I don't think XMonad is a poster child for advertising how great (read: awkward) haskell is for runtime extensibility. Sure it does the job, but it's based on a

I don't think XMonad is a poster child for advertising how great (read: awkward) haskell is for runtime extensibility. Sure it does the job, but it's based on a giant hack (invoking the compiler on your configuration, forking a new process, etc.), this is pretty specific to xmonad and is non-trivial, so you can't easily reproduce this kind of extensibility for other applications. It's completely different to the kind of extensibility offered by something like emacs/lisp, which is done purely in the runtime and doesn't require transferring some state to a new process.