hello cafe, I just want to do cursion to a fixpoint in an IO function, I write sth. like this, handle_ar::(Set String,Set String)->FilePath-> IO (Set String, Set String) handle_ar (def,undef) ar=do let gs = def gu = undef syms <- liftM (map (\x -> (symb x, x))) $ defined_syms ar usyms <- liftM (map (\x -> (symb x, x))) $ undefined_syms ar mfix (\(gs,gu) -> do case find_obj gu usyms of Nothing -> return (gs,gu) Just z -> do let fout = fromList . map fst . filter ((== (objf z)) . objf . snd) ls = fout syms lu = fout usyms handle_obj_ar (gs,gu) (ls,lu)) What I want to express is: gs and gu are initiliazed to def and undef, then do recursion until "find_obj gu usyms" is nothing, i.e, (gs,gu) reaches a fixpoint (suppose handle_obj_ar always change the pair). I am not sure I am on the right track since there is no room for def and undef stand in the function. Anyone can help? thanks a lot. Gang