如何在超時時重試http請求？

Question

我在下載方法往往已經超時了，我想在這裏補充一些重試：與ScopedTypeVariables

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

添加試算

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 

我發現方法不是什麼大問題，但似乎就像我不能隨便使用的東西都

response ← retryOnTimeout (http request manager) 

因爲它會導致奇怪的類型有衝突

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)) 

如何在HTTP get方法的Timeout上實現重試？

Answer 1

錯誤消息告訴你，你的代碼需要IO a，但實際上是給出ResourceT IO a。最容易做的事情是：

• 更改您的類型簽名匹配
• 使用catch從Control.Exception.Lifted（在lifted-base包），而不是從Control.Exception