Haskell/资料库/解决方案/IO

练习

编写一下我们的程序的变体，使其首先询问用户是否希望从文件读取、写入文件或退出。如果用户用“退出”做出回应，该程序应退出。如果他们用“读取”做出回应，该程序应要求他们提供一个文件名，然后将该文件打印在屏幕上（如果文件不存在，该程序可能会崩溃）。如果他们用“写入”做出回应，它应要求他们提供一个文件名，然后要求他们输入要写在文件中的文本，用“。”表示已完成。除了“.”，所有内容都应被写入文件。 例如，运行此程序可能会生成 Do you want to [read] a file, [write] a file, or [quit]? read Enter a file name to read: foo ...contents of foo... Do you want to [read] a file, [write] a file, or [quit]? write Enter a file name to write: foo Enter text (dot on a line by itself to end): this is some text for foo . Do you want to [read] a file, [write] a file, or [quit]? read Enter a file name to read: foo this is some text for foo Do you want to [read] a file, [write] a file, or [quit]? blech I don't understand the command blech. Do you want to [read] a file, [write] a file, or [quit]? quit Goodbye!

module Main
    where

import System.IO
import Control.Exception

main = doLoop

doLoop = do
    putStrLn "Do you want to [read] a file, [write] a file, or [quit]?"
    command <- getLine
    if command == "quit"
        then return()
    else if command == "read" || command == "write"
        then do putStrLn ("Enter a file name to " ++ command)
                filename <- getLine
                if command == "read"
                    then do doRead filename
                            doLoop
                    else do doWrite filename
                            doLoop
        else doLoop

doRead filename = do
    bracket (openFile filename ReadMode) hClose
            (\h -> do contents <- hGetContents h
                      putStrLn contents)

doWriteChars handle = do
    char <- getChar
    if not (char == '.')
        then do hPutChar handle char
                doWriteChars handle
        else return ()

doWrite filename = do
    bracket (openFile filename WriteMode) hClose
            (\h -> doWriteChars h)