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

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