[Haskell-cafe] Space leak with recursion

23 Apr 2015

      Hi,

I'm trying to make an observable (code below) that listens to two ZeroMQ
sockets and publishes the received messages on a third. Every message
that is received gets an incremented sequence number before it is send
on the publishing socket. To keep the current sequence number a state
object is passed to the polling function and after handling the message,
the polling function is called again with the updated state.

Unfortunately this recursive calling of the polling function creates a
space leak. Any suggestions how to fix this?

Note: for brevity I left the incrementing of the sequence number and the
sending on the publishing socket out of the code.

Kind regards,

Martijn Rijkeboer

--- Code ---

module Observable
    ( run
    ) where

import Control.Monad (void)
import Data.Int (Int64)
import System.ZMQ4

data State = State
    { nextSeqNum    :: !Int64
    , listenSocket  :: !(Socket Pull)
    , publishSocket :: !(Socket Pub)
    , snapSocket    :: !(Socket Router)
    }

run :: IO ()
run = do
    withContext $ \ctx ->
        withSocket ctx Pull $ \observer ->
        withSocket ctx Pub $ \publisher ->
        withSocket ctx Router $ \snapshot -> do
            setLinger (restrict (0::Int)) observer
            bind observer "tcp://*:7010"

            setLinger (restrict (0::Int)) publisher
            bind publisher "tcp://*:7011"

            setLinger (restrict (0::Int)) snapshot
            setSendHighWM (restrict (0::Int)) snapshot
            bind snapshot "tcp://*:7013"

            let state = State
                    { nextSeqNum = 0
                    , listenSocket = observer
                    , publishSocket = publisher
                    , snapSocket = snapshot
                    }

            pollSockets state

pollSockets :: State -> IO ()
pollSockets state =
    void $ poll (-1)
       [ Sock (listenSocket state) [In] (Just $ observerHandleEvts state)
       , Sock (snapSocket state)   [In] (Just $ snapshotHandleEvts state)
       ]

observerHandleEvts :: State -> [Event] -> IO ()
observerHandleEvts state _ = do
    void $ receiveMulti $ listenSocket state
    pollSockets state

snapshotHandleEvts :: State -> [Event] -> IO ()
snapshotHandleEvts state _ = do
    void $ receiveMulti $ snapSocket state
    pollSockets state