Generating with Discourse Combinatory Categorial Grammar

Abstract

This article introduces Discourse Combinatory Categorial Grammar (DCCG) and shows how it can be used to generate multisentence paraphrases, flexibly incorporating both intra- and intersentential discourse connectives. DCCG employs a simple, practical approach to extending Combinatory Categorial Grammar (CCG) to encompass coverage of discourse-level phenomena, which furthermore makes it possible to generate clauses with multiple connectives and — in contrast to approaches based on Rhetorical Structure Theory — with rhetorical dependencies that do not form a tree. To do so, it borrows from Discourse Lexicalized Tree Adjoining Grammar (D-LTAG) the distinction between structural connectives and anaphoric discourse adverbials. Unlike D-LTAG, however, DCCG treats both sentential and discourse phenomena in the same grammar, rather than employing a separate discourse grammar. A key ingredient of this single-grammar approach is cue threading, a tightly constrained technique for extending the semantic scope of a discourse connective beyond the sentence. As DCCG requires no additions to the CCG formalism, it can be used to generate paraphrases of an entire dialogue turn using the OpenCCG realizer as-is, without the need to revise its architecture. In addition, from an interpretation perspective, a single grammar enables easier management of ambiguity across discourse and sentential levels using standard dynamic programming techniques, whereas D-LTAG has required a potentially complex interaction of sentential and discourse grammars to manage the same ambiguity. As a proof-of-concept, the article demonstrates how OpenCCG can be used with a DCCG to generate multi-sentence paraphrases that reproduce and extend those in the SPaRKy Restaurant Corpus.