Haskell 命令行参数

import System.Environment
import System.Directory
import System.IO
import Data.List

main = do
  (command:args) <- getArgs                     -- get the command
  let (Just action) = lookup command dispatch   -- find the command
  action args                                   -- do something

add :: [String] -> IO ()
add [fileName, todoItem] = appendFile fileName (todoItem ++ "\n") -- add the todoItem

view :: [String] -> IO ()
view [fileName]     =  do                                              -- match the filename
  contents          <- readFile fileName                               -- read the file
  let todoTasks     =  lines contents                                  -- get the contents
      numberedTasks =  zipWith (\n line -> show n ++ ": " ++ line) [0..] todoTasks
  putStrLn $ unlines numberedTasks                                     -- show the items

remove :: [String] -> IO ()
remove [fileName, numberString] = do                              -- get the filename and the line number
  handle <- openFile fileName ReadMode
  (tempName, tempHandle) <- openTempFile "." "temp"               -- create a temp file
  contents <- hGetContents handle                                 -- get contents from the handle
  let number = read numberString                                  -- read the String to Number
      todoTasks = lines contents
      newTodoTasks = delete (todoTasks !! number) todoTasks       -- remove the line of the number
  hPutStr tempHandle $ unlines newTodoTasks                       -- putStr into the tempfile
  hClose handle                                                   -- close handles
  hClose tempHandle
  removeFile fileName
  renameFile tempName fileName

dispatch :: [(String, [String] -> IO ())]
dispatch = [
    ("add", add),
    ("view", view),
    ("remove", remove)
  ]

-- runhaskell arags-test.hs view newData.txt
-- runhaskell args-test.hs add newData.txt "Say Hello"
-- runhaskell args-test.hs remove newData.txt 4