scanl' :: (b -> a -> b) -> b -> [a] -> [b]
scanl' f b as = reverse $ foldl g [b] as
  where g (x:xs) y = (x `f` y):x:xs