Logic Programming with Perl and Prolog

Computing languages can be addictive; developers sometimes blame themselves for perceived inadequacies, making apologies for them. That is the case, at least, when one defends his or her language of choice against the criticism of another language's devotee. Regardless, many programmers prefer one language, and typically ground that preference in a respect for that language's strengths.

Perl has many strengths, but two most often cited are its adaptability and propensity to work as a "glue" between applications and/or data. However, Perl isn't the only advantageous language: programmers have used C or even assembly to gain speed for years, and intelligent use of SQL allows the keen coder to offload difficult data manipulations onto a database, for example. Prolog is an often overlooked gem that, when combined with the flexibility of Perl, affords the coder powerful ways to address logical relationships and rules. In this article, I hope to provide a glimpse of the benefits that embedded Prolog offers to Perl programmers. Moreover, I hope that my example implementation demonstrates the ease with which one can address complex logical relationships.

A Bit About Prolog

For the sake of demonstration, I would like to frame a simple problem and solution that illustrate the individual strengths of Perl and Prolog, respectively. However, while I anticipate that the average reader will be familiar with the former, he or she may not be as familiar with the latter. Prolog is a logic programming language often used in AI work, based upon predicate calculus and first developed in 1972. There are several excellent, free versions of Prolog available today, including GNU Prolog and the popular SWI Prolog. For the Prolog initiate, I recommend checking out some of the free Prolog tutorials, either those linked from Wikipedia or from OOPWeb.

Prolog and Perl aren't exactly strangers, however. There are several excellent Perl modules available to allow the coder to access the power of Prolog quite easily, including the SWI module developed by Robert Barta, the Interpreter module by Lee Goddard, the Yaswi modules developed by Salvador Fandino Garcia, and the AI::Prolog module written by Curtis "Ovid" Poe. Poe has also recently provided a rather nice introduction to Prolog-in-Perl in an online-accessible format.

The Problem

There are many advantages to using Prolog within Perl. In the general sense, each language has its own advantages, and can thus complement the other. Suppose that I am building a testing harness or a logic-based query engine for a web application, where neither language easily provides all of the features I need. In cases such as these, I could use Prolog to provide the logic "muscle," and Perl to "glue" things together with its flexibility and varied, readily available modules on CPAN.

In my simple demonstration, I am going to posit the requirement that I take genealogical information built by another application and test relationships based upon a set of rules. In this case, the rules are defined in a Prolog file (an interesting intersection here is that both Perl and Prolog typically use the suffix .pl), while the genealogical information is contained in a Dot file readable by Graphviz. As such, I am going to make certain assumptions about the format of the data. Next, I am going to assume that I will have a query (web-based, or from yet another application) that will allow users to identify relationships (such as brothers, cousins, etc.).

Here are my Prolog rules:

is_father(Person)        :- is_parent(Person, _),
                            is_male(Person).
is_father(Person, Child) :- is_parent(Person, Child),
                            is_male(Person).
is_mother(Person)        :- is_parent(Person, _),
                            is_female(Person).
is_mother(Person, Child) :- is_parent(Person, Child),
                            is_female(Person).
ancestor(Ancestor, Person) :- is_parent(Ancestor, Person).
ancestor(Ancestor, Person) :- is_parent(Ancestor, Child),
                              ancestor(Child, Person).
is_sibling(Person, Sibling) :- is_parent(X, Person),
                               is_parent(X, Sibling).
is_cousin(Person, Cousin) :- is_parent(X, Person),
                             is_parent(Y, Cousin),
                             is_sibling(X, Y).

One advantage to separating my logic is that I can troubleshoot it before I even write the Perl code, loading the rules into a Prolog interpreter or IDE such as XGP (for Macintosh users) and testing them. However, AI::Prolog conveniently provides its own solution: by typing aiprolog at the command line, I can access a Prolog shell, load in my file, and run some tests.

At this point, however, I am mostly interested in accessing these rules from Perl. While there are several options for accessing Prolog from within Perl, the AI::Prolog module is perhaps the easiest with which to start. Moreover, it is quite simple to use, the rules used to build the Prolog database being fed in when creating the AI::Prolog object. The ability to hand the object constructor a filehandle is not currently supported, but would indeed be a nice improvement. While there are other ways to accomplish the task of reading in the data, such as calling the Prolog command consult, I will read in the Prolog file (ancestry.pl) and provide a string representation of the contents.

open( PROLOGFILE, 'ancestry.pl' ) or die "$! \n";
local $/;
my $prologRules = <PROLOGFILE>;
close( PROLOGFILE );
my $prologDB = AI::Prolog->new( $prologRules );

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