此代码无法编译并出现此错误

src/JsonParser.hs:33:37-62: error:
    * Couldn't match type `b1' with `b'
      `b1' is a rigid type variable bound by
        the type signature for:
          resultantRunParse :: forall b1. String -> Maybe (String, b1)
        at src/JsonParser.hs:29:5-52
      `b' is a rigid type variable bound by
        the type signature for:
          fmap :: forall a b. (a -> b) -> Parser a -> Parser b
        at src/JsonParser.hs:27:11-42
      Expected type: Maybe (String, b1)
        Actual type: Maybe (String, b)

编码：

import Prelude

newtype Parser a = Parser
  { runParser :: String -> Maybe (String, a)
  }

instance Functor Parser where
  fmap :: (a -> b) -> Parser a -> Parser b
  fmap f (Parser p) = Parser resultantRunParse where
    -- THIS FUNCTION TYPE DECLARATION IS THE PROBLEM
    -- IF I REMOVE THE TYPE DECLARATION, IT WORKS
    resultantRunParse :: String -> Maybe (String, b)
    resultantRunParse input = 
      case p input of
        Nothing -> Nothing
        Just (remaining, parsed) -> Just (remaining, f parsed)

我尝试了resultantRunParse :: forall b1. String -> Maybe (String, b1)错误消息中建议的签名，但也没有用。

此外，制作resultantRunParse = undefined和保持类型签名也有效。很奇怪！

这个函数的正确签名是什么？

回答

{-# LANGUAGE ScopedTypeVariables #-}