[I sent this to -cafe by mistake]

most useful kind of instance would be symbolic booleans

Yes. Ersatz has this: https://hackage.haskell.org/package/ersatz-0.4.8/docs/Ersatz-Bit.html#t:Bool... I also use this class (re-exported) in https://hackage.haskell.org/package/obdd Then modules typically start with import Prelude hiding ( not, and, or, (&&), (||) ) -- possibly more import Ersatz slightly annoying but you get used to it ... - J.