Improving library documentation

If you find standard library documentation lacking in any way, please log it here. At the minimum record what library/module/function isn't properly documented. Please also suggest how to improve the documentation, in terms of examples, explanations and so on.

Example:

   package base
   Data.List
   unfoldr

   An example would be useful. Perhaps:

       -- A simple use of unfoldr:
       --
       -- > unfoldr (\b -> if b == 0 then Nothing else Just (b, b-1)) 10
       -- >  [10,9,8,7,6,5,4,3,2,1]
       --

Please add your comments under the appropriate package: