无法在 Network.URI 上匹配类型 Maybe 与 not Maybe

Can't match type Maybe vs not Maybe on Network.URI

假设我想解析一个环境变量,如果没有,则默认为 localhost,使用 https://hackage.haskell.org/package/network-2.3/docs/Network-URI.html

我可以这样写一个函数:

parseRabbitURI :: Text -> Maybe URI.URI
parseRabbitURI "" = URI.parseURI "amqp://guest:guest@127.0.0.1/"
parseRabbitURI uri = (URI.parseURI . toS) uri

这很好用。现在假设我想处理错误。我注意到 parseURI returns a Maybe 所以表面上我只需要对其进行模式匹配。所以我创建了一个自定义错误:

data CustomError = MyCustomError Text deriving(Show)

我创建了一个辅助函数:

parsedExtractor
  :: MonadError CustomError.MyCustomError m
  => Text
  -> Maybe URI.URI
  -> m(URI.URI)
parsedExtractor originalString Nothing = throwError $ FlockErrors.FailedToParseURI originalString
parsedExtractor _ (Just uri) = do
  pure uri

最后,我修改我的初始函数:

parseRabbitURI :: MonadError CustomError.MyCustomError m => Text -> m(URI.URI)
parseRabbitURI "" = URI.parseURI "amqp://guest:guest@127.0.0.1/" >>= parsedExtractor "amqp://guest:guest@127.0.0.1/"
parseRabbitURI uri = (URI.parseURI . toS) uri >>= parsedExtractor uri

编译失败:

• Couldn't match type ‘URI.URI’ with ‘Maybe URI.URI’
  Expected type: URI.URI -> Maybe URI.URI
    Actual type: Maybe URI.URI -> Maybe URI.URI
• In the second argument of ‘(>>=)’, namely ‘parsedExtractor uri’
  In the expression: (URI.parseURI . toS) uri >>= parsedExtractor uri
  In an equation for ‘parseRabbitURI’:
      parseRabbitURI uri
        = (URI.parseURI . toS) uri >>= parsedExtractor uri

| 23 | parseRabbitURI uri = (URI.parseURI .toS) uri >>= parsedExtractor uri |

我这辈子都弄不明白为什么。如果初始实现 returns 是 Maybe,为什么它会转换为我无法通过的解包器 URI.URI?

至关重要的是,当我更改 parsedExtractor 上的模式以期待一个字符串时,它 无法使用反向消息 (

Couldn't match expected type ‘URI.URI’
                      with actual type ‘Maybe URI.URI’

我觉得我一定错过了一些完全基本的东西。这是怎么回事?

And for the life of me I can't figure out why. If the initial implementation returns a Maybe, why is it converting to an unwrapper URI.URI which I can't then pass?

Control.Monad引用>>=的定义,它有类型签名:

(>>=) :: m a -> (a -> m b) -> m b

现在,比较表达式:

(URI.parseURI . toS) uri >>= parsedExtractor uri

我们有:

m a        ~ (URI.parseURI . toS) uri
(a -> m b) ~ parsedExtractor uri

由于 (URI.parseURI . toS) uri return 键入 Maybe URI.URI 并且 MaybeMonad 的实例,所以

m a ~ Maybe URI.URI 

(a -> m b) ~ (URI.URI -> m b) 

m b 可以推断为 m (URI.URI),因此 >>= 之后的函数(即 parsedExtractor uri)预期的类型为:

(URI.URI -> m (URI.URI))

但实际并非如此。