A complete indexing scheme for WAM-based abstract machines

Abstract

This paper introduces an indexing scheme for logic programming languages, complete in considering all parameters and nested terms. Nevertheless it does not lead to a code-size explosion, because the sets of valid clauses are not encoded by special control flow instructions. Instead, some kind of identifiers of the applicable clauses are stored in (clause) sets allowing the stepwise restriction of alternative clauses during the runtime analysis of the parameters. We present the compilation scheme to integrate this approach in existing compilers. It shows the cooperation of the new instructions, the strict separation of the indexing code from the code executing the compiled clauses, and it demonstrates the easiness of integrating a complete indexer in compilers. The compilation scheme was originally developed for the stack-based narrowing machine of the functional logic language BABEL. We describe, however, the slightly modified indexing scheme for the usual WAM-architecture and assume Prolog as the language platform.