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AI::Prolog - Perl extension for logic programming.


 use AI::Prolog;
 use Data::Dumper;

 my $database = <<'END_PROLOG';
   append([], X, X).
   append([W|X],Y,[W|Z]) :- append(X,Y,Z).

 my $prolog = AI::Prolog->new($database);
 my $list   = $prolog->list(qw/a b c d/);
 while (my $result = $prolog->results) {
     print Dumper $result;


 AI::Prolog is merely a convenient wrapper for a pure Perl Prolog compiler.
 Regrettably, at the current time, this requires you to know Prolog.  That will
 change in the future.


In Perl, we traditionally tell the language how to find a solution. In logic programming, we describe what a solution would look like and let the language find it for us.


For those who like to just dive right in, this distribution contains a Prolog shell called aiprolog and two short adventure games, and If you have installed the aiprolog shell, you can run either game with the command:

 aiprolog data/
 aiprolog data/

When the aiprolog shell starts, you can type start. to see how to play the game. Typing halt. and hitting return twice will allow you to exit.

See the bin/ and data/ directories in the distribution.

Additionally, you can read AI::Prolog::Article for a better description of how to use AI::Prolog. This document is an article originally published in The Perl Review ( and which they have graciously allowed me to redistribute.

See also Robert Pratte's article, "Logic Programming with Perl and Prolog" ( for more more examples.


AI::Prolog is a pure Perl predicate logic engine. In predicate logic, instead of telling the computer how to do something, you tell the computer what something is and let it figure out how to do it. Conceptually this is similar to regular expressions.

 my @matches = $string =~ /XX(YY?)ZZ/g

If the string contains data that will satisfy the pattern, @matches will contain a bunch of "YY" and "Y"s. Note that you're not telling the program how to find those matches. Instead, you supply it with a pattern and it goes off and does its thing.

To learn more about Prolog, see Roman Barták's "Guide to Prolog Programming" at Amongst other things, his course uses the Java applet that AI::Prolog was ported from, so his examples will generally work with this module.

Fortunately, Prolog is fairly easy to learn. Mastering it, on the other hand, can be a challenge.

USING AI::Prolog

There are three basic steps to using AI::Prolog.

Create the Prolog program.
Create a query.
Run the query.

For quick examples of how that works, see the examples/ directory with this distribution. Feel free to contribute more.

Creating a logic program

This module is actually remarkable easy to use. To create a Prolog program, you simply pass the Prolog code as a string to the constructor:

 my $prolog = AI::Prolog->new(<<'END_PROLOG');
    steals(PERP, STUFF) :-

Side note: in Prolog, programs are often referred to as databases.

Creating a query

To create a query for the database, use query.


Running a query

Call the results method and inspect the results object:

  while (my $result = $prolog->results) {
      # $result = [ 'steals', 'badguy', $x ]
      print "badguy steals $result->[2]\n";


See AI::Prolog::Builtins for the built in predicates.



This is the constructor. It takes a string representing a Prolog program:

 my $prolog = AI::Prolog->new($program_text);

See AI::Prolog::Builtins and the examples/ directory included with this distribution for more details on the program text.

Returns an AI::Prolog object.


One can "trace" the program execution by setting this property to a true value before fetching engine results:

 while (my $result = $engine->results) {
     # do something with results

This sends trace information to STDOUT and allows you to see how the engine is trying to satify your goals. Naturally, this slows things down quite a bit.

Calling trace without an argument returns the current trace value.


You can get access to the full, raw results by setting raw_results to true. In this mode, the results are returned as an array reference with the functor as the first element and an additional element for each term. Lists are represented as array references.

 $prolog->query('steals(badguy, STUFF, VICTIM)');
 while (my $r = $prolog->results) {
     # do stuff with $r in the form:
     # ['steals', 'badguy', $STUFF, $VICTIM]

Calling raw_results without an argument returns the current raw_results value.

This is the default behavior.


This method quotes a Perl string to allow AI::Prolog to treat it as a proper Prolog term (and not worry about it accidentally being treated as a variable if it begins with an upper-case letter).

 my $perl6 = AI::Prolog->quote('Perl 6'); # returns 'Perl 6' (with quotes)

At the present time, quoted strings may use single or double quotes as strings. This is somewhat different from standard Prolog which treats a double-quoted string as a list of characters.

Maybe called on an instance (the behavior is unchanged).


Turns a Perl list into a Prolog list and makes it suitable for embedding into a program. This will quote individual variables, unless it thinks they are a number. If you wish numbers to be quoted with this method, you will need to quote them manually.

This method does not add the list brackets.

 my $list = AI::Prolog->list(qw/foo Bar 7 baz/);
 # returns:  'foo', 'Bar', 7, 'baz'

May be called on an instance (the behavior is unchanged).



This method is useful when you wish to combine the query() and results() methods but don't care about the results returned. Most often used with the assert(X) and retract(X) predicates.


This is a shorthand for:

 1 while $prolog->results;

This is important because the query() method merely builds the query. Not until the results() method is called is the command actually executed.


After instantiating an AI::Prolog object, use this method to query it. Queries currently take the form of a valid prolog query but the final period is optional:

 $prolog->query('grandfather(Ancestor, julie).');

This method returns $self.


After a query has been issued, this method will return results satisfying the query. When no more results are available, this method returns undef.

 while (my $result = $prolog->results) {
     # [ 'grandfather', $ancestor, 'julie' ]
     print "$result->[1] is a grandfather of julie.\n";

If raw_results is false, the return value will be a "result" object with methods corresponding to the variables. This is currently implemented as a Hash::AsObject so the caveats with that module apply.

Please note that this interface is experimental and may change.

 $prolog->query('steals("Bad guy", STUFF, VICTIM)');
 while (my $r = $prolog->results) {
     print "Bad guy steals %s from %s\n", $r->STUFF, $r->VICTIM;

See raw_results for an alternate way of generating output.


See AI::Prolog::Builtins and AI::Prolog::Engine for known bugs and limitations. Let me know if (when) you find them. See the built-ins TODO list before that, though.


  • Why does this take so long to run?

     perl examples/ 3

    On my Mac that takes over an hour to complete.

  • Support for more builtins.

  • Performance improvements.

    I have a number of ideas for this, but it's pretty low-priority until things are stabilized.

  • Add "sugar" interface.

  • Better docs.

  • Tutorial.

  • Data structure cookbook.

  • Better error reporting.


None by default. However, for convenience, you can choose ":all" functions to be exported. That will provide you with Term, Parser, and Engine classes. This is not recommended and most support and documentation will now target the AI::Prolog interface.

If you choose not to export the functions, you may use the fully qualified package names instead:

 use AI::Prolog;
 my $database = AI::Prolog::Parser->consult(<<'END_PROLOG');
 append([], X, X).
 append([W|X],Y,[W|Z]) :- append(X,Y,Z).

 my $query  = AI::Prolog::Term->new("append(X,Y,[a,b,c,d]).");
 my $engine = AI::Prolog::Engine->new($query,$database);
 while (my $result = $engine->results) {
     print "$result\n";






Roman Barták's online guide to programming Prolog:


Curtis "Ovid" Poe, <moc tod oohay ta eop_divo_sitruc>

Reverse the name to email me.

This work is based on W-Prolog,, by Dr. Michael Winikoff. Many thanks to Dr. Winikoff for granting me permission to port this.

Many features also borrowed from X-Prolog with Dr. Jean Vaucher's permission.


Patches and other help has also been provided by: Joshua ben Jore and Sean O'Rourke.


Copyright 2005 by Curtis "Ovid" Poe

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.