Table of contents

Intro

Prolog is a logic-programming language that does somethings extremely, like searching, well. It’s easy to get confused if used to other-paradigm languages (that’s what happend to me in the beginning).
In the logic paradigm a program is a theory, consisting on facts and rules written as Horn clauses, then once the theory is written up, we can do some queries based on the theory.
If you have experience with SQL and relational databases, it looks really similar.

A Horn clause is a special kind of logical formula that has at most one positive (unnegated) literal.

In prolog:

• theory -> we write the theory/program in a .pl file
• queries -> the result of these queries is based on the .pl file, also called database

this is a really simple example

male(tommy).
male(muni).
male(ett).

female(gia).
female(lula).

male(roby) :- 
	female(lula).

male(andrea) :- 
	female(tommy).example.pl

for example the queries:

• male(ett). => yes
• female(muni). => no
• male(roby). => yes
• male(andrea). => no

Theory

Theory in prolog is the entire set of rules and facts that define the logical structure and knowledge base of a Prolog program, forming the ground upon which inference is performed.

Facts

Facts in Prolog are the simplest kind of statement, used to declare information that is unconditionally true in your program’s knowledge base. A fact asserts a relationship or property about specific objects or values, without any conditions attached.

parent(john, mary). %% John is parent of Mary
sunny. %% it is sunny
likes(alice, pizza). %% alice likes pizza
male(bob). %% bob is  a malefacts.pl

In facts:

• Atoms are constant symbols, starting with a lowercase letter or enclosed in single quotes.
• Variables are identifiers beginning with an uppercase letter or an underscore.
• Complex terms are structures consisting of a functor and arguments (like loves(Vincent, mia), where loves is the functor).

Rules

Rules in prolog are logical statements that define relationships between facts and allow deduction of new information from existing data. A rule, Head :- Body. is structured so Head is the the predicate (what i want to enstablish / find out if it’s true), and Body is a sequence of conditions that must all be true for the Head to be true. The :- can be read as an “if” so the rule can means “Head is true if Body is true”.

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

aunt(Aunt, Child) :-
    sister(Aunt, Parent),
    parent(Parent, Child).
rules.pl

Variables

Variables in Prolog are placeholders used to represent values that are not yet known. In prolog, any sequence of letters, digits, or underscores that starts with an uppercase letter or an underscore, is a variable. The most common convention is to use names like X, Y, Person, but _anonymous forms are also valid. Prolog variables are logical variables, then when not instatiated can match with anything.

Queries

Queries, also called goals, are the questions we put to the Prolog interpreter.
A query tells Prolog to try to prove something from the facts and rules currently loaded in it’s theory.

Syntax and Semantics

A query is written exactly like a predicate and ends with a dot.
At an interactive prompt you usually prepend ?-, but when we use a top-level prompt (for example in tuProlog) we don’t have to prepend it.

?- likes(sam, X).

Prolog attems to satisfy the query by matching it, via unification and backtracking, against clauses in the theory. If it finds a match/proof, the query succeds and any variable bindings are reported; otherwise it returns false

Types of queries