Introduction to Prolog
========================

What is Prolog?


Prolog (short for *PROgramming in LOGic*) is a logic programming language that is very different from languages like Python, Java, or C.  
Instead of telling the computer *how* to do something step by step, you tell it *facts* and *rules*, and then ask *questions*.

Prolog is commonly used in:
- Artificial Intelligence (AI)
- Expert systems
- Natural language processing
- Rule-based systems

Prolog works by using **logic, pattern matching, and backtracking** to find answers to questions.


Basic Concepts in Prolog


Facts


Facts are statements that describe relationships or truths. They end with a period.

Example:

.. code-block:: prolog

   likes(john, pizza).
   likes(mary, sushi).
   likes(john, pasta).

These facts say:
- John likes pizza
- Mary likes sushi
- John likes pasta


Rules


Rules define logic using conditions.  
They use `:-` which can be read as “if”.

Example:

.. code-block:: prolog

   food_lover(X) :- likes(X, pizza).

This rule means:
- A person `X` is a food lover **if** they like pizza


Queries


Queries are questions you ask Prolog.  
They start with `?-` when typed into the Prolog interpreter.

Example:

.. code-block:: prolog

   ?- likes(john, pizza).

Prolog will answer:

.. code-block:: text

   true.

You can also ask:

.. code-block:: prolog

   ?- likes(john, What).

Prolog will return all foods John likes.


Building Simple Prolog Scripts


Creating a Prolog File


Prolog scripts are usually saved with a `.pl` extension.

Example file name:

.. code-block:: text

   family.pl


Example 1: Family Relationships


Below is a simple Prolog script that defines family relationships.

.. code-block:: prolog

   parent(john, mary).
   parent(john, sam).
   parent(susan, mary).
   parent(susan, sam).

   father(X, Y) :- parent(X, Y), male(X).
   mother(X, Y) :- parent(X, Y), female(X).

   male(john).
   female(susan).

You can ask questions like:

.. code-block:: prolog

   ?- parent(john, mary).
   ?- father(john, mary).
   ?- mother(susan, sam).


Example 2: Simple Math Rule


Prolog can also perform simple logic-based math.

.. code-block:: prolog

   double(X, Y) :- Y is X * 2.

Query example:

.. code-block:: prolog

   ?- double(5, Result).

Prolog will respond:

.. code-block:: text

   Result = 10.


Example 3: Checking Conditions


This example checks if a number is even.

.. code-block:: prolog

   even(X) :- 0 is X mod 2.

Query:

.. code-block:: prolog

   ?- even(4).
   ?- even(7).

Prolog will return `true` for even numbers and `false` for odd numbers.


How Prolog Finds Answers


Prolog searches for answers by:
- Matching facts and rules
- Trying possible solutions
- Backtracking if a solution fails

This means Prolog can automatically find multiple answers without writing loops.


Conclusion


Prolog is a powerful language that focuses on logic rather than instructions.  
By defining facts and rules, you can build intelligent systems that answer complex questions.

Even simple Prolog scripts show how different logic programming is compared to traditional programming languages, making it especially useful for problem-solving and AI-related tasks.