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Title: HTN Planning with Semantic Attachments

Paper content:
Hierarchical Task Networks (HTN) generate plans using a decomposition process guided by extra domain knowledge to guide search towards a planning task.
While many HTN planners can make calls to external processes (e.g. to a simulator interface) during the decomposition process, this is a computationally expensive process, so planner implementations often use such calls in an ad-hoc way using very specialized domain knowledge to limit the number of calls.
Conversely, the few classical planners that are capable of using external calls (often called semantic attachments) during planning do so in much more limited ways by generating a fixed number of ground operators at problem grounding time.
In this paper we develop the notion of semantic attachments for HTN planning using semi co-routines, allowing such procedurally defined predicates to link the planning process to custom unifications outside of the planner.
The resulting planner can then use such co-routines as part of its backtracking mechanism to search through parallel dimensions of the state-space (e.g. through numeric variables).
We show empirically that our planner outperforms the state-of-the-art numeric planners in a number of domains using minimal extra domain knowledge.
Planning in domains that require numerical variables, for example, to drive robots in the physical world, must represent and search through a space defined by real-valued functions with a potentially infinite domain, range, or both.
This type of numeric planning problem poses challenges in two ways.
First, the description formalisms BID6 might not make it easy to express the numeric functions and its variables, resulting in a description process that is time consuming and error-prone for real-world domains BID17 .
Second, the planners that try to solve such numeric problems must find efficient strategies to find solutions through this type of state-space.
Previous work on formalisms for domains with numeric values developed the Semantic Attachment (SA) construct BID3 ) in classical planning.
Semantic attachments were coined by (Weyhrauch 1981) to describe the attachment of an interpretation to a predicate symbol using an external procedure.
Such construct allows the planner to reason about fluents where numeric values come from externally defined functions.
In this paper, we extend the basic notion of semantic attachment for HTN planning by defining the semantics of the functions used as semantic attachments in a way that allows the HTN search and backtracking mechanism to be substantially more efficient.
Our current approach focused on depth-first search HTN implementation without heuristic guidance, with free variables expected to be fullyground before task decomposition continues.Most planners are limited to purely symbolic operations, lacking structures to optimize usage of continuous resources involving numeric values BID9 .
Floating point numeric values, unlike discrete logical symbols, have an infinite domain and are harder to compare as one must consider rounding errors.
One could overcome such errors with delta comparisons, but this solution becomes cumbersome as objects are represented by several numeric values which must be handled and compared as one, such as points or polygons.
Planning descriptions usually simplify such complex objects to symbolic values (e.g. p25 or poly2) that are easier to compare.
Detailed numeric values are ignored during planning or left to be decided later, which may force replanning BID17 .
Instead of simplifying the description or doing multiple comparisons in the description itself, our goal is to exploit external formalisms orthogonal to the symbolic description.
To achieve that we build a mapping from symbols to objects generated as we query semantic attachments.
Semantic attachments have already been used in classical planning BID3 ) to unify values just like predicates, and their main advantage is that new users do not need to discern between them and common predicates.
Thus, we extend classical HTN planning algorithms and their formalism to support semantic attachment queries.
While external function calls map to functions defined outside the HTN description, we implement SAs as semi co-routines BID1 , subroutines that suspend and resume their state, to iterate across zero or more values provided one at a time by an external implementation, mitigating the potentially infinite range of the external function.Our contributions are threefold.
First, we introduce SAs for HTN planning as a mechanism to describe and evaluate external predicates at execution time.
Second, we introduce a symbol-object table to improve the readability of symbolic descriptions and the plans generated, while making it easier to handle external objects and structures.
Finally, we empirically compare the resulting HTN planner with a modern classical planner BID10 in a number of mixed symbolic/numeric domains showing substantial gains in speed with minimal domain knowledge.
We developed a notion of semantic attachments for HTN planners that not only allows a domain expert to easily define external numerical functions for real-world domains, but also provides substantial improvements on planning speed over comparable classical planning approaches.
The use of semantic attachments improves the planning speed as one can express a potentially infinite state representation with procedures that can be exploited by a strategy described as HTN tasks.
As only semantic attachments present in the path decomposed during planning are evaluated, a smaller amount of time is required when compared with approaches that precompute every possible value during operator grounding.
Our description language is arguably more readable than the commonly used strategy of developing a domain specific planner with customized heuristics.
Specifically, we allow designers to easily define external functions in a way that is readable within the domain knowledge encoded in HTN methods at design time, and also dynamically generate symbolic representations of external values at planning time, which makes generated plans easier to understand.Our work is the first attempt at defining the syntax and operation of semantic attachments for HTNs, allowing further research on search in SA-enabled domains within HTN planners.
Future work includes implementing a cache to reuse previous values from external procedures applied to similar previous states BID4 ) and a generic construction to access such values in the symbolic layer, to obtain data from explored branches outside the state structure, i.e. to hold mutually exclusive predicate information.
We plan to develop more domains, with varying levels of domain knowledge and SA usage, to obtain better comparison with other planners and their resulting plan quality.
The advantage of being able to exploit external implementations conflicts with the ability to incorporate such domain knowledge into heuristic functions, as such knowledge is outside the description.
Further work is required to expose possible metrics from a SA to heuristic functions.

assistant: text: An approach to perform HTN planning using external procedures to evaluate predicates at runtime (semantic attachments).