Editing Prolog (section)

=== Predicates and programs ===

A ''predicate'' (or ''procedure definition'') is a collection of clauses whose heads have the same name and arity. We use the notation ''name/arity'' to refer to predicates. A ''logic program'' is a set of predicates.  For example, the following Prolog program, which defines some family relations, has four predicates: <syntaxhighlight lang="prolog">
mother_child(trude, sally).

father_child(tom, sally).
father_child(tom, erica).
father_child(mike, tom).

sibling(X, Y)      :- parent_child(Z, X), parent_child(Z, Y), not(X = Y).

parent_child(X, Y) :- father_child(X, Y).
parent_child(X, Y) :- mother_child(X, Y).
</syntaxhighlight>

Predicate <code>father_child/2</code> has three clauses, all of which are facts, and predicate <code>parent_child/2</code> has two clauses, both are rules.

Due to the relational nature of many built-in predicates, they can typically be used in several directions. For example, <code>length/2</code> can be used to determine the length of a list (<code>length(List, L)</code>, given a list <code>List</code>), and to generate a list skeleton of a given length (<code>length(X, 5)</code>), and to generate both list skeletons and their lengths together (<code>length(X, L)</code>). Similarly, <code>append/3</code> can be used both to append two lists (<code>append(ListA, ListB, X)</code> given lists <code>ListA</code> and <code>ListB</code>), and to split a given list into parts (<code>append(X, Y, List)</code>, given a list <code>List</code>). For this reason, a comparatively small set of library predicates suffices for many Prolog programs.

As a general purpose language, Prolog also provides various built-in predicates to perform routine activities like [[input/output]], using graphics and otherwise communicating with the operating system. These predicates are not given a relational meaning and are only useful for the side-effects they exhibit on the system. For example, the predicate <code>write/1</code> displays a term on the screen.