Question

我正在尝试创建一个管道，它将通过管道源从HTTP流式传输数据。以下是我到目前为止的情况：

import qualified Network.HTTP.Client.Conduit as CC

getStream :: String -> IO (ConduitM () BS.ByteString IO ())
getStream url = do
  req <- parseUrl url
  return $  CC.withResponse req $ \res -> do
    responseBody res $= (awaitForever $ \bytes -> liftIO $ do
      putStrLn $ "Got " ++ show (BS.length bytes) ++ " but will ignore    them")

但我得到了

No instance for (Control.Monad.Reader.Class.MonadReader env0 IO) ‚Ä¶
      arising from a use of ‚ÄòCC.withResponse‚Äô
    In the expression: CC.withResponse req
    In the second argument of ‚Äò($)‚Äô, namely
      ‚ÄòCC.withResponse req
       $ \ res
           -> do { responseBody res $= (awaitForever $ \ bytes -> ...) }‚Äô
    In a stmt of a 'do' block:
      return
      $ CC.withResponse req
        $ \ res
            -> do { responseBody res $= (awaitForever $ \ bytes -> ...) }

为什么要MonadReader？这对我没有任何意义。

Answer 1

the example in the Network.HTTP.Conduit docs的这种变体如何：

{-# LANGUAGE OverloadedStrings #-}

module Lib2 () where

import Data.Conduit (($$+-), awaitForever)
import qualified Network.HTTP.Client.Conduit as CC
import Network.HTTP.Conduit (http, tlsManagerSettings, newManager)
import Control.Monad.IO.Class (liftIO)
import Control.Monad.Trans.Resource (runResourceT)
import Data.Conduit.Binary (sinkFile) -- Exported from the package conduit-extra

main2 :: IO ()
main2 = do
       request <- CC.parseUrl "http://google.com/"
       manager <- newManager tlsManagerSettings
       runResourceT $ do
           response <- http request manager
           CC.responseBody response $$+- (awaitForever $ \x -> liftIO $ putStrLn "Chunk")

原始回答

getStream的返回类型错误。尝试删除类型签名并使用FlexibleContexts，例如：

{-# LANGUAGE OverloadedStrings, FlexibleContexts #-}

module Lib () where

import Data.Conduit
import qualified Data.ByteString as BS
import qualified Network.HTTP.Client.Conduit as CC
import Control.Monad.IO.Class

getStream url = do
  req <- CC.parseUrl url
  CC.withResponse req $ \res -> do
   CC.responseBody res $= (awaitForever $ \x -> liftIO $ putStrLn "Got a chunk")

然后:t getStream报告：

getStream
  :: (monad-control-1.0.0.4:Control.Monad.Trans.Control.MonadBaseControl
        IO (ConduitM a c m),
      mtl-2.2.1:Control.Monad.Reader.Class.MonadReader env m, MonadIO m,
      CC.HasHttpManager env,
      exceptions-0.8.0.2:Control.Monad.Catch.MonadThrow m) =>
     String -> ConduitM a c m ()

表明返回类型的格式为ConduitM ...，而不是IO ...。

这也显示MonadReader如何进入图片...... monad m必须通过读者环境访问HTTP管理器，如以下约束所示：

CC.HasHttpManager env
MonadReader env m

所有这一切都说m有一个类型为env的读者环境，它本身就有一种访问HTTP管理器的方式。

特别是，m不能只是普通的IO monad，这就是错误消息所抱怨的内容。

在评论中回答问题

以下是如何从HTTP响应创建Producer的示例：

{-# LANGUAGE OverloadedStrings #-}

module Lib3 () where

import qualified Data.ByteString as BS
import qualified Network.HTTP.Client.Conduit as CC
import           Network.HTTP.Conduit (http, tlsManagerSettings, newManager)
import qualified Network.HTTP.Client          as Client (httpLbs, responseOpen, responseClose)
import           Data.Conduit (Producer, addCleanup)
import           Data.Conduit (awaitForever, await, ($$))
import qualified Network.HTTP.Client.Conduit  as HCC

import Control.Monad.IO.Class (liftIO, MonadIO)

getStream url = do
  request <- CC.parseUrl url
  manager <- newManager tlsManagerSettings
  response <- Client.responseOpen request manager
  let producer :: Producer IO BS.ByteString
      producer = HCC.bodyReaderSource $ CC.responseBody response
      cleanup _ = do liftIO $ putStrLn "(cleaning up)"; Client.responseClose response
      producerWithCleanup = addCleanup cleanup producer
  return $ response { CC.responseBody = producerWithCleanup }

test = do
  res <- getStream "http://google.com"
  let producer = CC.responseBody res
      consumer = awaitForever $ \_ -> liftIO $ putStrLn "Got a chunk"
  producer $$ consumer

通过haskell管道管道http流

1 个答案: