-- Stolen from https://www.haskellforall.com/2015/10/basic-haskell-examples.html
putTodo :: (Int, String) -> IO ()
putTodo (n, todo) = putStrLn (show n ++ ": " ++ todo)

prompt :: [String] -> IO ()
prompt todos = do
    putStrLn ""
    putStrLn "Current TODO list:"
    mapM_ putTodo (zip [0..] todos)
    command <- getLine
    interpret command todos

interpret :: String -> [String] -> IO ()
interpret ('+':' ':todo) todos = prompt (todo:todos)
interpret ('-':' ':num ) todos =
    case delete (read num) todos of
        Nothing -> do
            putStrLn "No TODO entry matches the given number"
            prompt todos
        Just todos' -> prompt todos'
interpret  "q"           todos = return ()
interpret  command       todos = do
    putStrLn ("Invalid command: `" ++ command ++ "`")
    prompt todos

delete :: Int -> [a] -> Maybe [a]
delete 0 (_:as) = Just as
delete n (a:as) = do
    as' <- delete (n - 1) as
    return (a:as')
delete _  []    = Nothing

main = do
    putStrLn "Commands:"
    putStrLn "+ <String> - Add a TODO entry"
    putStrLn "- <Int>    - Delete the numbered entry"
    putStrLn "q          - Quit"
    prompt []

func main() {
    e := echo.New()
    e.Use(middleware.Logger())

    data := newData()
    e.Renderer = newTemplate()

    e.GET("/", func(c echo.Context) error {
        return c.Render(200, "index", data)
    })

    e.POST("/contacts", func(c echo.Context) error {
        name := c.FormValue("name")
        email := c.FormValue("email")
        data.Contacts = append(data.Contacts, newContact(name, email))
        return c.Render(200, "display", data)
    })

    e.Logger.Fatal(e.Start(":42069"))
}