如何在超时时重试http请求？

Question

我的下载方法经常超时，我想在这里添加一些重试：

withSocketsDo $ do
    irequest  ← liftIO $ parseUrl url
    fileExist ← doesFileExist fname
    when fileExist $ do
        putStrLn " -> Removing old version"
        removeFile fname
    withManager $ \manager → do
        let request = irequest
             { method = methodGet
             , responseTimeout = Just 10000000 }
        response ← http request manager
        responseBody response C.$$+- sinkFile fname

我找到了 ScopedTypeVariables

的方法

retryOnTimeout ∷ IO a → IO a
retryOnTimeout action = catch action $ \ (_ :: HttpException) → do
    putStrLn "Timed out. Trying again."
    threadDelay 5000000
    action

添加尝试次数不是大问题，但似乎我不能只使用类似的东西

response ← retryOnTimeout ( http request manager )

因为会引起奇怪的类型冲突

Couldn't match type `IO'
              with `resourcet-1.1.3.3:Control.Monad.Trans.Resource.Internal.ResourceT
                      IO'
Expected type: resourcet-1.1.3.3:Control.Monad.Trans.Resource.Internal.ResourceT
                 IO
                 (Network.HTTP.Conduit.Response
                    (C.ResumableSource IO Data.ByteString.Internal.ByteString))
  Actual type: IO
                 (Network.HTTP.Conduit.Response
                    (C.ResumableSource IO Data.ByteString.Internal.ByteString))

如何为 HTTP get 方法实现超时重试？

Answer 1

错误消息告诉您，您的代码需要一个 IO a，但实际上得到的是一个 ResourceT IO a。最简单的做法是：

更改类型签名以匹配
使用 Control.Exception.Lifted 中的 catch（在 lifted-base 包中）而不是 Control.Exception

如何在超时时重试http请求？

How to retry http request on timeout?

haskell

httprequest