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haskell - Haskell - 尝试 Monad 实例 - 语法问题

问题描述

我有以下定义：

newtype Parser a = Parser { parse :: String -> [(a,String)] }


instance Functor Parser where
 fmap g (Parser pa) = Parser { parse = \s -> [(g a,s') | (a,s') <- pa s] }


instance Applicative Parser where
 pure x = Parser { parse = \s -> [(x,s)] }
 (<*>) (Parser pg) (Parser pa) = Parser { parse = \s -> [(g a,s'') | (g,s') <- pg s, (a, s'') <- pa s'] }



instance Monad Parser where
 return = pure
 (>>=) (Parser pa) g = Parser { parse = \s -> [(b,s'') | [(a, s')] <- (pa s), [(b,s'')] <- (g a) s'] }

问题在于绑定运算符的实现。我不确定为什么会出现类型错误。我试图在与 (<*>) 相同的行上定义绑定运算符

错误是：

parser.hs:17:58: error:
    • Couldn't match expected type ‘(a, String)’
                  with actual type ‘[(a, t0)]’
    • In the pattern: [(a, s')]
      In a stmt of a list comprehension: [(a, s')] <- (pa s)
      In the expression:
        [(b, s'') | [(a, s')] <- (pa s), [(b, s'')] <- (g a) s']
    • Relevant bindings include
        g :: a -> Parser b (bound at parser.hs:17:20)
        pa :: String -> [(a, String)] (bound at parser.hs:17:16)
        (>>=) :: Parser a -> (a -> Parser b) -> Parser b
          (bound at parser.hs:17:2)
   |
17 |  (>>=) (Parser pa) g = Parser { parse = \s -> [(b,s'') | [(a, s')] <- (pa s), [(b,s'')] <- (g a) s'] }
   |                                                          ^^^^^^^^^

parser.hs:17:92: error:
    • Couldn't match expected type ‘t0 -> [[(b, String)]]’
                  with actual type ‘Parser b’
    • The function ‘g’ is applied to two arguments,
      but its type ‘a -> Parser b’ has only one
      In the expression: (g a) s'
      In a stmt of a list comprehension: [(b, s'')] <- (g a) s'
    • Relevant bindings include
        s' :: t0 (bound at parser.hs:17:63)
        g :: a -> Parser b (bound at parser.hs:17:20)
        (>>=) :: Parser a -> (a -> Parser b) -> Parser b
          (bound at parser.hs:17:2)
   |
17 |  (>>=) (Parser pa) g = Parser { parse = \s -> [(b,s'') | [(a, s')] <- (pa s), [(b,s'')] <- (g a) s'] }

更新：为了完整起见，基于答案。这是有效的：

instance Monad Parser where
 return = pure
 (>>=) (Parser pa) g = Parser { parse = \s -> [(b,s'') | (a, s') <- (pa s), (b,s'') <- parse (g a) s'] }

标签： haskellmonads

解决方案

像这样的守卫模式[(a, s')] <- (pa s)意味着您期望pa s返回一个列表列表，其中每一对都恰好包含一对aand s，而实际上它只返回一个列表，其中包含任意数量的这些对。

基本上，放下括号（以及其他绑定），然后它应该可以工作。

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