学步园

Haskell的list类型应用很广泛，但不是做所有事情都适合用。List类型从头部取一个元素，或从头部插入一个元素的时间复杂度是O(1)，如果随机访问任意位置的元素，则时间复杂度为O(n)。List类型如果求它的长度用length函数，则时间复杂度为O(n)，所以如果判断一个List对象是否为空，最好用null函数，而不是判断length等于0。

Haskell在读写文件的时候，尤其大的文件的时候，最好不采用List数据类型。Haskell提供的Data.ByteString类型，可以用来比较有效率地读写大文件。

下面是一个应用Data.ByteString来分割合并文件的例子：

 import qualified Data.ByteString.Lazy as BS<br /> import System.Environment (getArgs, getProgName )<br /> import System.IO (IOMode(ReadMode), hClose, openFile, hFileSize, hGetLine,readFile,writeFile,appendFile, Handle(..) )<br /> import Data.String (fromString)<br /> import Data.Int<br /> import Control.Exception (handle)</p> <p>main = do<br /> progName <- getProgName<br /> args <- getArgs<br /> case args of<br /> -- two args, divide file<br /> ([_,_]) -> do<br /> let divideFileName = args !! 0<br /> let numberOfPart = read (args !! 1 ) :: Integer<br /> divideFile divideFileName numberOfPart<br /> -- three args, merge file<br /> ([_,_,_]) -> do<br /> let option = args !! 0<br /> if option == "-m"<br /> then do<br /> let partFileName = args !! 1<br /> let outFileName = args !! 2<br /> mergeFile partFileName outFileName<br /> else<br /> printUsage progName<br /> -- otherwise print usage<br /> (_) -><br /> printUsage progName</p> <p>{-<br /> partFileName is gemerated when dividing file;<br /> outFileName is the fileName for merged file<br /> -}<br /> mergeFile partFileName outFileName =<br /> do names<-partFileNames partFileName<br /> mergeFile' names outFileName</p> <p>{-<br /> fileNames is get from partFile which is generated when dividing file<br /> -}<br /> mergeFile' fileNames outFileName =<br /> case fileNames of<br /> (fileName:fs) -> do content <- BS.readFile fileName<br /> BS.appendFile outFileName content<br /> mergeFile' fs outFileName</p> <p> _ -> return ()</p> <p>partFileNames :: FilePath ->IO [FilePath]<br /> partFileNames partFileName = do<br /> fileHandle <- openFile partFileName ReadMode<br /> result <- parseFileName fileHandle<br /> hClose fileHandle<br /> return result</p> <p>parseFileName::Handle->IO [FilePath]<br /> parseFileName fileHandle =<br /> handle ( eofEncuntered ) $<br /> do line<-hGetLine fileHandle<br /> let fileName = line2FileName line<br /> fileNames<-parseFileName fileHandle<br /> return (fileName:fileNames)</p> <p>{-<br /> the second word in the line is file names<br /> -}<br /> line2FileName::String->String<br /> line2FileName s = words s !! 1</p> <p>eofEncuntered::IOError ->IO [FilePath]<br /> eofEncuntered e = return []</p> <p>printUsage progName = do<br /> mapM_ putStrLn lines<br /> where<br /> lines = [progName ++ " <divide-file-name> <number-of-parts> ",<br /> progName ++ " -m <parts-file-name> <out-file-name> "]</p> <p>filePartList :: Integer -> Integer -> [Integer]<br /> filePartList fileTotalSize numberOfPart =<br /> let eachSize = fileTotalSize `div` numberOfPart<br /> remain = fileTotalSize `mod` numberOfPart<br /> in if remain > 0 then<br /> filePartList' fileTotalSize (eachSize+1)<br /> else<br /> filePartList' fileTotalSize eachSize<br /> where filePartList' fileTotalSize eachSize<br /> | fileTotalSize <= eachSize = [fileTotalSize]<br /> | otherwise = eachSize : (filePartList' (fileTotalSize - eachSize) eachSize)</p> <p>divideFile :: FilePath -> Integer -> IO ()<br /> divideFile divideFileName numberOfPart = do<br /> fileTotalSize <- fileSize divideFileName<br /> content<-BS.readFile divideFileName<br /> let fileSizeList = filePartList fileTotalSize numberOfPart<br /> let fileMapList = zip [0..] fileSizeList<br /> saveIndividualFile divideFileName fileMapList content<br /> saveDivideInfo divideFileName fileMapList</p> <p>saveDivideInfo :: FilePath -> [(Integer,Integer)] -> IO ()<br /> saveDivideInfo fileName fileMapList = writeFile (fileName++".parts") (strLst2Str (divideInfo fileMapList))<br /> where divideInfo fileMapList=case fileMapList of<br /> ((fileIndex,fileSize):fm) -> ((show fileIndex) ++" "++ fileName ++ (show fileIndex) ++ " "++(show fileSize) ): (divideInfo fm)<br /> [] -> []</p> <p>strLst2Str :: [String] -> String<br /> strLst2Str (x:xs) = x++('/n':(strLst2Str xs))<br /> strLst2Str [] = ""</p> <p>saveIndividualFile :: FilePath -> [(Integer,Integer)] ->BS.ByteString->IO ()<br /> saveIndividualFile fileName fileMapList content =<br /> case fileMapList of<br /> ((fileIndex,fileSize):fm) -> do<br /> let prefix = BS.take (read(show(fileSize))::Int64) content<br /> let postfix = BS.drop (read(show(fileSize))::Int64) content<br /> BS.writeFile (fileName ++ (show fileIndex)) prefix<br /> saveIndividualFile fileName fm postfix<br /> [] -> return ()</p> <p>fileSize :: FilePath -> IO Integer<br /> fileSize fileName = do<br /> fileHandle <- openFile fileName ReadMode<br /> size <- hFileSize fileHandle<br /> hClose fileHandle<br /> return size