Haskell/資料庫/解決方案/IO

練習

編寫一下我們的程式的變體，使其首先詢問使用者是否希望從檔案讀取、寫入檔案或退出。如果使用者用“退出”做出回應，該程式應退出。如果他們用“讀取”做出回應，該程式應要求他們提供一個檔名，然後將該檔案列印在螢幕上（如果檔案不存在，該程式可能會崩潰）。如果他們用“寫入”做出回應，它應要求他們提供一個檔名，然後要求他們輸入要寫在檔案中的文字，用“。”表示已完成。除了“.”，所有內容都應被寫入檔案。 例如，執行此程式可能會生成 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)