Hi Daniel and everyone, Thanks for the reply! I thought of using waitForProcess, and in fact an earlier version of the program did. However, as the program got more complex, I don't really see how this is still possible. For example the runProgram function looks like this: runProgram :: [String] -> IO () runProgram [] = return () runProgram (c:cs) = do runCommand ("lalala " ++ c) runProgram cs It might be possible to write the function runProgram in a way, so that it returns an expression of type [IO ProcessHandle] and then try to work from there. But I have the feeling, that this will become messy very quickly, and there must be some more elegant way of doing this. The other thing is that actually not all functions are able to return ProcessHandles, e.g. writeFeatures :: [String] -> IO () writeFeatures cs = Exc.bracket (openFile training AppendMode) hClose (\h -> goo h) where goo h = go 1 cs where go :: Int -> [String] -> IO () go n [] = putStrLn "Features written." go n (c:cs) = do features <- makeFeatures n c -- makeFeatures :: Int -> String -> IO String hPutStr h features go (n+1) cs And the file that is produced here is needed in the next function. I hoped to do something with forkIO, as I would like to parallelize the whole program at the end. Especially the function runProgram would benefit hugely from this (I so don't have a clue how to do this yet;). I tried for example the following: main :: IO () main = do cs <- getArgs p1 <- forkIO $ writeData cs p2 <- forkIO $ runProgram cs p3 <- forkIO $ writeFeatures cs p4 <- forkIO $ runTestOnFeatures seq p1 (seq p2 (seq p3 (seq p4 (putStrLn "Done")))) But that of course doesn't work, because now I am not actually requesting anything. The program does in fact nothing, apart from printing out "Done". Any ideas? Cheers, Thomas Daniel Fischer wrote:

Am Donnerstag 07 Mai 2009 18:36:08 schrieb Thomas Friedrich:

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Hi everyone,

Each of the above function take a list of filenames, run certain command-line programs on them, which I invoke by runCommand, and each of them produce multiple output-files. Each function in main needs a couple of those output-files that are produced by the function directly above it. How do I get Haskell to wait, till all the data is written to the disk, before invoking the next command.

System.Process.waitForProcess

should do it, conveniently runCommand returns a ProcessHandle.

...

The way the program is currently written, Haskell doesn't see that the input of one function depends on the output of another, and tries to run them all at the same time.

Any ideas?

Thanks everyone for your help.

Cheers, Thomas

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