How to do it...

1. Create a new project called master-slave with the simple stack template:

        stack new master-slave simple

2. Add a dependency on the distributed-process and distributed-process-localnet libraries in the build-depends sub-section of the executable section:

        executable master-slave
          hs-source-dirs:      src
          main-is:             Main.hs
          default-language:    Haskell2010
          build-depends:       base >= 4.7 && < 5
                             , distributed-process
                             , distributed-process-simplelocalnet

3. Note that you might have to add the dependent library specifically to the extra-deps section, as these libraries are not part of stackage LTS at the time of writing this recipe. Add the following to stack.yaml:

        extra-deps:
          - distributed-process-0.6.6
          - distributed-process-simplelocalnet-0.2.3.3
          - syb-0.6

4. Open src/Main.hs. We will be adding our source here. Add the Main module, and the relevant imports:

        module Main where

        import System.Environment (getArgs)
        import Control.Distributed.Process
        import Control.Distributed.Process.Node
        import Control.Distributed.Process.Backend.SimpleLocalnet

5. Create the Process that we will run on the master node:

        masterTask :: Backend -> [NodeId] -> Process ()
        masterTask backend slaves = do
          liftIO $ putStrLn $ "Initial slaves: " ++ show slaves
          terminateAllSlaves backend

6. Use main to start either the master or slave node:

        main :: IO ()
        main = do
          args <- getArgs

          case args of

            "-m":h:p:[] -> do
              backend <- initializeBackend h p initRemoteTable
              startMaster backend (masterTask backend)
            "-s":h:p:[] -> do
              backend <- initializeBackend h p initRemoteTable
              startSlave backend  

7. Build and execute the project:

        stack build
        stack exec -- master-slave -s 127.0.0.1 10501 & 
        stack exec -- master-slave -s 127.0.0.1 10502 &
        stack exec -- master-slave -s 127.0.0.1 10503 &
        stack exec -- master-slave -s 127.0.0.1 10504 &
        stack exec -- master-slave -m 127.0.0.1 10505

You should see the following output: