Introduction to SWI-Prolog & Its Graphical Tracer Programming ...

Introduction to SWI-Prolog
&
Its Graphical Tracer
Programming Languages Midterm Project
Phuong T. Tran
March 8, 2011

What is Prolog?
●
●
Prolog is:
– A logic programming language
– A declarative language
– Comprised of:
●
●
●
Facts
Rules
Queries
Useful in specific problems like AI and
scheduling.

Facts
●
●
●
has(bicycle,wheel,2).
This fact states that a bicycle has wheel, two of
them.
Other examples of facts:
– has(wheel,spoke,32).
– has(bicycle, frame,1).

Rules
●
●
abs(X,Y) :- X >= 0, Y is X
This rule states that for the predicate
”abs(X,Y)” where X is the parameter inputed,
and Y is the result.
● If X >=0, then Y is X.
●
This abs predicate is part of defining the
absolute value of X. The other part is:
● abs(X,Y) :- X < 0, Y is X * -1

Queries
●
Using the Facts from the earlier slide, here is
an example of a query:
– partof(wheel,spoke).
– Is spoke part of wheel?
●
Using the abs predicate rule:
– abs(-1,Y).
– What is the absolute value of -1?

Is an IDE Necessary for Prolog?
●
●
●
Starting off with Prolog only needs two tools:
– The Prolog compiler/implementation
– Text Editor
IDEs typically provide the following tools:
– A ”project/explorer” view for multiple files &
dependencies
– Graphical debugger
– GUI development tools
Of the above, a graphical debugger may be the most
useful part of an IDE for teaching and learning.

Launching SWI-Prolog

Let's Use the abs(X,Y) Rules
abs(X,Y) :- X >= 0, Y is X
abs(X,Y) :- X < 0, Y is X * -1
And compile...

Result of a Query
● The following is the of the query that inputs -1
and asks for the absolute value of -1.
● The result is Y = 1.

Using trace.
●
Typing in ”trace.” at the Prolog prompt allows
you to trace the step of execution. This is very
useful as a debugging tool, especially for more
complex programs.

Tracing abs(-1,Y)

Graphical Tracer
●
Type in the following to launch the graphical
tracer and trace the abs(X,Y) predicate.

Parts of the Graphical Tracer
●
The graphical tracer has three main parts:
– Bindings
– Call Stack
– Program steps
●
Everything is color coded.

Step 1
●
The green highlights the first step.

Step 2.
●
The next step is to consider the first condition:
X > = 0.

Step 3.
●
Condition is false, as highlighted in red.

Step 4.
●
The debugger continues on to the next
condition: X < 0.

Step 5 & 6
●
Green indicates that the condition is true.
Evaluates Y = X * -1. Program then exits.

Other Components of Graphical
Tracer
●
Thread viewing for more advanced programs

Other Components (cont.).
●
●
●
A Navigator to view all components
of a program
Useful for larger programs that is
comprised of many files.
Query Insert:

Console vs. Graphical Tracer
●
●
The Console and Graphical Tracer providers
the same information.
The Graphical Tracer advantages
– Color coding debugger and text editor
– Easier to view longer, more complex predicates
– Other Functions:
●
●
●
Edit program within debugger
Set breakpoints
Run queries within debugger.

Graphical Tracer as a Learning
Tool
●
●
●
●
Dividing the tracing into three sub windows allow for
easier viewing of the steps and stack calls.
Color coding helps to track steps, especially with
complex predicates, such as recursions.
Ability to edit, query, and compile in debugger saves
time from editing in text editor and then compiling in
different window.
Better for visual learners to see the background of
stack calls and step throughs.

Another IDE – Visual Prolog
●
●
Other IDEs exist, some more complex than the
tools that come with SWI-Prolog.
Visual Prolog is an IDE for a Windows OS.
– Text Editor
– Compiler, Linker, and Debugger
– Additional tools for developing GUIs, such as a
graphics editor, toolbar editor.

Summary
●
●
A graphical debugger, such as provided with
SWI-Prolog can be a very useful
teaching/learning tool for Prolog, especially for
visual learners.
A more complex IDE, such as Visual Prolog,
probably is not necessary for teaching because
it has more functions not needed for just
learning.

Further Research
●
●
●
Analyze a more complex predicate in the
graphical debugger, such as recursion.
Compare the different graphical debuggers,
such as SWI-Prolog and Visual Prolog.
Use the more advanced functions of the
debugger, such as breakpoints.

References
●
●
●
●
My implementation of Prolog Assignment for
abs and partof code.
Textbook ”7 Languages in 7 Weeks”
Http://www.swi-prolog.org/
http://www.visual-prolog.com/

Hours Spent on This Project
● Researching SWI Prolog and gtrace and Visual Prolog: 1.5
hours
●
●
●
●
●
●
●
Slide Planning: 1 hour.
Learning how to use gtrace: 45 minutes
Running programs through trace and gtrace with screenshots:
45 minutes.
Compiling and editing presentation: 1.5 hour.
Total time: 5.5 hours.
Average time I study for a midterm: 3 hours.
Time taking a midterm: 2 hours or less.