Haskell todo-app

todo.hs

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

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

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

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


view :: [String] -> IO ()
view [filename] = do
  contents <- readFile filename

  let todoTasks = lines contents
      numberedTasks = zipWith (\num task -> show num ++ " - " ++ task) [0..] todoTasks

  putStrLn $ unlines numberedTasks

remove :: [String] -> IO ()
remove [filename, num] = do
  handle <- openFile filename ReadMode
  (tempName, tempHandle) <- openTempFile "." "temp"
  contents <- hGetContents handle
  let number = read num
      todoTasks = lines contents
      newTodoItems = delete (todoTasks !! number) todoTasks

  hPutStr tempHandle $ unlines newTodoItems
  hClose handle
  hClose tempHandle
  removeFile filename
  renameFile tempName filename

bump :: [String] -> IO ()
bump [filename, num] = do
  handle <- openFile filename ReadMode
  (tempName, tempHandle) <- openTempFile "." "temp"
  contents <- hGetContents handle

  let number = read num
      todoTasks = lines contents
      task = todoTasks !! number
      newTodoItems = task : delete task todoTasks

  hPutStr tempHandle $ unlines newTodoItems
  hClose handle
  hClose tempHandle
  removeFile filename
  renameFile tempName filename