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A Haskell library to simplify certain Networking tasks

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README.md

NetSpec

A Haskell library to simplify static Networking tasks

Certain networking tasks have a very simple format:

  • Set up a given number of connections and initial state
  • Loop until some condition is met
  • Wrap up and close connections

NetSpec provides a straightforward way to specify and run such tasks, and also provides convenience functions ! and receive in the spirit of Erlang's simple message passing. These two parts of NetSpec can be used separately, but are designed to fit nicely together.

When you use NetSpec, you choose whether you are a client or a server, and which message format you want to use.

You have 3 options for message formats: ByteString (Lazy), Text, and JSON. Text uses the newline character as a sentinel for the end of a message, while ByteString and JSON indicate the length of the remaining message in the first 64 bits. Although JSON is built on top of ByteString, when you use JSON you can automatically serialize to and from your own data types for messages, without worrying about ByteStrings at all. For this reason, JSON is the recommended approach. You can derive the necessary typeclass instances with $(deriveJson id ''MyDataType).

Regardless of your choice, NetSpec attempts to hide the implementation of message-passing from you (connecting NetSpec clients and NetSpec servers should be a breeze) but at the same time tries to use a sensible implementation that can easily work with other programming languages and systems.

{-# LANGUAGE OverloadedStrings #-}
import Network.NetSpec.ByteString

  -- or --

{-# LANGUAGE OverloadedStrings #-}
import Network.NetSpec.Text

  -- or --

{-# LANGUAGE TemplateHaskell #-}
import Network.NetSpec.Json

You select whether you are a server or a client based on which kind of spec you run.

import Network.NetSpec

main = runSpec ServerSpec { ... }

  -- or --

main = runSpec ClientSpec { ... }

The difference between a server and a client is that the server specifies ports to listen on, and waits for connections, while the client specifies hostnames and ports, and immediately demands connections. However, other than this detail, servers and clients are specified the same: how to start up, how to loop, and how to shut down.

The examples are found in the "examples" folder (surprise!). To run them, you'll need to install the NetSpec library.

$ pwd
blah/blah/netspec

$ cabal install
blah blah
Registering netspec-0.1.0.0...

$ cd examples

Or, assuming you have the Haskell Platform and the aeson library installed, you don't actually have to install NetSpec to run the examples. Just use the -i option to tell it where the NetSpec source is.

$ pwd
blah/blah/netspec/examples

$ runhaskell -i.:../src SomeExample.hs

To run the Relay example, open 3 terminals:

0$ runhaskell Relay.hs
1$ telnet localhost 5001
2$ telnet localhost 5002

Type things into terminal 1, and watch as they are relayed to terminal 2. You can run the Chat example in much the same way. These examples illustrate a stateless server, using Text messages. The termination string "bye\r" has that retarded \r because that's what telnet sends. Remember, Text uses only \n as the end-of-message sentinel, so it is up to you to handle \r if you are dealing with outside programs that send it. (NetSpec does not).

Contrast this with the Echo / Telnet example, which illustrates how nicely a NetSpec server and NetSpec client can communicate (with Text).

0$ runhaskell Echo.hs
1$ runhaskell Telnet.hs localhost 5001

To run the Blackjack example, you'll need the random-shuffle package

$ cabal install random-shuffle

Open 3 terminals:

0$ runhaskell BlackjackServer.hs
1$ runhaskell BlackjackClient.hs localhost 5001 bot
2$ runhaskell BlackjackClient.hs localhost 5002

The Blackjack example illustrates a stateful server, mostly statless clients, and uses JSON messages. It uses some of the StateT-related convenience functions exported by NetSpec.


(c) 2012 Dan Burton

danburton.email@gmail.com

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