I am going to use the function lookupInsertWithDefault :: Ord a => Map.Map a b -> a -> b -> (Map.Map a b, b) lookupInsertWithDefault tab a b' = case Map.lookup a tab of Just b -> (tab, b ) _ -> (Map.insert a b' tab, b') Its intention is as in the example: lookupInsertWithDefault ftable n (factor n) -> (ftable', factorization) (1) I hope that it will search in the table before trying to compute (factor n) by new. Also it stores each new value for future. (2) I wonder in what relation this may be to existing libraries (something like findWithDefault + insert) and whether the Map library may need such. What the people think of this? Regards, ----------------- Serge Mechveliani mechvel@botik.ru