Subject: [ANNOUNCE] Erlang-style Distributed Haskell
To: glasgow-haskell-users@haskell.org

This is the first release of an implementation of an Erlang-style
communication mechanism with runtime typing which provides a Haskell
equivalent for sending using "!" (pronounced "bang!") and "receive".

Distributed Haskell´s home is at

     http://www-i2.informatik.rwth-aachen.de/~stolz/dhs/

where you will find HTML-ified information about the package.

You can use Distributed Haskell to safely communicate data (derived from
classes Read & Show) across a network. A small example implementing
a global counter which accepts remote requests for {de,in}crementing and
getting its value looks like this:

Server
======
> import Distributed
>
> data Msg = Inc | Dec | GetValue Pid | Value Int
>   deriving (Show, Read)
>
> counter' n = do
>   receive (\v -> case v of
>     Inc -> counter' (n+1)
>     Dec -> counter' (n-1)
>     GetValue pid -> do
>       pid <!> (Value n)
>	counter' n		    

Client
======
> import Distributed
> 
> data Msg = Value Int | GetValue Pid | Inc | Dec | Hallo
>    deriving (Show, Read)
> 
> destHost = "remote.host" 
> destNode = "Counter-Node"
> 
> client :: Pid -> DIO Msg ()
> client pid = do 
>   proc $ putStrLn "i,d,v?"
>   str <- proc $ getLine
>   case str of
>     "i" -> do
>       remoteSend destHost destNode "Counter" Inc
> 	client pid
>     "d" -> do
>       remoteSend destHost destNode "Counter" Dec
> 	client pid
>     "v" -> do
>       me <- self
> 	remoteSend destHost destNode "Counter" (GetValue me)
> 	receive (\v -> case v of
> 	  Value v -> do
>	    proc $ putStrLn ("\n"++(show v))
>	    client pid
>	 )
>     _ -> client pid

Further features include:
=========================
 - optimization of network-usage (connection caching)
 - robustness (similar Erlang´s "monitor_node"/"spawn_link")
 - a preprocessor for easy program creation from .dhs files (desugars
      <!> & receive) 
 - a couple of example programs including Dish, the Distributed Haskell
     Shell for testing your Distributed Haskell programs. 
 - written entirely in Literate Haskell :-)

Future work
===========
Another implementation using a "ports-based" approach extending Channels
will be released some time later.

Feedback
========
We´re really interested in feedback, especially regarding all the
trouble caused by using yet another monad (DIO) which requires casting
normal IO-operations (see the "proc (putStrLn ..)" in the client code
above).

Bug reports are welcome, too! Please send them to
  Frank Huch <hutch@i2.informatik.rwth-aachen.de>
and
  Volker Stolz <stolz@i2.informatik.rwth-aachen.de>.

Feel free to contact us about any issues arising from using Erlang-style
Distributed Haskell.

Happy distributing,
   Volker