The Basics of Prolog Programming Language

Prolog is a logic programming language that is based on facts and rules. The idea is to create a set of facts related to specific domain. After that, we create rules that can be used to answer questions (or queries). For example, we may create a facts that can be used to identify a human if he is a man or a woman. In top of that, we can add more facts to state that this human is a father or a mother of another human. Once we have the given facts, we can create rules. For example, we can define a rule that can be used to find  the father of another human.


In this post, we will learn the basics of Prolog language in the simplest way. For this purpose, we will be using an online editor that can be used to write Prolog code and test it. The tool is called SWISH and can be fount at http://swish.swi-prolog.org/.

When we open the link, we will see that the page has 3 parts, one for creating new file, one for the output and one for asking questions.

The Layout of The Tool

To start writing Prolog code, simply click 'Program' and the code editor will appear.

Prolog Facts

In order to create Prolog rules, first we have to provide some facts. Suppose that we have 5 humans, one has the name 'john', another one has the name 'sara', another one has the name 'ali' and one has the name 'paul' and the last one with the name 'hannah'. The first facts that we are going to write is to state that a human can be a man or a woman. We can till from the names that 'john' is a man and 'sara' is a woman. Writing such facts in Prolog can be done as follows:


man(john).
man(ali).
man(paul).
woman(sara).
woman(hannah).



In Prolog, a fact is written as follows


fact_name(fact_attrs...).


Every fact we write in Prolog must end with a 'dot', and each fact can have one or more parameters (or attributes). Now that we know how to write facts, let's add more facts to our program. Let's say that 'john' is a father of 'ali', 'paul' and 'hannah'. Also let's say that 'sara' is the mother of 'ali', 'paul' and 'hannah'.


man(john).
man(ali).
man(paul).
woman(sara).
woman(hannah).
father(john,ali).
father(john,paul).
father(john,hannah).
mother(sara,ali).
mother(sara,paul).
mother(sara,hannah).



Once we have the given facts, we can ask questions about them and get answers. For example, we can ask the question 'Is john a man?'. Another question we could ask is 'Who is the mother of ali?'. A question in Prolog is called "query". In the next section, we will learn how to ask questions in Prolog.

Prolog Queries

One of the simplest queries in Prolog are the queries that are applied to the facts. The result of such queries will be either "true" or "false" if all the attributes of the query are known. For example, the result of running a query in one of the first 5 facts with one of the 5 names will be either "true" or "false".

In the example that we will see, we want to know if a person is a man or not. Prolog query has the following basic form: "?- rule_or_fact(vars...)". It is possible to construct more complex queries but we will do it later. For now, let's focus at the basics. The "?-" part is provided by the editor by default. All what we have to do is to write the other part.

In SWISH, we write the query in the place where it says " Your query goes here ...". After writing the query, All what we have to do is to click "run" and the result will appear. Let's give it a try.

As we can see, "ali" is a man since we got "true". Now we want to try something else. Suppose that we would like to get All women. To do that, we have to learn about new concept in Prolog which is variables.

Prolog Variables

A variable in Prolog can be used to answer queries. Variables are passed as an arguments to rules and facts to get specific values. Any parameter that starts with a capital letter is considered as a variable. For example, if we say 'woman(Sara)', 'Sara' is now a variable not an attribute. The  final value of the variable is based on the provided facts and rules. Now, to get all women, we pass a variable to the fact 'woman()'. Prolog will set the value of that variable to the name of the woman.

 As we can see, we have got our first woman. To get the rest, all what we have to do is to click "Next" till the execution is finished.

Now let's try something new. Suppose that we would like to know who is the father of 'ali'. To ask such as query, we write it as 'father(X, ali)'. In this case, 'X' will be equal to the father of 'ali'.

Suppose that we have a person that has unknown father. What will be the value of 'X' in this case? Let's try it. To do that, we will add new man called 'ibrahim' and try to find his father.

Simply we got 'false'. Whenever we try to apply a rule that does not exist, we will get 'false'. Also if we try the rule with a name that does not exist, we will get false. For example, if we try 'father(X, dania)', this will also give false.