http-conduit, snap and lazy IO

Question

I have two http-servers working with a json api using the snap framework

my first prototype contains a handler similar to this example handler

import           Data.ByteString (ByteString)
import           Data.ByteString.Char8 as B (unwords, putStrLn)
import           Data.ByteString.Lazy.Char8 as L (putStrLn)
import           Control.Monad.IO.Class (liftIO)
import           Data.Monoid ((<>))
import           Snap.Core (getParam, modifyResponse, setHeader, writeLBS) 
import           Network.HTTP.Conduit
import           Network.HTTP.Client (defaultManagerSettings)


exampleHandler :: AppHandler ()
exampleHandler = do resp <- liftIO
                         $ do L.putStrLn "Begin request ..."
                              initReq <- parseUrl "http://localhost:8001/api"
                              manager <- newManager defaultManagerSettings
                              let req  = initReq { method = "GET"
                                                 , proxy = Nothing}
                              r <- httpLbs req manager
                              L.putStrLn "... finished request."
                              return $ responseBody r
                    liftIO . L.putStrLn $ "resp: " <> resp
                    modifyResponse $ setHeader "Content-Type" "application/json"
                    writeLBS $ "{ \"data\": \""<> resp <>"\" }"

If I issue an ajax-request, the response is sent and received - i see this when the server writes resp: testdata on the console, but the response sent to the browser with writeLBS is not. Now if I change the last line to

                    writeLBS $ "{ \"data\": \""<> "something fixed" <>"\" }"

everything works like a charm. I think I am meeting one of the pitfalls of lazy IO, but I don't know how to remedy this.

I also tried a few variations with no singe liftIO -block but putting liftIO where necessary.

EDIT

based on the comment by @MichaelSnoyman I did some research regarding writeLBS and tried to

               modifyResponse $ setBufferingMode False
                              . setHeader "Content-Type" "application/json"
               writeLBS resp

as I thought maybe buffering could be the problem - no it is not

Furthermore I tried to write explicitly a setResponseBody

               let bb = enumBuilder . fromLazyByteString $ "{ \"data\": \""<> resp <>"\" }"
               modifyResponse $ setBufferingMode False
                              . setHeader "Content-Type" "application/json"
                              . setResponseBody bb

Which showed also no success.

Answer 1

I have solved this issue - it actually was a problem with the javascript getting the handwritten json (note to self: never do that again). There was a non-breaking space at the end of the input data that was not encoded correctly, and I as I am a newbie at JS I didn't get that from the error message.

The intermediate solution is to add urlEncode and make a strict ByteString

               let respB = urlEncode . L.toStrict $ C.responseBody resp
               modifyResponse $ setBufferingMode False
                              . setHeader "Content-Type" "application/json"
               writeBS $ "{ \"data\": \"" <> respB <> "\" }"

of course you have to change imports accordingly.

The long term solution is: write a proper from/toJSON instance and let the library deal with this.