On Mon, 2007-04-09 at 21:24 -0400, Jefferson Heard wrote:
It is indeed! Is that to be found in Control.Monad, I take it?
Yes. Other common derivatives in that module include: mapM f as = sequence (map f as) mapM_ f as = sequence_ (map f as) forM_ = flip mapM_ forM = flip mapM however, personally, I think it's better style to use sequence/sequence_ directly since it shows more nicely how the IO is only going on at the top level whereas forM_ starts to feel very imperative. A nice style, imho, is to use sequence and list comprehensions, eg, reformulating your original program: main = do files <- getArgs sequence_ [ print . map (mean . map read . split ',') . lines =<< readFile file | file <- files ] sequence_ [ print . map (variance . map read . split ',') . lines =<< readFile file | file <- files ] Note, that like in your original we read each file twice, once for the mean and once for the variance. In your program you share the action to do the reading, but not the result of the action, so you do execute those file reading actions twice. Let's try fixing that: main = do files <- getArgs items <- sequence [ return . map (map read . split ',') . lines =<< readFile file | file <- files ] sequence_ [ print (map mean item) | item <- items ] sequence_ [ print (map variance item) | item <- items ] The next problem to note is that we read all the files before calculating the info we want from each file. So opening 100 files might be ok but 100,000 might not, so lets refactor again: main = do files <- getArgs mvs <- sequence [ return . calc . map (map read . split ',') . lines =<< readFile file | file <- files ] let (ms, vs) = unzip mvs mapM_ print ms mapM_ print vs calc items = (map mean items, map variance items) However this doesn't quite cut it either since nothing forces the mean and variance to be calculated until the very end where they get printed. This means the file will need to be held open until then too. We need to force the calculation of the mean and variance which will consume the file contents and allow the file to be closed. We could do this by printing the mean and variance out earlier (ie as soon as we've read the file) but that's changing the order of the lines in the output of your program (you print all the means followed by all the variance lines) so instead lets add a bit of strictness: import Control.Exception (evaluate) main = do files <- getArgs mvs <- sequence [ evaluate . calc . map (map read . split ',') . lines =<< readFile file | file <- files ] let (ms, vs) = unzip mvs mapM_ print ms mapM_ print vs calc items = m `seq` v `seq` (m, v) where m = map mean items v = map variance items We've changed two bits. We make sure that calc calculates the two values before returning the pair and we force the evaluation of calc itself using evaluate. Both are necessary. The evaluate action forces a value as an IO action, so we can be sure about when the forcing happens in relation to other IO actions. Duncan