Two Types of Planning in Language Generation

As our understanding of natural language generation has increased, a number of tasks have been separated from realization and put together under the heading atext planning I . So far, however, no-one has enumerated the kinds of tasks a text planner should be able to do. This paper describes the principal lesson learned in combining a number of planning tasks in a planner-realiser: planning and realization should be interleaved, in a l imited-commitment planning paradigm, to perform two types of p]annlng: prescriptive and restrictive. Limited-commitment planning consists of both prescriptive (hierarchical expansion) planning and of restrictive planning (selecting from options with reference to the status of active goals). At present, existing text planners use prescriptive plans exclusively. However, a large class of p]anner tasks, especially those concerned with the pragmatic (non-literal) content of text such as style and slant, is most easily performed under restrictive planning. The kinds of tasks suited to each planning style are listed, and a program that uses both styles is described. 1 I n t r o d u c t i o n PAULINE (Planning And Uttering Language In Natural Environments) is a language generation program that is able to realize a given input in a number of different ways, depending on how its pragmatic (interpersonal and situation-specific) This work was done while the author was at the Yale University Computer Science Departmentt New Haven This work was supported in part by the Advanced Research Projects Agency monitored by the Office of Naval Research under contract N00014-82-K-0149. It was also supported by AFOSR contract F49620-87-C-0005. goals are set by the user. The program consists of over 12,000 lines of T, a dialect of LISP developed at Yale University. PAULINE addresses simultaneously a wider range of problems than has been tried in any single language generation program before (with the possible exception of [Clippinger 74]). As is to be expected, no part of PAULINE provides a satiefactorily detailed solution to any problem; to a larger or smaller degree, each of the questions it addresses is solved by a set of s impl~ed, somewhat ad ho¢ methods. However, this is not to say that the program does not provide some interesting insights about the nature of language generation and the way that generators of the future will have to be structured. One insight pertains to the problems encountered when the various tasks of generation both of text planning and of realization ~ are interleaved to provide plannlng-on-demand rather than strict top-down planning (which has been the approach taken so far). The planning tasks that are best performed on demand tend to have shortrange effects on the text (compared to those best performed in full before realization). In order to achieve the types of communicative goals such tasks usually serve, the planner must ensure that they work together harmoniously so that their effects support one another rather than conflict. This requirement imposes constraints on the organlzation and architecture of a generation system. This paper describes PAULINE's architecture, the text planning tasks implemented, and how the tasks are managed. Unfortunately many details have to be left unsaid; the interested reader is referred to relevant material at appropriate points. Overview descriptions appear in [Hovy 87a, 87b].