2 Prolog Source Code

Just like most other computer languages, Prolog allows you to store programs and data in text files. The builtin predicates consult/1, reconsult/1, read/1, and their variants will translate the textual representation for programs and data into the internal representations used by the Prolog system. This section describes what kinds of syntactic objects can appear in source files and how they are interpreted.

###2.1 Source Terms

Every Prolog source file must be a sequence of zero or more Prolog terms, each term followed by a period (.) and a white space character. A period followed by a white space character is called a full stop. Full stops are needed in source files to show where one term ends and another begins. Each term in a file is treated as a closed logical formula. This means that even though two seperate terms have variable names in common, each term’s variables are actually distinct from the variables in any other term. Most variables are quantified once for each term. However, there is a special variable, the anonymous variable (written ‘_ ’), which is quantified for each occurrance. This means that within a single term, every occurrance of ‘_’ is a different variable.

###2.1.1 Rules

A rule is the most common programming construct in Prolog. A rule says that a particular property holds if a conjunction of properties holds. Rules are always written with :-/2 as the principal functor. The first argument of the :- is called the head, and the second argument is called the body. Here are some examples of rules:

a(X) :- b(X).  
blt :- bacon, lettuce, tomato.  
test(A,B,C) :- cond1(A,B), cond2(B,C).  

consult/1 and reconsult/1 load rules (and facts - see below) from a source file into the internal run-time Prolog database in the order they occur in the source file.

###2.1.2 Facts

A fact is any term which is not a rule and which cannot be interpreted as a directive or declaration. For example,

module mymodule.

would not be interpreted as a fact since it is a module declaration - see Chapter 3 Modules.{ADD LINK} More specifically, a fact is any term that cannot be interpreted as a declaration and whose principal functor is not :-/1, ?-/1, or :-/2. One way to understand a fact is to say that it is a rule without a body, or a rule with a trivial body (one that is always true). These are example facts:

mortal(socrates).  
big(ben).  
identical(X,X).  

As with rules, consult/1 and reconsult/1 load facts (and rules)from a source file into the internal database in the order they occur in the source file.