I'm not currently doing much web development in Haskell, but I've done a lot in Ruby over the last few years, including writing a couple of frameworks, so I have some general comments.
1. Send the original, unmodified, unparsed request! This minimzies the chances that bugs on the web server side will cause pain for the appplication server side, and also gives you the best chance of recovering from these bugs. For example, one of the major problems with CGI and related protocols is that every web server seems to deal with things like the REQUEST_PATH differently, so switching web servers, or even making changes to a web server configuration, may break your application's dispatch. There's no reliable way, in CGI, to recover the URL that was originally used to access the resource.
Feel free to supply a library for parsing cookie headers or whatever, but make that part of the Haskell side, both to minimize the amount of work needed to be done by the web server implementor and to let us fix the bugs in once place, rather than several.
It's reasonable, if the web server has further configuration information to send (such as what the CGI "prefix path" is, if it has such a thing, or if we want to generate things like REQUEST_PATH for backward compatability) to send an environment (presumably as a dictionary) along with each request. But this should be separate from the request itself.
2. Consider how the protocol will work over a network, for those that need that functionality. Issues include:
- How do you send back error messages to be logged by the web server? - How do you set up and tear down connections. (One hopes not one per request!) - Do you multiplex requests in an async manner over a single connection? If so, do you force all clients to have to deal with multiplexed requests, or do you specify that it can be disabled?
Incidently, if it turns out to be a decent protocol, I may well contribute a set of Ruby bindings, since I'm reaching the point where I am soon going to have to rewrite from scratch the FastCGI library I'm using in Ruby, and that still won't eliminate all the problems I'm having.