(gobject:define-genum "GdkScrollDirection" scroll-direction
  (:export t
   :type-initializer "gdk_scroll_direction_get_type")
  (:up 0)
  (:down 1)
  (:left 2)
  (:right 3)
  (:smooth 4))  

:up

The window is scrolled up.

:down

The window is scrolled down.

:left

The window is scrolled to the left.

:right

The window is scrolled to the right.

:smooth

The scrolling is determined by the delta values in the gdk:scroll-event event. See the gdk:scroll-event-deltas function.