John,

Your change makes just the server function to return the same number that it receives. My server implementation multiplied the incoming number by two.

My question was why does the output from writer shows first all output from server, then followed by all output from client.  I would like to see output from client and server to alternate in the order in which the computation occurs, namely [server, client, server, client, server, client, …].

I know that for finite sequences, it is easily possible to reorder the output from writer. However, it is not possible to reorder the output if the sequence is infinite.  That can be seen in my example program if it is modified by removing the statement “take 10”. In that case the program never prints output from clients, the output printer will be only from the server.

Jan

From: John Vogel [mailto:jpvogel1@gmail.com]
Sent: Monday, January 14, 2008 8:33 PM
To: Jan Stranik
Cc: haskell@haskell.org
Subject: Re: [Haskell] Simulating client server communication with recursive monads

If you redefine as follows:

server :: [Integer] -> Writer [String] [Integer]
server [] = return []
server (a:as) = do
tell ["Server " ++ show a]
rs <- server as
return (a:rs)

You get this for the output:


["Server 0","Server 1","Server 2","Server 3","Server 4","Server 5","Server 6","Server 7","Server 8","Server 9","Server 1
0","Client 0","Client 1","Client 2","Client 3","Client 4","Client 5","Client 6","Client 7","Client 8","Client 9"]

Then you just need to alternate the pattern.  Though a real simulation of sever traffic would account for the packets 
not being recieved, rerequested, or recieved out of order.  It also depends whether you are simulating TCP or UDP.

On 1/14/08, Jan Stranik <janstranik@yahoo.de> wrote:

Hello,

I am trying to simulate a client server traffic using recursive lazy evaluation. I am trying to do that in a recursive writer monad.

Following code is my attempt to simulate client server interaction and collect its transcript:

{-# OPTIONS -fglasgow-exts #-}

module Main where

import Control.Monad.Writer.Lazy

simulation:: Writer [String] ()

simulation = mdo

        a <- server cr

        cr <- client $ take 10 a

        return ()

server:: [Integer] -> Writer [String] [Integer]

server (a:as) = do

  tell ["server " ++ show a]

  rs <- server as

  return ((a*2):rs)

server [] = return []

client:: [Integer] -> Writer [String] [Integer]

client as = do

  dc <- doClient as

  return (0:dc)

    where

      doClient (a:as) = do

              tell ["Client " ++ show a]

              as' <- doClient as

              return ((a+1):as')

      doClient [] = return []

main = return $ snd $ runWriter simulation

The problem that I see is that the transcript collected contains first all output from the server, and then output from the client.

Here is an example of output that I see:

:["server 0","server 1","server 3","server 7","server 15","server 31","server 63","server 127","server 255","server 511","server 1023","Client 0","Client 2","Client 6","Client 14","Client 30","Client 62","Client 126","Client 254","Client 510","Client 1022"]

I would like to collect the output like:

:["client 0","server 0", "client 1",…]

This would allow me to remove the ending condition in simulation (take 10), and instead rely fully on lazy evaluation to collect as many simulation steps as needed by my computation.  

I am still relatively new to the concepts of recursive monadic computations, so I would appreciate any suggestions from experts on this mailing list.

Thank you

Jan

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