Functional equivalent of decorator pattern?

Question

What is the functional programming equivalent of the decorator design pattern?

For example, how would you write this particular example in a functional style?

Answer 1

Something like this:

class Window w where
    draw :: w -> IO ()
    description :: w -> String

data VerticalScrollingWindow w = VerticalScrollingWindow w

instance Window w => Window (VerticalScrollingWindow w) where
    draw (VerticalScrollingWindow w)
       = draw w >> drawVerticalScrollBar w  -- `drawVerticalScrollBar` defined elsewhere
    description (VerticalScrollingWindow w)
       = description w ++ ", including vertical scrollbars"