3

我正在尝试去了解 Haskell 中的导管。这听起来很有趣，但是类型很快变得非常混乱。

到目前为止，我已经编写了以下文件：https ://gist.github.com/anonymous/7991727

据推测，它会做的是，给定一系列股票代码，它会去雅虎财经并下载相关的数据文件。听起来很简单。

getLink效果extractLink很好。那里没有问题。

downloadContent给我各种编译问题。这个源代码：

downloadContent manager = do
  mx <- await
  case mx of
    Nothing               -> return ()
    Just (sym, Nothing)   -> return ()
    Just (sym, Just link) -> do
      req <- parseUrl link
      res <- http req manager
      responseBody res $$+- sinkFile sym

我得到的确切错误是：

src/suckyahoo.hs:58:21:
    No instance for (MonadBaseControl
                       IO
                       (ConduitM
                          [Char] ([Char], Maybe String) (ResourceT (ResourceT IO))))
      arising from a use of `getLink'
    Possible fix:
      add an instance declaration for
      (MonadBaseControl
         IO
         (ConduitM
            [Char] ([Char], Maybe String) (ResourceT (ResourceT IO))))
    In the second argument of `($=)', namely `getLink manager'
    In the first argument of `($$)', namely
      `sourceSyms $= getLink manager'
    In a stmt of a 'do' block:
      sourceSyms $= getLink manager $$ downloadContent manager

src/suckyahoo.hs:58:40:
    No instance for (MonadBaseControl
                       IO
                       (ConduitM
                          (FilePath, Maybe String)
                          void-0.6.1:Data.Void.Void
                          (ResourceT (ResourceT IO))))
      arising from a use of `downloadContent'
    Possible fix:
      add an instance declaration for
      (MonadBaseControl
         IO
         (ConduitM
            (FilePath, Maybe String)
            void-0.6.1:Data.Void.Void
            (ResourceT (ResourceT IO))))
    In the second argument of `($$)', namely `downloadContent manager'
    In a stmt of a 'do' block:
      sourceSyms $= getLink manager $$ downloadContent manager
    In the expression:
      do { sourceSyms $= getLink manager $$ downloadContent manager }
Failed, modules loaded: none.

这里有什么见解可以让我完成代码的意图？

4

1 回答 1

1

你将不得不将你的行为提升到你的管道的底层单子：

downloadContent manager = do
  mx <- await
  case mx of
    Nothing               -> return ()
    Just (sym, Nothing)   -> return ()
    Just (sym, Just link) -> lift $ do
      req <- parseUrl link
      res <- http req manager
      responseBody res $$+- sinkFile sym
于 2013-12-16T19:32:02.790 回答