haskell - 重复追加到字符串haskell

Question

嗨，我是Haskell的新手，不确定如何处理它。到目前为止，我有以下代码

ask :: String -> IO ()
ask prompt =
  do
  putStrLn prompt
  line <- getLine
  if line == ""
    then ask "please try again"
    else putStrLn ("you said: " ++ reverse line)

main :: IO ()
main =
  do
  ask "please say something"

我正在尝试做到这一点，以便当有人按Enter键而不输入任何内容时，重复的提示将有一个额外的“!”在最后。因此，每次重复时，都会添加一个额外的“!”

示例输出:
请说些什么

请再试一次!

请再试一次!!

请再试一次!!!

Answer 1

您可以在方法中添加一个计数器，例如:

ask :: String -> IO ()
ask = ask' 1

ask' :: Int -> String -> IO ()
ask' n prompt = do
  putStrLn prompt
  line <- getLine
  if null line
    then ask' (n+1) ("please try again" ++ replicate n '!')
    else putStrLn ("you said: " ++ reverse line)