Happstack.Server provides a self-contained HTTP server and a rich collection of types and functions for routing Requests, generating Responses, working with query parameters, form data, and cookies, serving files and more.

A very simple, "Hello World!" web app looks like:

 import Happstack.Server
 main = simpleHTTP nullConf $ ok "Hello World!"

By default the server will listen on port 8000. Run the app and point your browser at: http://localhost:8000/

At the core of the Happstack server we have the simpleHTTP function which starts the HTTP server:

 simpleHTTP :: ToMessage a => Conf -> ServerPart a -> IO ()

and we have the user supplied ServerPart (also known as, ServerPartT IO), which generates a Response for each incoming Request.

A trivial HTTP app server might just take a user supplied function like:

 myApp :: Request -> IO Response

For each incoming Request the server would fork a new thread, run myApp to generate a Response, and then send the Response back to the client. But, that would be a pretty barren wasteland to work in.

The model for ServerPart is essential the same, except we use the much richer ServerPart monad instead of the IO monad.

For in-depth documentation and runnable examples I highly recommend The Happstack Crash Course http://happstack.com/docs/crashcourse/index.html.