Haskell中Hangman的递归错误

Question

我的haskell代码有点问题：

import Prelude
hangman :: String -> IO ()
hangman [] = return ()
hangman s = do putStr ("Secret: " ++ text s ++ "n")
putStr "Enter a character: "
charr <- getChar 
function s charr (text s)
function :: String -> Char -> String -> IO ()
function [] _ _          = return ()
function (a:bc) charr (g:jt) = do putStr ("Secret: " ++ (textBack (a:bc) charr (g:jt)) ++ "n")
putStr "Enter a character: "
charbaby <- getChar
putStr "n" 
function (a:bc) charbaby (textBack (a:bc) charbaby (g:jt))

text :: String -> String
text []     = ""
text (x:ys) = "*" ++ text ys
textBack :: String -> Char -> String -> String
textBack [] _ _              = ""
textBack (a:bc) charr (g:jt) = if charr == a
then charr : textBack bc charr jt 
else g : textBack bc charr jt

这是输出：

*Main> hangman "hello"
Secret: *****
Enter a character: h
Secret: h****
Enter a character:
Secret: h****
Enter a character: e
Secret: he***
Enter a character:
Secret: he***
Enter a character: l
Secret: hell*
Enter a character:
Secret: hell*
Enter a character: o
Secret: hello
Enter a character:
Secret: hello
Enter a character:

正如你所看到的，有些线路被多次调用，我不知道为什么。我希望你能帮助我：(

谢谢你，

Finn

Answer 1

我认为有两件事让你自己变得困难：

1. 您在hangman和function中复制了逻辑。这使得事情很难保持同步；以及
2. 您使用了不必要的模式匹配，使功能不那么优雅

对于真正的问题，我认为这是由于使用了getChar：getChar从stdin中获取一个字符，如果您编写例如H和

ENTER因此，您最好使用getLine，然后只使用第一个字符，或者在用户写入多个字符时要求再次输入。例如：

readGuess :: String -> Maybe Char
readGuess [x] = Just x
readGuess _ = Nothing
getGuess :: IO Char
getGuess = do
putStrLn "Enter a character:"
cs <- readGuess <$> getLine
case cs of
Just c-> pure c
Nothing -> putStrLn "Invalid guess" >> getGuess

因此，这将检索一个猜测：

Prelude> getGuess 
Enter a character:
Invalid guess
Enter a character:
hello
Invalid guess
Enter a character:
foo
Invalid guess
Enter a character:
b
'b'

您可以将hangman简化为：

hangman :: String -> IO ()
hangman s = hangman' s (text s)
hangman' :: String -> String -> IO ()
hangman' sa sb
| sa == sb = putStrLn "You won"
| otherwise = do
putStr ("Secret: " ++ sb ++ "n")
g <- getGuess
hangman' sa (textBack sa g sb)