Domain Metatheories: Enabling User-Centric Planning Karen L. Myers Artificial Intelligence Center SRI International 333 Ravenswood Ave., Menlo Park, CA 94025 [email protected] Abstract planning knowledge base could completely capture the sub- tleties of a particular domain, users often want to participate Substantial improvements must be made in the usabil- in the plan development process to some extent, in order to ity of AI planning technology in order for it to achieve ensure that they understand both the structure of a plan and widespread adoption. In particular, approaches are re- quired that better serve the needs of users, who gen- the process by which planning decisions were made. erally want to play a central role in the plan develop- For these reasons, the future of automated planning lies in ment process. This user-centric perspective introduces the development of user-centric planning tools that can as- new requirements for the design of planning systems. sist human decision-makers rather than replace them. These In this paper, we argue that increased usability requires tools should enable a user to understand the complexity of a new representational layer that captures metatheo- the underlying problem, as well as providing guidance in retic properties of the planning application. A domain the determination of a solution that is well-suited to their metatheory would provide an abstracted characteriza- specific needs. tion of the planning domain that highlights key seman- tic differences among planning elements. This paper The main thesis of this paper is that effective interaction presents a candidate model for a domain metatheory, between users and planning technology requires an augmen- as well as an instantiation of that model for a simple tation of standard planning models to include an explicit travel domain. In addition, it describes three capabil- domain metatheory. A standard planning domain is mod- ities for user-centric planning that are enabled by this eled in terms of three types of elements: individuals cor- model, namely user directability of planning, genera- responding to real or abstract objects in the domain, rela- tion of qualitatively different plans, and plan summa- tions that describe characteristics of the world and individual rization. world states, and operators that describe ways to achieve ob- jectives. The domain metatheory would capture high-level Introduction semantic attributes of planning elements, thus providing a rich vocabulary for describing characteristics of solutions Artificial intelligence (AI) planning technology provides and problem-solving processes. As argued in this paper, powerful tools for solving problems that require the coor- the availability of such a language would greatly facilitate dination of actions in the pursuit of specified goals. The AI a broad range of user-planner interactions. community has produced several planning systems whose Different types of metatheories may suit different pur- demonstrations on realistic problems attest to the value of poses. This paper presents a candidate model for a plan- automated planning techniques. Nevertheless, there has ning metatheory, along with an instantiation of that model been limited success in transitioning this technology to user for a simple travel domain (Mayer 1997). The paper de- communities. A major reason for the lack of technology scribes how this metatheory was used to support two critical transfer lies with the difficulty of using planning systems. user-centric planning capabilities: user directability of the AI planners have traditionally been designed to operate as planning process and the generation of qualitatively differ- “black boxes”: they take a description of a domain and a set ent plans. In addition, a proposal is put forth for using the of goals and automatically synthesize a plan for achieving metatheory in support of plan summarization. the goals. This design explicitly limits the amount of influ- ence that a user can have on the generated plans. In many domains, users are reluctant to relinquish full A Domain Metatheory control of the planning process. This reluctance stems from Overview several factors. One is a belief that human creativity and Our candidate domain metatheory is built around three main experience are essential for effective planning in complex constructs: roles, features,andmeasures. domains. Transparency is another: even in the event that a A feature designates an attribute of interest for an oper- Copyright c 2000, American Association for Artificial Intelli- ator that distinguishes it from other operators that could be gence (www.aaai.org). All rights reserved. applied to the same task. For example, among operators that g can be used to refine tasks of moving from location X to lo- Vacation-Scope = fOverseas National Regional cation Y, there can be some that involve travel by air, land, AFFORDABILITY: (Overseas National Regional) (Overseas National Regional) or water; each of these media could be modeled as a fea- TIME-EFFICIENCY: ture. Because there can be multiple operators that apply to g Accommodation = fHotel Motel Camp a particular task, features provide a way of abstracting from COMFORT: (Camp Motel Hotel) the details of an operator up to distinguishing attributes that AFFORDABILITY: (Hotel Motel Camp) might be of interest to users. Note that features differ from g operator preconditions in that they do not directly restrict Transport-Media = fAir Land Water use of operators by the planner. AFFORDABILITY: (Water Air Land) Related features are grouped into feature categories.For TIME-EFFICIENCY: (Water Land Air) g example, the features fAir Land Water mentioned above g define a Transport-Media category. Feature categories Land-Transport-Mode = fAuto Bus Shuttle Taxi Train Limo AFFORDABILITY: (Limo Train Auto Taxi Shuttle Bus) themselves can have interesting properties. Just as planning TIME-EFFICIENCY: (Bus Shuttle Auto Limo Taxi Train) operators reflect a hierarchical structure, features and fea- COMFORT: (Bus Shuttle Taxi Train Auto Limo) ture categories can be organized hierarchically. Certain cat- g egories may be mutually exclusive in that at most one feature Transit-Ownership = fPublic Private from the category can be assigned to any given operator; this COMFORT: (Public Private) is the case for the feature category Transit-Ownership con- AFFORDABILITY: (Private Public) g taining the elements fPublic Private . Other categories TIME-EFFICIENCY: (Public Private) may support overlapping features; for example, there may g be an operator that involves both Air and Land travel. Transit-Capacity = fSolo Shared A role corresponds to a capacity in which a domain COMFORT: (Shared Solo) AFFORDABILITY: (Solo Shared) object is to be used within an operator. Roles map to individual variables within a planning operator. For in- stance, an air transportation operator could have variables Figure 1: Feature Categories and Associated Measures from location.1 and location.2 that fill the roles of the Travel Domain Origin and Destination,aswellasavariableairline.1 that fills the role of Carrier. A comparable sea transporta- tion operator may have these same roles, although with the ranks higher than Ritz with respect to AFFORDABILITY. planning variable cruise-ship.1 for the role Carrier. We define the domain of a measure to be the set of (par- Feature categories can have associated measures.A tially) ordered values employed by the measure. For mea- measure corresponds to an ordering (possibly partial) of sures defined over feature categories, the domain is the set features within the category with respect to some desig- of features that comprise the feature category. For measures nated criteria. For example, consider the feature category defined over instances, the domain is the set of measure val- g Transit-Ownership with features fPublic Private .For ues that can be assigned to instances. the measure COMFORT, the feature Private would rank higher than Public; for the measure AFFORDABILITY,the order would be reversed. Discussion Figure 1 presents an excerpt from the metatheory for the There is no ‘correct’ formulation of a metatheory: as with travel domain that shows sample feature categories and as- the underlying planning domain, its design involves an ex- sociated measures. Each block defines a feature category, plicit modeling process. Different potential users may care with the first line listing the name of the feature category about different metatheoretic properties: for example, af- followed by its constituent features. The remaining lines de- fordability may be an issue when designing a system to be clare a measure associated with that feature category, and used for students, but not for business travelers. provide the ranking of the features for that measure and cate- gory. For simplicity, we show only measures that completely The value of the domain metatheory lies with its provi- order the features; however, partial orders are possible. sion of a semantically-grounded abstraction of the under- Just as measures can be employed to rank features (and lying planning domain that enables meaningful, high-level hence operators with those features), they can also be em- descriptions of plans and planning processes. As part of the ployed to rank instances. For measures on instances, an abstraction process, the metatheory provides semantic link- ordered set of measure values is defined. For each mea- age among different elements within a planning domain: sure, a given individual can