I would really like to help you, but without your imports, packages, etc,
it is really hard to interpret your program. Like where does decodeUtf8
come from, or receive, or TCPSocket? If they are functions you wrote, I
don't need their code, the types would be sufficient.
On Wed, Oct 12, 2016 at 10:15 PM, Sumit Raja
Hello,
I am trying to get my head around free monads by developing a simple network abstraction DSL. I've made good progress before adding TCP/IP semantics of accepting connections. I'm now stuck with the creation of monadic functions.
I've defined the following:
data NetworkActivity chan next = Accept chan next (chan -> next) | Send chan ByteString (Bool -> next) | Recv chan (ByteString -> next) | Close chan (() -> next)
clse :: a -> Free (NetworkActivity a) Text clse chan = liftF (Close chan (const "Quit"))
chatterServer :: a -> Free (NetworkActivity a) Text chatterServer svrchan = Free $ Accept svrchan (chatterServer svrchan) chatterLoop
chatterLoop :: a -> Free (NetworkActivity a) Text chatterLoop chan = Free $ Recv chan $ \bs -> case BS.uncons bs of Nothing -> clse chan Just x -> if bs == "Bye" then Free $ Close chan (\_ -> Pure "Quit") else Free (Send chan bs (\_ -> chatterLoop chan))
This works fine with the interpretTCP interpreter below accepting multiple connections:
interpretTCP :: Free (NetworkActivity TCPSocket) r -> IO r interpretTCP prg = case prg of Free (Accept serverSock svrLoop acceptProc) -> bracket (return serverSock) (\s-> interpretTCP (clse s)) (\s-> do (ss, sa) <- accept s forkIO $ do _ <- interpretTCP (acceptProc ss) return () interpretTCP svrLoop ) Free (Recv sock g) -> do bs <- receive sock 4096 mempty putStrLn (decodeUtf8 bs) interpretTCP (g bs) Free (Close sock g) -> do close sock putStrLn ("Server bye!" :: Text) interpretTCP (g ()) Pure r -> return r Free (Send sock pl g) -> do sent <- send sock pl mempty interpretTCP (g (sent > 0))
Where I'm stuck is defining the monadic version of accept and I'm beginning to think my original data type defined above may be wrong. As an initial step I've defined the following:
recv :: a -> Free (NetworkActivity a) ByteString recv chan = liftF (Recv chan identity)
sendit :: a -> ByteString -> Free (NetworkActivity a) Bool sendit chan pl = liftF (Send chan pl identity)
mchatterServer :: a -> Free (NetworkActivity a) Text mchatterServer chan = Free $ Accept chan (mchatterServer chan) (\s -> return (identity s) >>= mchatterLoop)
mchatterServer works as is, the interpreter accepts multiple connections. Similarly all good with recv and sendit. I am struggling with converting the Accept in mchatterServer into a function to use in the do syntax. The signature I think I should be using is
acc :: a -> NetworkActivity a Text -> Free (NetworkActivity a) (NetworkActivity a Text)
What I can't figure out is why it can't follow the pattern of recv and sendit above:
acc chan next = liftF $ Accept chan next identity
Which results in error on identity (using Protolude):
Expected type: a -> NetworkActivity a Text Actual type: NetworkActivity a Text -> NetworkActivity a Text
I can't really see how to get the types to line up and have now can't see through the type fog. What am I missing in my reasoning about the types?
Help much appreciated!
Thanks
Sumit _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners