CLASP
The Centre for Linguistic Theory and Studies in Probability

An Abstract Categorial Grammar Approach to the Discourse Modeling

Various theories have been proposed in order to analyze a discourse in terms of rhetorical (discourse) relations. The main assumption in those theories is that every meaningful piece of a felicitous discourse is related to some piece of that discourse with a rhetorical relation. This gives rise to a notion of a rhetorical (discourse) structure. In order to analyze a discourse, both from the parsing and structural points of view, formal grammars of discourse, D-STAG and D-LTAG, were proposed. They build their discourse grammars on top of sentence-level grammars. Discourse connectives are main lexical means for expressing rhetorical relations. They play a similar role in discourse grammars as words do in sentence-level grammars. A discourse connective may appear inside a clause (a clause-medial position) or in front of a clause (a clause-initial position). The grammars of D-STAG and D-LTAG are capable of modeling cases where discourse connectives occupy only clause-initial positions. To process discourses where a discourse connective appears at a clause-medial position, D-STAG and D-LTAG make use of preprocessing of a discourse, which involves moving connectives from clause-medial positions to clause-initial ones. Afterwards, the grammars of D-STAG and D-LTAG can be employed to parse the discourse and simultaneously construct its rhetorical structure. Thus, D-STAG and D-LTAG, each makes use of a two-step process to analyze a discourse. We develop a single-step, purely grammatical approach for analyzing a discourse. Our framework is Abstract Categorial Grammars (ACGs). Our encoding falls into the class of second-order ACGs, which guarantees that the tasks of discourse parsing and generation are of polynomial complexity. In addition, our encoding puts together the discourse-level and sentence-level grammars within a single grammar. This makes our approach beneficial for reducing problems related to ambiguity that arise in the case of treating the discourse-level and sentence-level grammars separately.