[Haskell-cafe] reifying is-a

Hi, all! I guess what I am about to ask is currently impossible, but as you haskellers always manage to amaze me here it goes. Given two type classes A t and B t, I'd like to derive (two) different A t instances depending exactly on whether t is an instance of B. In other words, is it possible to define a type-level function/class IsB t f such that: IsB t HTrue <=> (B t) exists IsB t HFalse <=> otherwise? If not, is this wish intrisically pointless? Thanks in advance. Cheers, Jorge M. Pelizzoni ICMC - Universidade de São Paulo