[xmonad] list of visible workspaces

[Bryan Huh]

How do you get a list of visible workspaces (only those workspaces that 
are displayed on the monitors)? Currently I get the entire list of 
workspaces in my xmonad config with (XMonad.workspaces conf). It looks 
like this command returns a list of strings, each of which denote a 
workspace (i.e. ["1", "2"... ] ). But not sure how to get a list of only 
those which are visible (I could also probably work with a list of 
hidden workspaces).

Sorry, my Haskell is weak and I do not understand how to use the 
documentation here: 
http://xmonad.org/xmonad-docs/xmonad/XMonad-StackSet.html

Thanks!