Using a Textual Representation Level Component in the Context of Discourse and Dialogue Generation

A natural language generation system is typically constituted by two main components: a content planning component (e.g., text planner or dialogue act planner) and a linguistic realization component. But, this is not sufficient since, on the one hand, the message built by the content pldnning component is generally not adequately detailed in order to control the many possibilities for its expression and, on the other hand, the content planner cannot influence the way in which the message will be verbalized. Generation systems require a third component, called the micro-planning (or sentence planning or phrasing) component, which acts as an intermediary between the pragmatico-semantic level and the purely syntactic level. The micro-planner is responsible for transforming the message into a textual structure. For this transformation to be achieved, grammatical and lexical resources must be selected. 1 I n t r o d u c t i o n Traditionally, the architecture of a generation system is either pipelined (e.g., TEXT [McKeown 85]), interleaved (e.g., PAULINE [Hovy 90]) or integrate.d (e.g., KAMP [Appelt 85]). Whatever the approach, the system is divided into two main components: a message planning component and a linguistic realization component. The former is responsible for the selection and organization of the information to be conveyed; it builds a structure often called message . The latter involves the formulation of the message in grammatically correct sentences. In a context of discourse, the message planning component is a text planner and the message is a t e x t p l an *This work was realized at the lstituto per la Ricerca Scientifica e Tecnolo#ica ( IRST) in Trento (Italy). (e.g., see [Maier & Not 93]). In a context of dialogue, the message planner is a 'fictitious' component consisting of a dialogue act planner, which precedes the generation system, and a planner for surface linguistic acts, which is a component of the generation system. The dialogue act planner builds plans of dialogue acts. A plan of dialogue acts is a tree-like structure in which non-leaf nodes are. macro-acts (e.g., "disjunction" or "sequence" of dialogue acts) and leaf nodes are dialogue acts (see [Sadek 91]). A dialogue act conveys information about its type (e.g., "inform", "yn-question" or "confirm"), its propositional content (i.e., a semantico-conceptual representation of the information to be communicated), and the interlocutors. The surface linguistic act planner transforms a plan of dialogue acts into a p l a n o f su r face l ingu i s t i c ac t s (i.e., the message). In particular, the dialogue act type is transformed into a surface linguistic act type such as "assertion", "answer" or "question", and the semanticoconceptual content into a semantic propositional content (cf. figure 1 and section 3). One of the problems which appear in traditional generation systems concerns the message itself: it is underspecified with respect to the language. On the one hand, the message planner cannot influence the way in which the message will be verbalized, and, on the other hand, the message is not detailed enough for the many possibilities for its expression to be controlled. Figure 1 shows several messages (a text plan and three surface linguistic acts corresponding to the same dialogue act) and their different verbalizations. A second problem is related to the fact that most generation systems produce texts successfully since they implicitly ensure that the messages are expressible. For instance, in the case where the message is a text plan, each atomic unit in the message must be a proposition, and thus can always be realized as a clause. Each unit can be independently translated into the language using the linguistic realization component, since there are