W-Prolog

Links2Go Key Resource Prolog Topic

Table of Contents

1. Introduction

W-Prolog has a nicer user interface than most Prolog systems (which typically provide a command line interface). It provides simple tracing and has an (optional) occur check.

The W-Prolog system is small and comparatively simple. However it is not particularly fast.

W-Prolog has been:

• Used as part of Roman Barták's Guide to Prolog Programming
• Used to compute kinship relationships at the center for social anthropology and computing at the University of Kent.
• Extended by Jean Vaucher with arithmetic, assert/retract, and cut. The result is XProlog.
• Used by Fernando Koch as the basis for the inference engine of AgentLight, a light-weight multi-agent platform running on small devices (PDAs, phones).
• Used by Fernando Koch as the basis for M-Prolog, part of 3APL-M, a light-weight implementation of the 3APL ("Triple-A-P-L") agent-oriented programming language.
• Ported to C# by Hugo Jose Ferreira, who also added a GUI to enable it to run on the PocketPC.
• Ported to Perl by Curtis 'Ovid' Poe.
• Extended by Boris van Schooten into YProlog (Boris extended XProlog)
• This page has been translated to Serbo-Croatian by Anja Skrba from Webhostinggeeks.com.

1.1 The Distribution

NEW W-Prolog is now also available as a clickable jar file (wp.jar)

If you want to try W-Prolog now (and are viewing this with a Java capable browser) just click the button below.

2. The Language

Program ::= Rule | Rule Program
Query ::= Term

Rule ::= Term . | Term :- Terms .
Terms ::= Term | Term , Terms
Term ::= Number | Variable | AtomName | AtomName(Terms)
                | [] | [Terms] | [Terms | Term]
                | print(Term) | nl | eq(Term , Term)
                | if(Term , Term , Term) | or(Term , Term ) | not(Term) | call(Term) | once(Term)
Number ::= Digit | Digit Number
Digit ::= 0 | ... | 9
AtomName ::= LowerCase | LowerCase NameChars
Variable ::= UpperCase | UpperCase NameChars
NameChars ::= NameChar | NameChar NameChars
NameChar ::= a | ... | z | A | ... | Z | Digit

Comments begin with a % and terminate at the end of the line or begin with /* and terminate with */.

Note that as of version 1.2 code of the form p(X) :- X will not work. Instead, use the new builtin call as follows: p(X) :- call(X).

Note also, that the clauses defining a predicate need to be contiguous. So, for example, use:

a :- b.
a :- c.
b :- defn.
c :- defn.

a :- b.
b :- defn.
a :- c.
c :- defn.

3. Usage

To run W-Prolog as an applet construct an HTML file containing the tag:

<center>
<h1>W-Prolog</h1>
<applet code=WProlog.class width=120 height=65>
</applet>
</center>

Alternatively, just use the address http://winikoff.github.io/software/wp/.

To run W-Prolog as a standalone application you need to have the Java Development Kit (JDK). The command is java WProlog where the directory containing the class files must be in the CLASSPATH.

3.1 Embedding the W-Prolog Engine

4. The User interface

The top row (labeled Query) contains a text field into which a Prolog atom can be typed (for example append(X,Y,[1,2,3,4]) - note that no terminating full stop is necessary). The first button (Run Query) will start the query running and the second (More) will seek alternative solutions to a succesful query.

The top text area (labeled Program) contains the program text. The bottom text area (labeled Results) contains the output from W-Prolog. The button marked Clear clears the results text area. The two toggles (Occur Check and Trace) control respectively whether the occur check is performed and whether tracing information is output.

To quit W-Prolog either click on the close window button (if your window manager supplies one or if you're using Windows) or select Quit from the menu.

4.1 Applet

The applet version of W-Prolog has four buttons:

Benchmark:

Runs the standard naive reverse benchmark. The formula 496 / runtime gives the speed in logical inferences per second.

About...:

Puts some brief information about the author (Michael Winikoff) and version (currently 1.2) into the program text area.

Append:

Puts a simple append program into the text area.

Documentation:

Opens a browser window on this file.

4.2 Application

5. Comments

Another example of undesired behavior under netscape concerns threads. Each query in W-Prolog runs in its own thread. This means that a long running (or non terminating) query won't freeze the interface (although it will slow subsequant queries down). This works, but not under netscape.

5.1 Bugs

5.2 Further Work

• Arithmetic
  NEW Joachim Wolf Laudien has suggested that the following predicates could be used to provide arithmetic:

  sum(X, zero, X).
  sum(X, succ(Y), succ(Z)) :- sum(X, Y, Z).
  
  product(X, zero, zero).
  product(X, succ(Y), Z) :- sum(X, P, Z), product(Y, X, P).
  

• Characters and strings
• Input
• Graphics and other access to Java libraries (eg. networking)
• Iterative Deepening
• Example Programs

6. Related Work

Another Prolog interpreter in Java can be found here. Unfortunately, it appears to use Lisp-ese syntax and has no documentation!

Update: Since I wrote W-Prolog and this page a number of other logic programming languages implemented using Java have appeared (including LL, JavaLog, Jinni, and MINERVA). You can find links to all of these Robert Tolksdorf's page on Languages for the Java VM.

A. History

Version 1.0

Version 1.1

• Changed user interface.
• Added ability to save file if run as an application.
• If run as an application W-Prolog uses a file requester for loading and saving.
• If run as an applet W-Prolog starts up in its own window.
• Changed default font.
• Removed all options.
• Fixed performance bug - the occur check was always being performed ...
• Fixed bug: if the query can't be parsed then the part of the query which caused the problem is highlighted. Previously, if the query had a syntax error then the program would be highlighted.
• Some optimisation - looking up clauses is now done once rather than at each execution step.
• Added indication of memory being used.
• Changed Engine to make embedding in another application easier.

Version 1.2

• Changed user intergace (again!) -- removed loading option in applets.
• Fixed bug with indexing.
• Added call.
• Added handling of QUIT event.

Version 1.2.1

• Fixed bug where a variable unified to itself created a circular binding rather than doing nothing.

B. Legalese

When making copies, please keep all files together (including this documentation) and do not change anything.