[Haskell] Simulating client server communication with recursive
janstranik at yahoo.de
Tue Jan 15 20:27:25 EST 2008
I appreciate your quick answer.
Do you know what is the theoretical foundation for having mfix process side-effects in the lexical order as opposed to execution order? Could you point me to some papers, if you know of any off top your head?
Thanks a lot,
From: Chris Kuklewicz [mailto:haskell at list.mightyreason.com]
Sent: Tuesday, January 15, 2008 11:07 AM
To: Jan Stranik
Subject: Re: [Haskell] Simulating client server communication with recursive monads
Jan Stranik wrote:
> My question was why does the output from writer shows first all output
> from server, then followed by all output from client.
The effect of "mfix" is that the side-effects of the calculation still occur in
the lexical order.
Since server is before client, the output of the server precedes the 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, …].
Then you have to restructure the code slightly. This works:
> import Control.Monad
> import Control.Monad.Writer.Lazy
> client:: [Integer] -> Writer [String] [Integer]
> client as = do
> dc <- doClient as
> return (0:dc)
> doClient (a:as) = do
> tell ["Client " ++ show a]
> as' <- doClient as
> return ((a+1):as')
> doClient  = return 
> server :: [Integer] -> Writer [String] [Integer]
> server  = return 
> server (a:as) = do
> tell ["Server " ++ show a]
> rs <- server as
> return (2*a:rs)
> simulation :: [(String,String)]
> simulation =
> let (clientOut,clientLog) = runWriter (client serverOut)
> (serverOut,serverLog) = runWriter (server clientOut)
> in zip serverLog clientLog
> main = print (take 10 simulation)
More information about the Haskell