http - 在 Haskell 中学习 Conduits 时与 MonadBaseControl 错误混淆

Question

我正在尝试学习 Haskell 中的 Conduits。这听起来很有趣，但类型很快就会变得非常困惑。

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，它会转到雅虎财经并下载相关的数据文件。听起来很简单。

getLink 和 extractLink 效果很好。那里没有问题。

downloadContent 给我各种编译问题。此源代码:

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.

这里有什么洞察力可以让我实现代码的意图？

Answer 1

您必须将您的操作提升到管道的底层 monad:

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