Extension table built-ins for Prolog

Changguan Fan, Suzanne Dietrich

Research output: Contribution to journalArticle

11 Scopus citations


The ET* algorithm is a complete evaluation strategy for Datalog programs, which are logic programs without function symbols. The ET* algorithm uses extension tables and depth-first iterative deepening to provide the evaluation of pure function-free logic programs as declarative specifications. Extension tables are a memo facility that the algorithm uses both to cut infinite derivation paths for complete evaluation and to optimize the evaluation of logic programs. The original implementation of the ET* algorithm incorporated extension tables as part of the Prolog database using the built-in predicates assert and retract. The advantage of implementing the extension table using the Prolog database is the portability of the ET* algorithm. There are several disadvantages, however, with this approach. One disadvantage is the cost associated with the built-in predicates assert and retract, which are known to be expensive operations in most current Prolog systems. Another disadvantage is the differences across implementations in the semantics that these built-ins provide for dynamic predicates. This paper presents an efficient implementation of extension tables as a global data structure in Prolog, which includes a set of built-in primitives for manipulating the extension table. The ET* algorithm is updated to reflect the utilization of the global extension table data structure. The implementations of the ET* algorithm are compared using time and space performance on a variety of benchmark programs.

Original languageEnglish (US)
Pages (from-to)573-597
Number of pages25
JournalSoftware - Practice and Experience
Issue number7
Publication statusPublished - Jul 1992


ASJC Scopus subject areas

  • Computer Graphics and Computer-Aided Design
  • Software

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