操作缓存文件,熟悉File操作

-- file: ch07/tempfile.hsimport System.IOimport System.Directory(getTemporaryDirectory, removeFile)import Control.Exception(finally)main :: IO ()main = withTempFile "mytemp.txt" myAction{- The guts of the program.  Called with the path and handle of a temporaryfile.  When this function exits, that file will be closed and deletedbecause myAction was called from withTempFile. -}myAction :: FilePath -> Handle -> IO ()myAction tempname temph =    do -- Start by displaying a greeting on the terminal        putStrLn "Welcome to tempfile.hs"        putStrLn $ "I have a temporary file at " ++ tempname        -- Let's see what the initial position is        pos <- hTell temph        putStrLn $ "My initial position is " ++ show pos        -- Now, write some data to the temporary file        let tempdata = show [1..10]        putStrLn $ "Writing one line containing " ++            show (length tempdata) ++ " bytes: " ++               tempdata        hPutStrLn temph tempdata        -- Get our new position.  This doesn't actually modify pos        -- in memory, but makes the name "pos" correspond to a different        -- value for the remainder of the "do" block.        pos <- hTell temph        putStrLn $ "After writing, my new position is " ++ show pos        -- Seek to the beginning of the file and display it        putStrLn $ "The file content is: "        hSeek temph AbsoluteSeek 0        -- hGetContents performs a lazy read of the entire file        c <- hGetContents temph        -- Copy the file byte-for-byte to stdout, followed by \n        putStrLn c        -- Let's also display it as a Haskell literal        putStrLn $ "Which could be expressed as this Haskell literal:"        print c{- This function takes two parameters: a filename pattern and anotherfunction.  It will create a temporary file, and pass the name and Handleof that file to the given function.The temporary file is created with openTempFile.  The directory is the oneindicated by getTemporaryDirectory, or, if the system has no notion ofa temporary directory, "." is used.  The given pattern is passed toopenTempFile.After the given function terminates, even if it terminates due to anexception, the Handle is closed and the file is deleted. -}withTempFile :: String -> (FilePath -> Handle -> IO a) -> IO awithTempFile pattern func =    do -- The library ref says that getTemporaryDirectory may raise on       -- exception on systems that have no notion of a temporary directory.       -- So, we run getTemporaryDirectory under catch.  catch takes       -- two functions: one to run, and a different one to run if the       -- first raised an exception.  If getTemporaryDirectory raised an       -- exception, just use "." (the current working directory).       tempdir <- getTemporaryDirectory       (tempfile, temph) <- openTempFile tempdir pattern       -- Call (func tempfile temph) to perform the action on the temporary       -- file.  finally takes two actions.  The first is the action to run.       -- The second is an action to run after the first, regardless of       -- whether the first action raised an exception.  This way, we ensure       -- the temporary file is always deleted.  The return value from finally       -- is the first action's return value.       finally (func tempfile temph)               (do hClose temph                   removeFile tempfile)

上面的代码还是好理解的,不过看和写是两码事(以此告诫要多动手),函数式处理File文件稍微不一样的如上面openTempleFile返回Handle,Handle可以不立即处理,这是常见的IO处理,haskell还有更好的惰性IO