f.hs

doFoo :: (a -> Maybe a) -> Foo a -> ([a], Foo a)
doFoo f (Foo int list) = 
  let (old, new) = foldr go ([],[]) list
  in  (old, Foo int new)
    where go item (old,new) = case f item of
                                Nothing       -> (old     , item:new)
                                Just new_item -> (item:old, new_item:new)
   
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   VV
   
   
doFoo :: (a -> Maybe a) -> Foo a -> ([a], Foo a)
doFoo f (Foo int list) = (old, Foo int new)
    where 
      (old, new) = foldr go ([],[]) list
      go item (old,new) = case f item of
                                Nothing       -> (old     , item:new)
                                Just new_item -> (item:old, new_item:new)