deletetodo.hs

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

main = do
  contents <- readFile "todo.txt"
  let todoTasks = lines contents
      numberedTasks = zipWith (\n line -> show n ++ " - " ++ line) [0..] todoTasks

  putStrLn "There are your TO-DO items:"
  mapM_ putStrLn numberedTasks
  putStrLn "Which one do you want to delete?"
  numberString <- getLine
  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")

すごいHaskellたのしく学ぼう！ 『第9章 もっと入力、もっと出力』より

$ cat todo.txt
Iron the dishes
Dust the dog
Append TODO !!
$ ./deletetodo
There are your TO-DO items:
0 - Iron the dishes
1 - Dust the dog
2 - Append TODO !!
Which one do you want to delete?
2
$ cat todo.txt
Iron the dishes
Dust the dog

入出力が出始めた途端に手続き型っぽくなってきた