Two Types of Planning in Language Generation

TWO TYPES OF PLANNING IN LANGUAGE GENERATION Eduard H. Hovy USC/Informat|on Sciences Institute 4676 Ar]miralty Way, Suite 1001 Marina del Rey, CA 90292-6695, U.S.A. HOVY@VAXA.ISI.EDU Abstract As our understanding of natural language gener- ation has increased, a number of tasks have been separated from realization and put together un- der 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 num- ber of planning tasks in a planner-realiser: plan- ning and realization should be interleaved, in a limited-commitment planning paradigm, to per- form two types of p]annlng: prescriptive and re- strictive. Limited-commitment planning consists of both prescriptive (hierarchical expansion) plan- ning and of restrictive planning (selecting from op- tions 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 prag- matic (non-literal) content of text such as style and slant, is most easily performed under restric- tive planning. The kinds of tasks suited to each planning style are listed, and a program that uses both styles is described. 1 Introduction 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 Re- search 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 devel- oped at Yale University. PAULINE addresses simultaneously a wider range of problems than has been tried in any sin- gle language generation program before (with the possible exception of [Clippinger 74]). As is to be expected, no part of PAULINE provides a sat- iefactorily detailed solution to any problem; to a larger or smaller degree, each of the questions it addresses is solved by a set of simpl~ed, somewhat ad ho¢ methods. However, this is not to say that the program does not provide some interesting in- sights 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 encoun- tered when the various tasks of generation -- both of text planning and of realization ~ are inter- leaved to provide plannlng-on-demand rather than strict top-down planning (which has been the ap- proach taken so far). The planning tasks that are best performed on demand tend to have short- range 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 …

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