My problem is how to I translate an algebraic data type into Prolog. I visited this idea in these post (A, B), but never made it past the point of getting it off the back burner.
Right now the cleanest way I see to put this into working Prolog code is to add state to the predicates with the state containing a compound term for the result, one part for the valid (happy) path and one part for the exception/error (unhappy) path.