I agree this would be very useful. There are a number of datalog idioms (demand transformations is a big one for encoding functional programs, adding provenance, inlining relations, doing some light compile time backwards proof search) that it would be nice to have a good meta-programming/macro language to express. Prolog seems like a natural choice. I briefly tried going this route programming in prolog syntax so prolog could parse it, but generating souffle syntax out of the prolog metaprogram.