Abstract bug description

WebHandler.hs

import           Control.Concurrent.Async       (mapConcurrently)
import           Control.Exception.Enclosed     (catchAny)
import qualified Network.Wreq                   as W
import           System.Timeout                 (timeout)

someHandler :: Handler ()
someHandler = do
    -- we use single http-client's Manager
    mgr <- _appWreqManager
    candidates <- getCandidates
    res <- liftIO $ fmap catMaybes $ mapConcurrently (fetch mgr) candidates
    log "Got responses!"
    renderResponses res
  where
    fetch :: Manager -> IO (Maybe CandidateData)
    fetch mgr candidate = ctch $ tm $ do
      log ("Provider: " <> show candidate)
      case candidate of
        CandidateProvider1 -> do
          let opts = defaults & manager .~ (Right mgr)
          r <- W.getWith opts "http://candidate1api.example.com/get-data"
          log ("CandidateProvider1 responded with: " <> show r)
          return (CandidateData1 r)
        CandidateProvider2 -> do
          log ("CandidateProvider2 doesn't need to fetch anything, returning pure value")
          return CandidateData2
    tm f = do
        res <- timeout (100000 * 5) f
        case res of
          Nothing -> do
            logErr ("Got timeout while requesting " <> show candidate)
            return Nothing
          Just r -> return r
    ctch f = catchAny f (\e -> logErr ("Exception while fetching " <> show candidate
                                       <> ": " <> show e)
                               >> return ())

data CandidateData
getCandidates = undefined
renderResponses = undefined