在Snap中捕获HTTPException

Question

在我的一个快照处理程序中，我使用HTTP客户端调用外部资源，它可以抛出一个HttpException。在最简单的情况下，我只想在调试时捕获错误并写入std；但是，我很难将类型排列起来。

assignCategories' :: String -> [String] -> Handler App (AuthManager App) String
assignCategories' l_id cats  = do
            let baseAttribs = M.fromList [("id",l_id)]                                                            
            let saveQuery = WMSaveObject {baseAttributesSO= baseAttribs ,relatedSO=relatedObjects}

            -- this is the function that can throw an HTTPException            
            -- runWMSave :: WMQueryObj -> Handler App (AuthManager App) String

            saveresponse <- try $ H.runWMSave saveQuery
            return $ case saveresponse of
                Left  e -> show (e :: HttpException)
                Right r -> r

我玩过这些类型，但通常都会出现以下错误.是否有在处理程序中调用HTTP并捕获异常的最佳实践？

我在用Control.Monad.Catch。我试着使用Control.Exception的try，但在排列类型时遇到了更多的困难。

   No instance for (Control.Monad.Catch.MonadCatch
                       (Handler App (AuthManager App)))
      arising from a use of ‘try’
    In the expression: try
    In a stmt of a 'do' block:
      saveresponse <- try $ H.runWMSave saveQuery
    In the expression:
      do { liftIO $ putStrLn l_id;
           let baseAttribs = M.fromList ...;
           let relatedObjects = concatMap makeRelatedRow cats;
           let saveQuery = ...;
           .... }

谢谢，尼尔

Answer 1

问题解决了，我使用了MonadCatchIO变压器的try，这就奏效了。