sugoiH9.5.hs

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

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

main = do
  args <- getArgs
  case args of 
    [] -> putStrLn "todo needs one of add, view, remove command"
    (command:argList) -> dispatch command argList

add :: [String] -> IO ()
add [fileName,todoItem] = appendFile fileName (todoItem ++ "\n")
add _ = putStrLn "The add command takes exactly two arguments"

view :: [String] -> IO ()
view [fileName] = do
  contents <- readFile fileName
  let todoTasks = lines contents
      numberdTasks = zipWith (\n line -> show n ++ " - " ++ line) [0..] todoTasks
  putStr $ unlines numberdTasks
view _ = putStrLn "The view command takes exactly one argument"

remove :: [String] -> IO ()
remove [fileName, numberString] = do
  contents <- readFile fileName
  let todoTasks = lines contents
      numberdTasks = zipWith (\n line -> show n ++ " - " ++ line) [0..] todoTasks
  let number = read numberString
      newTodoItems = unlines $ delete (todoTasks !! number) todoTasks
  bracketOnError (openTempFile "." "temp")
    (\(tempName,tempHandle) -> do
      hClose tempHandle
      removeFile tempName)
    (\(tempName,tempHandle) -> do
      hPutStr tempHandle newTodoItems
      hClose tempHandle
      removeFile fileName
      renameFile tempName fileName)
remove _ = putStrLn "The remove command takes exactly two arguments"

doesntExist :: String -> [String] -> IO ()
doesntExist command _ =
  putStrLn $ "The " ++ command ++ " command doesn't exist"