Hello apfelmus, Thursday, October 12, 2006, 4:42:14 PM, you wrote:
A better solution would be to begin output before the the whole input is read, thus making things more lazy. This can be done the following way: from the input, construct a lazy list of (date,line) pairs. Then, let foldM thread a map from dates to corresponding output file pointers through the list and, at the same time, use the file pointers to output the line in question via appendFile. This way, every line consumed is immediately dispatched to its corresponding output file and things should only require memory for the different dates, besides buffering.
In a setting without IO, the task corresponds to the "Optimization Problem" discussed at length in September on this list. The problem here is that writeFile currently cannot be interleaved lazily, this has to be simulated with appendFile. We can read files lazily but we cannot output them lazily. Can this be remedied? Can there be a version of writeFile which is, in a sense, dual to getContents?
this can be solved in other way. here is a program that reads stdin and puts to stdout lines starting with '>' and to stderr the rest. note that our main processing function is pure: main = do a <- getContents let b = map (processing stdout stderr) (lines a) mapM_ (\(file,line) -> hPutStrLn file line) b processing file1 file2 line = if ">" `isPrefixOf` line then (file1,line) else (file2,line) -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com