Datalog for CSE 350

Datalog
CSE 350, Spring 2003

Datalog is a subset of Prolog so obtaining a release of Prolog will allow you to do the Datalog examples in the text book

Obtaining Prolog

There are numerous releases of Prolog, many are free. One free distribution is GNU Prolog. There are source and binary versions available, including Windows and Linux versions. See http://gnu-prolog.inria.fr for more information or just get the Windows setup binary from ftp://ftp.inria.fr/INRIA/Projects/contraintes/gprolog/setup-gprolog-1.2.16.exe. Once you have downloaded it, double click it to start setup. It will create a shortcut in a program group and a desktop icon if you want it to. You may want to change the working directory (where it will look for your datalog programs) by editing the shortcut properties.

GNU Prolog is loaded on the computers in ITEB C27 (the lab says "Unix" on the outside, but the computers are Windows 2000). Also, it can be installed on your Z drive on the learning center computers. If you load it on your Z drive on one machine, it will be on your Z drive on all of them, in E2 and ITEB learning centers. The Z drive is entirely yours so it stays until you explicitly delete it. It takes ~10 meg. Here is what to do:

create a directory Z:\private_home\Class
download the Windows setup file from the GNU Prolog site
run the setup program
when it asks for a drive, say Z:\private_home\Class
when it asks what program group, say don't make a program group
when it asks to make a desktop icon, say no desktop icon
after setup (which reports an error writing to autoexec.bat), create a file named startGnu.bat in the directory
Z:\private_home\Class\Gnu-Prolog , containing only the following two lines:

set Path=%path%;Z:\private_home\Class\GNU-Prolog\bin
Z:\private_home\Class\GNU-Prolog\bin\gprolog.exe
To run Datalog, double click the batch file.

Once you have Prolog installed, you'll want the to download at least the sample "college database" (DatalogCollege.txt) from the class handout. You may also want the "student database" (DatalogBook.txt) example from chapter 8 in the textbook; this file contains both facts (data) and queries. You can save them and work with them directly as text files with extension ".txt", but many people prefer to give them an extension of ".p" for Prolog. Either works fine.

Learning how it works

There are on line manuals and tutorials that can give you a "crash course" in Prolog. One very detailed tutorial, available at http://www.csupomona.edu/~jrfisher/www/prolog_tutorial/pt_framer.html, is written for SWI Prolog but is applicable to GNU Prolog as well.

The GNU Prolog web site has a reference manual on line.

Sample session

If you want to work with the DatalogBook.txt file discussed above, here is how a sample session might look. Blank lines have been removed to save space and comments (marked with %%) have been added, but your output should look very similar.

 %% When the session begins I load my program, the "singleton
variables" 

%% warnings just mean I could have been more efficient by using 

%% anonymous variables when I don't use their contents. 

?- consult('C:/usr/rs/sch/CSE350/Working/DatalogBook.txt').

 

compiling C:\usr\rs\dev\prolog\DatalogTest.txt for byte code... 

C:\usr\rs\dev\prolog\DatalogBook.txt:14 warning: singleton variables
[Major,Grade1,Grade2] for firstreq/1

C:\usr\rs\dev\prolog\DatalogBook.txt:15 warning: singleton variables
[Major] for scndreq/1 C:\usr\rs\dev\prolog\DatalogBook.txt:19 warning:
singleton variables [Grade] for hastaken/2

C:\usr\rs\dev\prolog\DatalogBook.txt compiled, 20 lines read - 3638
bytes written, 150 ms 

yes 

%% End of their report, begin my typing queries. 

| ?- firstreq(Name). 

Name = 'Jim Jones' ? a 

Name = 'Jim Black' 

no 

%% observe that Prolog only gave me one match, 

%% I typed 'a' to get all the rest (just 1 in this case)

| ?- scndreq(Name). 

Name = 'Jim Jones' a 

Name = 'Jim Black'

(20 ms) no 

| ?- scndreq2(Name). 

Name = 'Jim Jones' ? a 

Name = 'Jim Black' 

no 

| ?- lacks_cs143(Name). 

no 

| ?- lacks_cs123(Name). 

Name = 'Jim Jones' ? a 

Name = 'Jim Black' 

yes 

%% I'm done, I want to quit 

| ?- halt.

Important Notes

Syntax is changed slightly from the book examples. Use ":-" instead of "<-" to separate the head from the body of predicates, and use "\+" for negation.
Although ad hoc queries can be typed at the command prompt, they must use predicates whose definitions come from an external file that is "consulted". (EMacs is nice for this because it has syntax highlighting for Prolog and leaves the file closed, and therefore readable while you are editing it.)
Consulting a new program does not remove the old one from memory. It may redefine some predicates, but it doesn't remove old facts and rules. This can be to your advantage because you can have one file with common data and other files with the rules or queries you are testing against the data.
All rules, facts, and queries must end with periods (".") or Prolog will keep "waiting for more". You can put the period on the next line (or even after a bunch of "Enters") if you forgot.
Most punctuation is insensitive to whitespace but when you use the :- sequence, make sure to keep space around it so the "-" isn't interpreted as a unary minus.
The order you define predicates in is important. If you define the terminating case after the recursive case, your program might loop forever.
A query usually returns just the first matching tuple. To get subsequent tuples type ";" after each solution to get the next match, or type "a" to get all of the remaining matches.
All variables must begin with a capital letter, all predicates must begin with a lower case letter. Constants may either begin with a lower case letter and not contain spaces, or be enclosed in single quotes.
Any line beginning with two adjacent percent signs (%%) is a comment and Prolog ignores it.
The command prompt for GNU Prolog is "| ?- ". When you see it in examples, don't try to type it. If you don't see it on your screen, it means Prolog isn't ready for your input. You can interrupt Prolog by typing CTRL-C and when it asks what to do, press the key "a" to abort the current action.
When working with files, the path separator is a forward slash (like in UNIX). File names that begin with upper case letters must be in quotes.
You can use the command change_directory('dirname') to change your working directory, to avoid retyping long paths if you are frequently consulting a program not in your current working directory. You can use the command working_directory('dirname') to see the current directory.
When pasting text into GNU Prolog, using the right-mouse-click and picking "paste" puts the text on the screen, but Prolog will not evaluate it. Instead, you MUST use the Ctrl-Alt-v or the "edit" - "paste" menu.

This page last updated on 9/29/03

Datalog CSE 350, Spring 2003

Datalog
CSE 350, Spring 2003