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 "text planning". 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-realizer: planning and realization should be interleaved, in a limited-commitment planning paradigm, to perform two types of planning: 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 planner 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.