scanl is similar to foldl, but returns a list of successive reduced values from the left:
> scanl f z [x1, x2, ...] == [z, z `f` x1, (z `f` x1) `f` x2, ...]
> last (scanl f z xs) == foldl f z xs.
scanl1 is a variant of scanl that has no starting value argument:
> scanl1 f [x1, x2, ...] == [x1, x1 `f` x2, ...]