Haskell Pipes and Branching

Question

Problem

I\'m attempting to implement a simple web server with Haskell and the Pipes library. I understand now that cyclic or diamond topologies aren\'t possible wi

Answer 1

I was wrong when I originally said you could not handle diamond topologies. I later discovered a sensible way to do this using an ArrowChoice-like interface and included the solution in pipes-3.2.0 in the form of the leftD and rightD combinators. I'll explain how it works:

Instead of nesting proxy transformers, you wrap the result with a Left or Right

routeRequest ::
    (Monad m, Proxy p)
    => () -> Pipe p HTTPRequest (Either HTTPRequest HTTPRequest) m r
routeRequest () = runIdentityP $ forever $ do
    httpReq <- request ()
    let method = getMethod httpReq
    let (URI uri) = getURI httpReq
    respond $ case method of
      GET  -> Left  httpReq
      POST -> Right httpReq

Then you can selectively apply each handler to each branch and then merge the branches:

routeRequest >-> leftD handleGET >-> rightD handlePOST >-> mapD (either id id)
    :: (Monad m, Proxy p) => () -> Pipe p HTTPRequest ByteString IO r

If you have more than two branches then you will have to nest Eithers, but that is just a limitation of how ArrowChoice works.