[Haskell-cafe] how to create a pipes producer from websockets?

[Daniel Díaz]

One option is to use pipes-concurrency 
<http://hackage.haskell.org/package/pipes-concurrency>. 

Create 
<http://hackage.haskell.org/package/pipes-concurrency-2.0.3/docs/Pipes-Concurrent.html#v:spawn-39-> 
a mailbox, and pass to *runClient *a callback that reads from the 
Connection and writes to the mailbox 
<http://hackage.haskell.org/package/pipes-concurrency-2.0.3/docs/Pipes-Concurrent.html#t:Output>
. 

Build 
<http://hackage.haskell.org/package/pipes-concurrency-2.0.3/docs/Pipes-Concurrent.html#v:fromInput>a 
*Producer *from the other end of the mailbox and consume it in another 
thread.

Coordinate the threads with something like concurrently 
<http://hackage.haskell.org/package/async-2.0.2/docs/Control-Concurrent-Async.html#v:concurrently> from 
the async package.

On Thursday, October 22, 2015 at 1:42:37 AM UTC+2, Dimitri DeFigueiredo 
wrote:
>
> Hello All,
>
> I am trying to use both the websockets library and the pipes library. They 
> seem like a natural fit. However, I wanted to use the websockets library to 
> build a pipes 'Producer' and that does not seem possible without some heavy 
> lifting.
>
> The websockets library does not give me a connection I can read from and 
> write to. Instead, I need to supply an IO action (called a clientApp). And 
> execute it using something like:
>
> --runClient :: String       -- ^ Host
> --          -> Int          -- ^ Port
> --          -> String       -- ^ Path
> --          -> ClientApp a  -- ^ Client application -- type ClientApp a = 
> Connection -> IO a
> --          -> IO a
>
> main :: IO ()
> main = withSocketsDo $ runClient "echo.websocket.org" 80 "/" app
>
> The problem happens when you couple this restriction with the requirement 
> that ClientApps have type 'Connection -> IO a'. Pipes producers are Monad 
> transformer stacks. For example, I would like to build something like: 
> messageProducer :: Producer' WebsocketMessage IO ()
>
> But this does not run in the IO monad and at the same time I cannot get a 
> 'Connection' object unless I use runClient.
>
> So, it seems I have a catch22 situation. Or have I overlooked something? 
> I'd rather not have to do "un-lifting" on the monad transformer stack.
>
> Has anyone done this before? Any suggestions on how to go about it?
>
>
> Thanks,
>
>
> Dimitri
>
>
>
>
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