使用StateT循环:为什么此循环不循环

Question

我不明白为什么这段代码只循环一次然后退出？在Ghci我只能回答第一个循环,然后似乎变量cont设置为false,我没有提示回答.

结果是:

*Main> testLoop1 td10
test
Do you want to continue? (y/N)
y
we continue
test
Do you want to continue? (y/N)
We stop

---

码:

type TDeckSTIO    = StateT TableDecks IO

continue = do
putStrLn "Do you want to continue? (y/N)"
c <- getChar
return $ c == 'y'


loop1 :: TDeckSTIO () 
loop1 = do 
    liftIO $ putStrLn "test"
    cont<- liftIO continue 
    if cont
    then do 
        liftIO $ putStrLn "we continue"
        liftIO $ testLoop1 td

    else liftIO $ putStrLn "We stop"

testLoop1 td =  runStateT (loop1 ) td   >> return ()

Answer 1

问题是当你输入y并按Enter键时,实际上输入了两个字符:'y'本身,以及通过按返回键发送的换行符.第一次循环,循环看到'y',但下一次,它看到'\n',并且'\n'从不'y',它退出.

您可以hSetBuffering stdin NoBuffering在进入循环之前执行(您需要导入System.IO),这样您可以在不等待换行符的情况下处理字符,或者一次专门处理行:

continue = do
  putStrLn "Do you want to continue? (y/N)"
  s <- getLine
  return $ s == "y"

顺便说一下,liftIO $ testLoop1 td你可以保持相同的状态monad 而不是写作:你可以替换它,loop1它将完全相同.

另外,testLoop1写得更好:

testLoop1 = evalStateT loop1

evalStateT就像runStateT,但不包括最终状态,因此您不必明确地丢弃该值>> return ().