DCGs: Parsing as Deduction?

Abstract

The idea of viewing parsing as deduction has been a powerful way of explaining formally the foundations of natural language processing systems. According to this view, the role of grammatical description is to write logical axioms from which the well-formedness of sentences in a natural language can be deduced.

However, this view is at odds with work on unification grammars, where categories are given complex descriptions and the process of building satisfying models is at least as relevant as that of building deductive proofs. In some work feature logics are even used to replace the context-free component of grammars. From this work emerges the view that grammatical description is more like writing down a set of constraints, with well-formed sentences being the possible solutions to these constraints. In this paper, we concentrate on Definite Clause Grammars (DCGs), the paradigm example of “parsing as deduction”. The fact that DCGs are based on using deduction (validity) and unification grammar approaches are based on constructing models (satisfiability) seems to indicate a significant divergence of views. However, we show that, under some plausible assumptions, the computation involved in using deduction to derive consequences of DCG clauses produces exactly the same results as would be produced by a process of model building using a set of axioms derived syntactically from the original clauses.

This then suggests that there is a single view of parsing (and generation) that reconciles the two approaches. This is a view of parsing as model-building, not a view of parsing as deduction. Even in the original paradigm case there is some doubt as to whether “parsing as deduction” is the best, or only, explanation of what is happening.