On Tue, Jun 26, 2012 at 8:22 PM, Nicolas Trangez
Hello Cafe,
Some time ago I tried to implement a network service using iteratee (or enumerator, can't remember), but gave up in the end. More recently I wanted to create something similar (a similar protocol), but failed again.
So I'm looking for some example code or something similar (Google only helped slightly).
First of all, I don't care which API/library to use, I guess for my purpose all of enumerator, iteratee, iterIO, pipes, conduits,... are OK, so all feedback is welcome.
Here's the catch. Most examples out there implement some server which accepts a single client request, interprets it, creates a response, returns this, and closes the connection (or something alike, think HTTP).
The protocol I'd like to implement is different: it's long-running using repeated requests & responses on a single client connection. Basically, a client connects and sends some data to the server (where the length of this data is encoded in the header). Now the server reads & parses this (binary) data, sets up some initial state for this client connection (e.g. opening a file handle), and returns a reply. Now the client can send another request, server parses/interprets it using the connection state, sends a reply, and so on.
Might sound easy (and actually it's pretty easy in most other languages I know, including an OCaml implementation), yet I fail to figure out how to get this done using some enumerator-style library.
With the current development version of pipes-core (https://github.com/pcapriotti/pipes-core/tree/devel) I would write something like the following (completely untested) code: import qualified Control.Pipe.Binary as B ... request :: PipeL IO ByteString ByteString u () request = do h <- header let n = hdrSize h B.take n -- I assume a fixed-size header for simplicity header :: PipeL IO ByteString b u Header header = do h <- B.take headerSize >+> fold (<>) ByteString.empty return $ parseHeader h -- the function doing the actual parsing handler :: Pipe IO ByteString ByteString u () handler = do -- server logic here -- just echo the input data as an example void idP server hInput hOutput -- read from the socket = B.handleReader hInput -- process all requests >+> forever (withUnawait $ request >+> handler) -- write to socket >+> B.handleWriter hOutput Requests are handled sequentially by the `forever` loop. This whole pipeline works with chunks of data represented as `ByteString`s, and `PipeL` (a new feature of pipes-core 0.2.0) is used to pass leftover data along. In a real implementation, you would also probably need to wrap `handler` in something like: catch handler $ \e -> liftIO $ logException e discard so that failures (or early termination) in `handler` don't bring the whole pipeline down. Sorry for the not very practical reply, involving experimental unreleased code. :) BR, Paolo