Answer Sources in Prolog

Nan.System.Sources
Nan.System.Sources/Prolog 1.2.0-beta
Answer Sources in Prolog
Copyright 2015-2017 Julio P. Di Egidio
Licensed under GNU GPLv3.
http://julio.diegidio.name/Projects/Nan.System.Sources/
https://github.com/jp-diegidio/Nan.System.Sources-Prolog/

This library implements Answer Sources in Prolog.
This is an open and free project.
Source code available and pull requests accepted.

What are answer sources
How to use this library
Still to be done

1. What are answer sources

Answer sources can be seen as generalized iterators, allowing a given program to control answer production in another. Each source works as a separate program interpreter, and, in our implementation, in its own process, via multi-threading.

For the rationale and design, we have followed almost exactly Paul Tarau on fluents [1], with at least the following differences:

Our sources use multi-threading unlike Tarau's;
Our sources execute arbitrary Prolog goals, i.e. are not restricted to Horn clauses;
Thanks to multi-threading, our next operation (get in Tarau's) supports an asynchronous pattern: e.g. this allows for easy implementation of parallelism in combinators;
Our sources implement a reset operation that restarts the enumeration, so Tarau's split combinator is not necessary;
The overhead of threading being unnecessary for combinators, support for combined sources is built into our source object module;
Our answer term extends Tarau's to report exact determinism.

TODO: A still needed final step is to extend our sources from fluents to interactors [2], which requires implementation of a yield operation (return in Tarau's) to be available to the target predicate, i.e. in the context of the source worker loop.

A "Preview" version of Answer Sources was initially presented here [3]. A flow diagram for the yield operation (denoted return in the diagram) was later presented here [4].

[1] P. Tarau, "Fluents: A Refactoring of Prolog for Uniform Reflection and Interoperation with External Objects":
http://www.cse.unt.edu/~tarau/research/LeanProlog/RefactoringPrologWithFluents.pdf
[2] P. Tarau, A. Majumdar, "Interoperating Logic Engines":
http://www.cse.unt.edu/~tarau/research/LeanProlog/InteroperatingLogicEngines.pdf
[3] J.P. Di Egidio, "Answer Sources in Prolog (SWI) - Preview":
http://seprogrammo.blogspot.it/2015/09/answer-sources-in-prolog-swi-preview.html
[4] J.P. Di Egidio, "Answer Sources: from Fluents to Interactors":
http://seprogrammo.blogspot.co.uk/2015/09/answer-sources-from-fluents-to.html

2. How to use this library

This library comprises the following modules:

sources (nan/system/sources.pl)
Provides the predicates that implement Answer Sources in Prolog.
sources_types (nan/system/sources_types.pl)
Provides type testing predicates to validate arguments in user code.

For detailed documentation on the available modules and predicates, please see the code docs accompanying the Prolog code files.

This library was developed and tested with SWI-Prolog 7.3.25.

Basic usage example:

?- pack_install(nan_system_sources).
true.

?- use_module(library(nan/system/sources)).
true.

?- using_source(I, between(1, 2, I), _S,
   (   repeat,
       source_next(_S, answer(_Det, the(I))),
       (_Det == last -> !; true)
   )).
I = 1 ;
I = 2.

3. Still to be done

Extend from fluents to interactors (implement yield/1).
Improve compatibility with other Prolog systems (implement a compatibility layer).
Improve documentation and test cases.