Question

这个小函数检查（有限的）Brainfuck字符串的有效性。它检查[和]是否平衡。代码非常简单，并且编写为尾递归：

-- checks Brainfuck for validity.
validateBrainfuck :: Monad m => String -> m String
validateBrainfuck s = maybe (return s) (fail . fromJust) (validate s 0) where
  validate :: String -> Int -> Maybe String -- Here inversed: String means error
  validate (']':_ ) 0 = Just "Too many closing brackets"
  validate (']':xs) c = validate xs (pred c)
  validate ('[':xs) c = validate xs (succ c)
  validate ( x :xs) c = validate xs       c
  validate []       0 = Nothing
  validate []       _ = Just "Too many opening brackets"

现在，GHC抱怨打字问题：

Brainfuck.hs:62:58:
    Couldn't match expected type `Maybe String'
           against inferred type `[Char]'
      Expected type: Maybe (Maybe String)
      Inferred type: Maybe String
    In the third argument of `maybe', namely `(validate s 0)'
    In the expression:
        maybe (return s) (fail . fromJust) (validate s 0)

也许我太傻了，无法弄清楚出了什么问题，但对我来说这看起来很奇怪。

Answer 1

查看maybe的类型，并考虑它应该做什么：

maybe :: b -> (a -> b) -> Maybe a -> b

如果maybe值不包含结果（即Nothing），maybe将返回b参数。

否则 - 当给出Just a时 - 它将给定的函数应用于有效结果。我们这里不需要fromJust提取。

您的代码变为

maybe (return s) fail (validate s 0)

为什么GHC抱怨错误的类型？

1 个答案: