I'd like to propose an addition to the FiniteMap module in the form of a monadic version of plusFM_C. The proposed implementation is pretty much a copy of the existing implementation within a do block. [[ -- | Monadic version of plusFM_C. -- -- The combiner function returns a monadic value, which is threaded though -- the combined elements in key order, yielding a FiniteMap that is bound -- to the same monadic type. The intended use for this is with a Maybe -- monad, allowing result returned to be Nothing if any of the attempted -- combinations of key values return Nothing. Could also be usefully used -- with an error or state monad. plusFM_CM :: ( Ord key, Monad m ) => (elt -> elt -> m elt) -> FiniteMap key elt -> FiniteMap key elt -> m (FiniteMap key elt) plusFM_CM combiner EmptyFM fm2 = return fm2 plusFM_CM combiner fm1 EmptyFM = return fm1 plusFM_CM combiner fm1 (Branch split_key elt2 _ left right) = do { new_l <- plusFM_CM combiner (splitLT fm1 split_key) left ; new_elt <- case lookupFM fm1 split_key of Nothing -> return elt2 Just elt1 -> combiner elt1 elt2 ; new_r <- plusFM_CM combiner (splitGT fm1 split_key) right ; return $ mkVBalBranch split_key new_elt new_l new_r } -- And some test cases: fm1 = listToFM [(1,["a"]),(2,["b","c"]),(3,["d"])] fm2 = listToFM [(1,["b","c"]),(4,["e"])] fm3 = listToFM [(1,["d","e"]),(2,["c","d"]),(4,["f"])] fm12 = Just $ listToFM [(1,["a","b","c"]),(2,["b","c"]),(3,["d"]),(4,["e"])] fm13 = Nothing fm23 = Just $ listToFM [(1,["b","c","d","e"]),(2,["c","d"]),(4,["e","f"])] -- Test function returns Nothing if list values have a member in common: comb ovs nvs | null (intersect ovs nvs) = Just (ovs++nvs) | otherwise = Nothing fmt1 = plusFM_CM comb fm1 fm2 == fm12 fmt2 = plusFM_CM comb fm1 fm3 == fm13 fmt3 = plusFM_CM comb fm2 fm3 == fm23 fmtall = and [fmt1,fmt2,fmt3] ]] ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact