.. $Id: examples.txt bd350590fc64 2009-11-03 mtnyogi $
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    page-description:
        An overview of the examples provided with Pyke.
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    filedate: $Id: examples.txt bd350590fc64 2009-11-03 mtnyogi $
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========
Examples
========

Several examples are included to help you become familiar with Pyke.  These
are all in an ``examples`` directory; which forms the top-level Python package
for importing::

    >>> from examples.towers_of_hanoi import test
    >>> test.test(2)
    got 1: ((0, 1), (0, 2), (1, 2))
    got 2: ((0, 2), (0, 1), (2, 0), (1, 2), (0, 2))

Thus the source directory (containing the ``examples`` directory) must be on
your Python path.  This is most easily accomplished by running the examples
from the source directory.

Each example is in its own sub-directory and has a README.txt file to get you
started.  They all have `.krb files`_ and a Python module to run the example
that also demonstrates `how to call Pyke`_ from your Python program.


Family_relations
================

This is a very good basic example to start with.

The family_relations example takes an initial set of facts_ about people
(stated in a `.kfb file`_)::

    son_of(david_r2, david_r, sarah_r)
    daughter_of(shirley, david_r, sarah_r)

And figures out how any two people are related::

    david_r2, shirley are ('brother', 'sister')

This same problem is solved in four different ways so that you can compare
them:

- Forward-chaining_ only
- Backward-chaining_ only
- Backward-chaining only with a few rule optimizations that make the rules
  run 100 times faster!
- A mix of forward-chaining and backward-chaining with some use of plans_ added
  too.

The test.py program also demonstrates how to use krb_traceback_ and the
print_stats_ function.


Knapsack
========

At the `PyCon 2008`_ conference, somebody asked about the `knapsack problem`_.
We found a solution in Prolog here__ (starting on page 19), and rewrote it in
Pyke.  This is a quick simple little example.

.. __: http://www.ise.gmu.edu/~duminda/classes/fall03/set3.ppt


Sqlgen
======

Pyke was originally developed as the control component for a web framework.
This example shows how Pyke can automatically generate SQL SELECT statements,
given a set of tables that the calling program has keys to and a tuple of the
desired column names.  Column names specified at the top-level in this tuple
are expected to have a single value each.  Nested tuples are used when
multiple rows are expected.  The column names in nested tuples make up the
columns in the result rows.

The top-level goal returns a plan_ that takes the key values for the initial
set of tables given to the goal and returns an immutable dictionary mapping
the column names to the values retrieved from the database.  The plan may be
used repeatedly without re-running the rules each time to figure out the
SELECT statements.  Thus, this acts like a SELECT statement compiler resulting
in queries with virtually no extra overhead.  It is *not*, however, an Object
Relational Mapper (ORM).

The data model used for the requested columns is that tables inherit the
columns from tables they link to.  So if there is a 1-many relationship
between tables A and B (1 A row has many B rows), the B table inherits the
columns from the A table through it's link to table A.  The Pyke rules will
automatically figure out the table joins for this.

The program automatically introspects the schema information.  For this
example, it assumes that ``id`` is the primary key for each table, and that
when one table links to another, it uses the target table name suffixed with
``_id`` as the column name.

This example was originally done using MySQL_ and includes the .sql files to
create the database, tables, and example data.  The example has since been
converted to use the Sqlite3 database to make it easier to run, as Sqlite3
does not require any setup (the Sqlite3 database file is included in the
example).

Sqlgen lacks more general capabilities that would be required for real use,
but may serve as a starting point for another project that's more complete.

This example also has much more elaborate rules than the prior two examples
and is a very real example of generating plans_.


Web_framework
=============

This example completes the Python web framework demo by adding rules to
automatically generate code to render HTML templates from the HTMLTemplate_
package (you can run ``easy_install HTMLTemplate`` to install the HTMLTemplate
package).  This uses the sqlgen_ example, above, to generate the SQL statements.

An HTMLTemplate does not include anything resembling program code in it, so
that your graphics designers can completely own the html files without the
developers having to modify them in any way.

Note that the code generated here is fully cooked_ code, custom built for
that specific schema and HTML template.  This runs extremely fast because
there is nothing left at run-time concerning parsing and figuring out the
HTML template, or constructing the SQL statements.

A test was done comparing this web framework example to the same example
done in `TurboGears 2`_ running against the same MySQL database.  The results
of the siege_ benchmark tests show that Pyke is just over 10 times faster than
TurboGears 2::

- Pyke: 791 trans/sec 
- TurboGears 2: 76 trans/sec

The demo is packaged as a WSGI_ application.  It also demonstrates the use of
multiple `rule bases`_ by using the sqlgen example above, as well as the
caching and reuse of plans_ to achieve the order of magnitude improvement in
performance over current practice.


