[Haskell] Re: Probably a trivial thing for people knowing Haskell

[apfelmus]

Friedrich wrote:
> Paul Johnson writes:
>>
>> -- Concatenate all the files into one big string.  File reading is
>> lazy, so this won't take all the memory.
>> getAllFiles :: [String] -> IO String
>> getAllFiles paths = do
>>   contents <- mapM getFile paths
>>   return $ concat contents
>>
>> Then use "lines" to split the result into individual lines and process
>> them using "filter", "map" and "foldr".  Because file reading is lazy,
>> each line is only read when it is to be processed, and then gets
>> reaped by the garbage collector.  So it all runs in constant memory.
>
> Would you mind to elaborate a bit about it. What's so terrible to open
> one file after the other, reading it line by line and close the file
> thereafter.

It's not beautiful.

Here's a more idiomatic version

    {-# LANGUAGE BangPatterns #-}
    module Main where

    import Control.Monad
    import System.Directory
    import Text.Regex
    import Data.List
    import Data.Maybe

    main = do
        files <- filter_reg "[0-9].*" `liftM` getDirectoryContents "."
        (sum,count) <- sumcount `liftM` mapM run_file files
        let dd = fromIntegral sum / fromIntegral count
        putStrLn $ "Download = " ++ show sum
                ++ " in " ++ show count
                ++ " days are " ++ show dd ++ " downloads/day"

    sumcount :: [(Integer,Int)] -> (Integer,Int)
    sumcount = foldl' (\(!s,!c) (ds,dc) -> (s+ds,c+dc)) (0,0)

    run_file name =
        (sumcount . map check_line . lines) `liftM` readFile' name

    readFile' name = unsafeInterleaveIO $
        openFile name ReadMode >>= hGetContents

    regexp = mkRegex "([0-9]+) Windows ex"
    check_line line = case matchRegex regexp line of
        Just (s:_) -> (read s,1)
        Nothing    -> (0,0)

    filter_reg pat = let reg = mkRegex pat in
        filter $ isJust . matchRegex reg


It's much shorter and should run in constant memory as well.



Regards,
apfelmus