A Meta-Level Extension of Prolog

Prolog has many attractive features as a programming tool for artificial intelligence. These include code that is easy to understand, programs that are easy to modify, and a clear relation between its logical and procedural semantics. Moreover, it has proved possible to create clear and efficient implementations. Nonetheless, we perceive several shortcomings. Chief among these is difficulty representing dynamic databases (databases which change in time) and an apparent restriction to backward chaining, backtracking, and depth-first search. Our intent in this paper is to present an extension to Prolog, called metaProlog, which preserves the virtues of Prolog while introducing powerful constructions to attack these problems. This work is a direct continuation of the investigation into meta-level programming in logic begun by Bowen and Kowalski [1982]

Disjunctive Delimited Control

Delimited control is a powerful mechanism for programming language extension which has been recently proposed for Prolog (and implemented in SWI-Prolog). By manipulating the control flow of a program from inside the language, it enables the implementation of powerful features, such as tabling, without modifying the internals of the Prolog engine. However, its current formulation is inadequate: it does not capture Prolog's unique non-deterministic nature which allows multiple ways to satisfy a goal. This paper fully embraces Prolog's non-determinism with a novel interface for disjunctive delimited control, which gives the programmer not only control over the sequential (conjunctive) control flow, but also over the non-deterministic control flow. We provide a meta-interpreter that conservatively extends Prolog with delimited control and show that it enables a range of typical Prolog features and extensions, now at the library level: findall, cut, branch-and-bound optimisation, probabilistic programming, . . .Comment: New version of paper is available at: arXiv:2108.0297

Use of Higher-Order Unification for Implementing Program Transformers

Source-to-source program transformers belong to the class of meta-programs that manipulate programs as objects. It has previously been argued that a higher-order extension of Prolog, such as λProlog, makes a suitable implementation language for such meta-programs. In this paper, we consider this claim in more detail. In λProlog, object-level programs and program schemata can be represented using simply typed λ-terms and higher-order (functional) variables. Unification of these λ-terms, called higher-order unification, can elegantly describe several important meta-level operations on programs. We detail some properties of higher-order unification that make it suitable for analyzing program structures. We then present (in λProlog) the specification of several simple program transformers together with a more involved partial evaluator. With the depth-first control strategy of λProlog for both clause selection and unifier selection all the above mentioned specifications can be and have been executed and tested

Approaches to Interpreter Composition

In this paper, we compose six different Python and Prolog VMs into 4 pairwise compositions: one using C interpreters; one running on the JVM; one using meta-tracing interpreters; and one using a C interpreter and a meta-tracing interpreter. We show that programs that cross the language barrier frequently execute faster in a meta-tracing composition, and that meta-tracing imposes a significantly lower overhead on composed programs relative to mono-language programs.Comment: 33 pages, 1 figure, 9 table

On the Implementation of GNU Prolog

GNU Prolog is a general-purpose implementation of the Prolog language, which distinguishes itself from most other systems by being, above all else, a native-code compiler which produces standalone executables which don't rely on any byte-code emulator or meta-interpreter. Other aspects which stand out include the explicit organization of the Prolog system as a multipass compiler, where intermediate representations are materialized, in Unix compiler tradition. GNU Prolog also includes an extensible and high-performance finite domain constraint solver, integrated with the Prolog language but implemented using independent lower-level mechanisms. This article discusses the main issues involved in designing and implementing GNU Prolog: requirements, system organization, performance and portability issues as well as its position with respect to other Prolog system implementations and the ISO standardization initiative.Comment: 30 pages, 3 figures, To appear in Theory and Practice of Logic Programming (TPLP); Keywords: Prolog, logic programming system, GNU, ISO, WAM, native code compilation, Finite Domain constraint

Experimenting with independent and-parallel prolog using standard prolog

This paper presents an approximation to the study of parallel systems using sequential tools. The Independent And-parallelism in Prolog is an example of parallel processing paradigm in the framework of logic programming, and implementations like <fc-Prolog uncover the potential performance of parallel processing. But this potential can also be explored using only sequential systems. Being the spirit of this paper to show how this can be done with a standard system, only standard Prolog will be used in the implementations included. Such implementations include tests for parallelism in And-Prolog, a correctnesschecking meta-interpreter of <fc-Prolog and a simulator of parallel execution for <fc-Prolog