This is fantastic! This is the kind of connection between OO and FP that I think should be heralded much more.

Finally tagless style is an important architectural style for Haskell programming since it allows you to pass algebra joining up into the Prolog solver. This is more than adequate for simple combinations of algebras and workable (with a little elbow grease and commentary) for more complex ones.

Edit: Also, that boilerplate reduction on slide 30-32!