[xmonad] Can I switch layouts based on number of windows in a workspace?

[Jeffrey David Johnson]

I'd like to have a different layout depending on how many windows are 
visible. Something like:

one?            --> Full
under four? --> Tall
otherwise    --> Grid

I couldn't find an existing function, and didn't make much headway 
writing my own either. (I tried to use sendMessage $ JumpToLayout "Full" 
etc. in a managehook) Is there an obvious way to do this?
Thanks
Jeff