Question

Using the websockets library in the following way

{-# LANGUAGE OverloadedStrings #-}

module Main where

import System.IO
import System.IO.Unsafe
import Network.Socket hiding (recv)
import Network.WebSockets
import Network.Socket.ByteString
import qualified Data.ByteString.Char8 as B
import Debug.Trace
import Control.Applicative

fetch :: IO B.ByteString
fetch = do
  B.putStrLn "connected"
  [v4] <- getAddrInfo Nothing (Just "127.0.0.1") (Just "3000")
  c <- socket (addrFamily  v4) Stream 0x6
  c `connect` (addrAddress v4)
  recv c 512000

proxy :: TextProtocol p => WebSockets p ()
proxy = sendTextData . unsafePerformIO $! fetch

app :: Request -> WebSockets Hybi00 ()
app r = acceptRequest r >> r `traceShow` proxy

main :: IO ()
main = withSocketsDo $! runServer "0.0.0.0" 4000 app

causes fetch to occur only once and all websocket clients receive the same not fresh data.

• How can I do arbitrary IO with websockets?
• How can I get the above example to work with fresh fetches?

I would love to hear any suggestions or complete solutions. A way of doing it without touching iteratee would be exceptionally appreciated.

Answer 1

The WebSockets monad is an instance of the MonadIO typeclass, so you can do arbitrary IO-operations with the liftIO function.

In this case I’m guessing you want to do

proxy = liftIO fetch >>= sendTextData

You also need to add the import

import Control.Monad.IO.Class