[web-devel] Getting the http request header in Yesod?

I'm looking to detect ajax requests sent to my yesod application using the technique detailed in http://davidwalsh.name/detect-ajax Is it possible to gain access to the request header directly or would yesod require modification to get at it? If someone could just point me in the right direction, I would be grateful. I imagine it would be a function returning GHandler? Thanks, Rehno