Definite-Clause Set Grammars: A Formalism for Problem Solving

D First, we present definite-clause set grammars (DCSG), a DCG-like formalism for free-word-order languages. The DCSG formalism is well suited for problem solving. By dealing with DCSG as a generalized parsing problem, we avoid a certain type of looping problem in backward chaining. Next, we extend DCSG by viewing grammar rules as definitions for set conversions. In order to realize inverse conversions, we introduce an inverse operator into DCSG syntax. This operator enables partial bottomup analyses in the DCSG top-down parsing process. Next, we discuss a looping problem called “left recursion” in top-down parsing. The looping problem is avoided by the bottom-up mechanism of the extended DCSG. The bottom-up mechanism can be viewed as the top-down controlled firing of production rules. Unlike most production systems, production systems written in extended DCSG can backtrack and produce alternative solutions. DCSG is a simple but powerful tool for generalized parsing problems which involve finding structures in a given data set. a