Prolog Basic Terminology - Prolog Basics #01

In this video, we will introduce the basic Prolog terminology. Transcription In this new video, we will see the common terminology adopted in Prolog. # Glossary Everything in Prolog is represented with *terms*. There are several types of terms: *atoms*, *numbers*, *variables*, and *compound terms*. The basic unit of information is the *atom*, a sequence of character that starts with a lowercase letter. For example, ``` house. a. fly. ``` are atoms. Atoms and numbers are *constants*. *Variables* in Prolog start with an uppercase letter or an underscore and, differently form other programming languages, once they are assigned to a value (in Prolog terminology, *bound*) this value be changed only through a special technique called *backtracking*. These are examples of variables: ``` Abc House Car _a ``` The variable `_` is called the *anonymous* variable. *Compound terms* are composed by an atom together with one or more arguments. For example, ``` friend(a,b). ``` is a compound term. In this case, it can be interpreted as a relation called `friend` between `a` and `b` (i.e., `a` is friend with `b`). However, note that not all compounds denote relations. The name of a term is an atom and it is called *functor*. The number of arguments of a compound term is called *arity* and compound terms are compactly referred with the notation `functor/arity`, so, for this example, `friend/2`. A term is called *ground* if it does not contain variables. This is a ground term ``` friend(a,b). ``` while this is not ``` friend(A,b). ``` A Prolog program is composed by a set of *rules*. Every rule is composed by a *head* and a *body* separated by the neck operator `:-`. The head is an atom or a compound term. The body can be a conjunction of terms (or empty). These are examples of rules: ``` grass:- sun. friend(a,b):- knows(a,b). ``` The intuitive meaning is: if the body is true then the head is also true. We can insert multiple terms in the body, by separating them with commas ``` a:- b,c. ``` So, `a` is true if both `b` and `c` are true. Commas between terms stand for conjunction (all the terms in the body must be true to make the head true). Each term in the body of a rule is also called *goal*. If the body is empty, the neck operator is omitted and the rule is called *fact*. A *clause* is a rule or a fact. ## Useful References - [https://sicstus.sics.se/sicstus/docs/4.3.1/html/sicstus/Glossary.html](https://sicstus.sics.se/sicstus/docs/4.3.1/html/sicstus/Glossary.html) - [https://www.swi-prolog.org/pldoc/man?section=glossary](https://www.swi-prolog.org/pldoc/man?section=glossary) - [http://www.let.rug.nl/bos/lpn//lpnpage.php?pagetype=html&pageid=lpn-htmlse2](http://www.let.rug.nl/bos/lpn//lpnpage.php?pagetype=html&pageid=lpn-htmlse2)