todo.hs

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

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

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

main = do
    (command:argList) <- getArgs
    dispatch command argList

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

view :: [String] -> IO ()
view [fileName] = do
    contents <- readFile fileName
    let todoTasks = lines contents
        numberedTasks = zipWith (\n line -> show n ++ " - " ++ line) [0..] todoTasks
    putStr $ unlines numberedTasks

remove :: [String] -> IO ()
remove [fileName, numberString] = do
    contents <- readFile fileName
    let todoTasks = lines contents
        numberedTasks = zipWith (\n line -> show n ++ " - " ++ line) [0..] todoTasks

    putStrLn "These are your TODO items:"
    mapM_ putStrLn numberedTasks
    putStrLn "Which one do you want to delete?"
    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 "todo.txt"
        renameFile tempName "todo.txt")

$ ./todo view todo.txt
0 - Iron the dishes
1 - Dust the dog
$ ./todo add todo.txt "Pick up children from dry cleaners"
$ ./todo view todo.txt
0 - Iron the dishes
1 - Dust the dog
2 - Pick up children from dry cleaners
$ ./todo remove todo.txt 2
These are your TODO items:
0 - Iron the dishes
1 - Dust the dog
2 - Pick up children from dry cleaners
Which one do you want to delete?
$ ./todo view todo.txt
0 - Iron the dishes
1 - Dust the dog
$ ./todo viewfoobar todo.txt
The viewfoobar command doesn't exist

doesntExist 関数は少なくとも main の下に書いたほうが良かった