IxSet filter by key

gistfile1.hs

-- | Returns the subset of an index whose key when run against the provided
-- function returns True.
filterByKey :: (Indexable a, Typeable a, Ord a, Typeable k)
      => (k -> Bool) -> IxSet a -> IxSet a
filterByKey func ixset@(IxSet indexes) = collect indexes
    where
    collect [] = [] -- FIXME: should be an error
    collect (Ix index _:is) = maybe (collect is) f $ cast index
        where
        f index'' = insertList (concatMap Set.elems [ val | (key, val) <- (Map.toList index''), (func key)]) empty