{-# LANGUAGE OverloadedStrings #-}

import qualified Data.ByteString.Lazy as BL

nats <- connect "nats://user:password@localhost:4222"

sid <- subscribe nats "news" Nothing $ \_ _ msg _ -> putStrLn $ show msg

publish nats "news" "I got news for you"

unsubscribe nats sid

subscribe nats "gift" Nothing $ \_ _ msg mreply -> do
    putStrLn $ show msg
    case mreply of
       Nothing -> return ()
       Just reply -> publish nats reply "I've got a gift for you."
 
reply <- request nats "gift" "Do you have anything for me?"

putStrLn $ show reply

The connect call connects to the NATS server and creates a receiver thread. The callbacks are run synchronously on this thread when a server messages comes. Client commands are generally acknowledged by the server with an +OK message, the library waits for acknowledgment only for the subscribe command. The NATS server usually closes the connection when there is an error.

Compared to API in other languages, the Haskell binding is very sparse. It does not implement timeouts and automatic unsubscribing, the request call is implemented as a synchronous call.

The timeouts can be easily implemented using Timeout module, automatic unsubscribing can be easily done in the callback function.

The connect function tries to connect to the NATS server. In case of failure it immediately fails. If there is an error during operations, the NATS module tries to reconnect to the server. During the reconnection, the calls subscribe and request will block. The calls publish and unsubscribe silently fail (unsubscribe is handled locally, NATS is a messaging system without guarantees, publish is not guaranteed to succeed anyway). After reconnecting to the server, the module automatically resubscribes to previously subscribed channels.

If there is network failure, the nats commands subscribe and request may fail on a network exception. The subscribe command is synchronous, it waits until the server responds with +OK. The commands publish and unsubscribe are asynchronous, no confirmation from server is required.