I had the same problem, and I found a fix somewhere. Don't reember what exactly I had to change so I'll just send you my Haskell section of xemacs init.el.

By the way I get an error whenever I type "undefined". Anyone else noticed it? (1) (error/warning) Error in `post-command-hook' (setting hook to nil): (wrong-type-argument listp a )

I'm officially the maintainer of the emacs Haskell mode since Reuben Thomas left, but I deleted the section of my brain that contained my knowledge of emacs lisp in order to make more room for GHC, so I'm not a very effective maintainer I'm afraid :( Offers to take over the maintainership are entirely welcome, but in the meantime I'm happy to add any fixes for these kinds of problems to the web page and/or the source. Cheers, Simon