Refined todo.hs in Chap 9 of "Learn You a Haskell for Great Good!": http://learnyouahaskell.com/

todo.hs

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

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

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

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

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
         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
       let number = read numberString
--           newTodoItems = unlines $ delete (todoTasks !! number) todoTasks
           removedTask = todoTasks !! number
           (h,t) = splitAt number todoTasks
           newTodoItems = unlines $ h++tail t
       updateTodo fileName newTodoItems

bump :: [String]->IO ()
bump [fileName, numberString] = do
     contents <- readFile fileName
     let todoTasks = lines contents
         number = read numberString
--         newTodoItems = unlines $ (todoTasks !! number):(delete (todoTasks !! number) todoTasks)
         popTask = todoTasks !! number
         (h,t) = splitAt number todoTasks
         newTodoItems = unlines $ popTask:h++tail t
     updateTodo fileName newTodoItems

updateTodo :: String->String->IO ()
updateTodo fileName todoItems = do
           bracketOnError (openTempFile "." "_temp")
                          (\(tempName, tempHandle)->do
                                 hClose tempHandle
                                 removeFile tempName)
                          (\(tempName, tempHandle)->do
                                 hPutStr tempHandle todoItems
                                 hClose tempHandle
--                               removeFile "todo.txt"
--                               renameFile tempName "todo.txt"
                                 removeFile fileName
                                 renameFile tempName fileName)

• Fixed to handle duplicate elements correctly in function "remove"
• Added bump function