Hello! On Sun, Oct 22, 2006 at 12:27:05AM +0400, Bulat Ziganshin wrote:
as Udo said, it should be better to evaluate thunks just when they are created, by using proper 'seq' calls.
While I understand why you and Udo are right, still it is difficult for me to related this discussion to my code. So I wrote a small example that reproduces my problem, with the hope that this will help me understand your point. This is my specific problem, I believe. There is a StateT monad with a list of string as a state. The list is populated with the lines of a file entered by the user. The user may read some lines of this file or request another one: - lFilename will load a file - sNumber will show a line number. The input file is evaluated at the very beginning (in my case that is forced by the xml parser, as far as I understand) and stored as the state. Now, the state will not be entirely consumed/evaluated by the user, and so it will not become garbage. Am I right? Where should I force evaluation? Is it clear my confusion (sorry for this kind of nasty recursion...;-)? Thanks for your kind attention. Best regards, Andrea here's the code: -------------- module Main where import Control.Monad.State import IO data Mystate = Mystate {mystate :: [String]} type SL = StateT Mystate IO getState :: SL [String] getState = do s <- get return $ mystate s setState ns = modify (\s -> s {mystate = ns}) getFile :: String -> SL () getFile p = do f <- liftIO $ readFile p let lns = lines f -- forces evaluation of lns liftIO $ putStrLn $ "Number of lines: " ++ show (length lns) setState lns promptLoop showLine :: Int -> SL () showLine nr = do s <- getState liftIO $ putStrLn $ s !! nr promptLoop promptStr = "lFilename [load the file Filename] - sNr [show the line Nr of Filename] - q to quit" promptLoop :: SL () promptLoop = do liftIO $ putStrLn promptStr str <- liftIO getLine case str of ('l':ss) -> getFile ss ('s':nr) -> showLine (read nr) ('q':[]) -> liftIO $ return () _ -> promptLoop main = evalStateT promptLoop $ Mystate []