src/Misc.hs

   1 -- | Miscellaneous utility functions
   2 module Misc (
   3   double_just )
   4 where
   5
   6 -- | If given 'Nothing', return 'Nothing'. Otherwise wrap our argument
   7 --   in another 'Just'. This is used when handling optional XML
   8 --   elements that are optionally empty. If the element is missing, we
   9 --   want 'Nothing'. And if the contents are missing, we want
  10 --   'Nothing' then too. But if something is present, we need @Just
  11 --   (Just foo)@ for the types to match up.
  12 --
  13 --   ==== __Examples__:
  14 --
  15 --   >>> double_just Nothing
  16 --   Nothing
  17 --   >>> double_just (Just 2)
  18 --   Just (Just 2)
  19 --
  20 double_just :: (Maybe a) -> Maybe (Maybe a)
  21 double_just val =
  22   case val of
  23     Nothing -> Nothing
  24     just_something -> Just just_something