Ergo: A Quest for Declarativity in Logic Programming

E rgo is a higher-level logic programming system developed by Coherent Knowledge Systems as a successor to Flora-2 [39]. From the start, Flora-2 and E rgo were designed with the explicit requirement of declarativity and usability using novel technologies developed over the years by the authors and their colleagues. Although E rgo programs are compiled into XSB [29] and they adopt many Prolog features, E rgo is altogether a different language. For instance, E rgo ’s core execution strategy is not the SLDNF of Prolog, but is instead based on the Well-Founded Semantics [31] and its core syntax is a combination of HiLog [6] and F-logic [20]. E rgo supports object-oriented modeling, logical meta-reasoning, defeasible reasoning, fully semantic update operators as in Transaction Logic [2, 3], explanations, and a variety of other features not found in Prologs. In this paper, we describe some of these novel features of E rgo with special emphasis on their relation to Prolog and how they contribute to the high degree of declarativeness of E rgo.