todo.hs

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

main = do
    (command:args) <- getArgs
    case command of
        "view" -> view args
        "add" -> add args
        "remove" -> remove args
    
readEntries :: String -> IO [String]
readEntries fileName = do
    contents <- readFile fileName
    return $ lines contents
    
writeEntries :: String -> [String] -> IO ()
writeEntries fileName entries = writeFile fileName $ unlines entries
    
view :: [String] -> IO ()
view [fileName] = do
    entries <- readEntries fileName
    putStr $ unlines $ zipWith (\n entry -> (show n) ++ " - " ++ entry) [0..] entries

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

remove :: [String] -> IO ()
remove [fileName, index] = do
    entries <- readEntries fileName
    writeEntries fileName $ delete (entries!!(read index :: Int)) entries