Advanced method matching with MatchMethod

The method routing functions use a class (MatchMethod method) instead of the concrete type Method.

#ifdef HsColour > class MatchMethod m where > matchMethod :: m -> Method -> Bool > > instance MatchMethod Method where ... > instance MatchMethod [Method] where ... > instance MatchMethod (Method -> Bool) where ... > instance MatchMethod () where ... #endif

This allows us to easily match on more than one method by either providing a list of acceptable matches, or by providing a function which returns a boolean value. We can use this feature to support the HEAD method. When the client does a HEAD request, the server is supposed to return the same headers it would for a GET request, but with an empty response body. Happstack includes special support for handling this automatically in most cases.

> module Main where > > import Control.Monad (msum) > import Happstack.Server (Method(GET, HEAD), dir, method, nullConf, ok, simpleHTTP) > > main :: IO () > main = simpleHTTP nullConf $ msum > [ do method [GET, HEAD] > ok $ "Hello, World\n" > ] >

[Source code for the app is here.]

We can now use curl to do a normal GET request, or we can use the -I flag which does a HEAD request:

 $ curl http://localhost:8000/
Hello, World
 $ curl -I http://localhost:8000/
HTTP/1.1 200 OK
Connection: Keep-Alive
Content-Length: 13
Content-Type: text/plain; charset=UTF-8
Date: Tue, 15 Jun 2010 19:56:07 GMT
Server: Happstack/0.5.0

Happstack automatically notices that it is a HEAD request, and does not send the body.