Haskell : todo

make.cmd

 ghc --make todo.hs

todo.hs

{-
This is referenced from "Learn You a Haskell for Great Good! "
-}
{- example
> ./todo add todo.txt "buy mild."
> ./todo view todo.txt
> ./todo remove todo.txt 2
-}
{-
>TODO
Test filename that is a argument if it is existed or not. 
-}

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

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

main = do
     args <- getArgs
     let command = if null args then [] else head args
     	 argList = if null args then [] else tail args
     dispatch command argList


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

-- add something to todolist.
add :: [String] -> IO ()
add [filename , todoItem ] =  appendFile filename ( todoItem ++ "\n" )
add _                      =  putStrLn "The add command takes exactly two arguments."

-- view todolist
view :: [String] -> IO ()
view [filename] = do
     contents <- readFile filename
     let todoItems = lines contents
         outputs   = zipWith makeStr [0..] todoItems
         	     where makeStr = (\n item -> show n ++ " - " ++ item )

     putStrLn $ unlines outputs
view _ = putStrLn "The view command takes exactly one arguments."

remove :: [String] -> IO ()
remove [filename , numberString ] = do
       contents <- readFile filename
       let todoTasks = lines contents
       	   index     = read numberString
	   newTasks  = unlines $ delete ( todoTasks !! index ) todoTasks

       bracketOnError (openTempFile "." "temp")
       		      (\(path,h) -> do
		      	      	 hClose h
				 removeFile path) 
       		      (\(path,h) -> do
		      	      	 hPutStr h newTasks
				 hClose h
				 removeFile filename
				 renameFile path filename )
remove _ = putStrLn "The remove command takes exactly two arguments."


help :: [String] -> IO ()
help _ = do
     putStrLn "Suppoted command is ..."
     putStrLn "view <filename>"
     putStrLn "add <filename> <task>"
     putStrLn "remove <filename> <number>"