Posts tagged ‘zeromq’

0MQ and Haskell

2012-03-06, 16:26

Ever since I heard the FLOSS weekly episode on 0MQ I’ve been looking for a reason to take a look at it. Well, to hell with reason, I’ll have a first look without any specific goal in mind.

I found a simple introduction to it in Nicholas Piël’s post ZeroMQ an introduction. The only issue was that it was based on Python, and Python2 at that. So here are my attempts at translating two of the clients to Haskell (using zeromq-haskell).

req-rep

Here’s the client in Python3 first:

1
2
3
4
5
6
7
8
9
10
11

import zmq
 
ctx = zmq.Context()
socket = ctx.socket(zmq.REQ)
socket.connect('tcp://127.0.0.1:5000')
 
for i in range(10):
    msg = "msg %s" % i
    socket.send_unicode(msg)
    print('Sending', msg)
    msg_in = socket.recv()

And here in Haskell:

1
2
3
4
5
6
7
8
9
10

import System.ZMQ
import Data.ByteString.Char8 as CBS
 
main = withContext 1 $ \ ctx -> withSocket ctx Req $ \ soc -> do
    connect soc "tcp://127.0.0.1:5000"
    let msgs = [pack ("msg " ++ show i) | i <- [0..9]]
    flip mapM_ msgs $ \ msg -> do
        send soc msg []
        CBS.putStrLn msg
        receive soc []

pub-sub

In Python3:

1
2
3
4
5
6
7
8
9
10

import zmq
 
ctx = zmq.Context()
socket = ctx.socket(zmq.SUB)
socket.connect('tcp://127.0.0.1:5000')
socket.setsockopt(zmq.SUBSCRIBE, b'sweden')
socket.setsockopt(zmq.SUBSCRIBE, b'denmark')
 
while True:
    print(socket.recv())

Haskell:

1
2
3
4
5
6
7
8
9

import System.ZMQ
import Control.Monad
import Data.ByteString.Char8 as CBS
 
main = withContext 1 $ \ ctx -> withSocket ctx Sub $ \ soc -> do
    connect soc "tcp://127.0.0.1:5000"
    subscribe soc "sweden"
    subscribe soc "denmark"
    forever $ receive soc [] >>= CBS.putStrLn

Two comments on the Haskell code here:

  • I’m not sure why, but the Haskell client dies after receiving just a few messages (they are properly filtered though).
  • The API for subscribe is a bit strange, it would make more sense if it took a ByteString rather than a String.
Tags: , ,
Category: Posts  |  6 Comments