Editing Prolog (section)

=== Data types ===
Prolog's single [[data type]] is the ''term''. Terms are either ''[[symbol (programming)#Prolog|atom]]s'', ''numbers'', ''variables'' or ''compound terms''.<ref group=note>    The Prolog terminology differs from that of  [[First-order logic|logic]].
    A term of Prolog is (depending on the context) a [[Term (logic)|term]] or an [[atomic formula]] of logic.
    An atom in a standard logic terminology means an [[atomic formula]];  
    an atom of Prolog (depending on the context) is a constant, function
    symbol or predicate symbol of logic.
</ref>
* An '''atom''' is a symbol name starting with a lower case letter or guarded by quotes. Examples of atoms include <code>x</code>, <code>red</code>, <code>'Taco'</code>, <code>'some atom'</code>, and <code>'p(a)'</code>.
* '''Numbers''' can be [[floating-point arithmetic|floats]] or [[integer]]s.  Most of the major Prolog systems support arbitrary length integer numbers.
* '''Variables''' are denoted by a string consisting of letters, numbers and underscore characters, and beginning with an upper-case letter or underscore. Variables closely resemble variables in logic in that they are placeholders for arbitrary terms.
* A '''compound term''' is composed of an atom called a "functor" and a number of "arguments", which are again terms. Compound terms are ordinarily written as a functor followed by a comma-separated list of argument terms, which is contained in parentheses. The number of arguments is called the term's [[arity]]. An atom can be regarded as a compound term with [[arity]] zero. An example of a compound term is <code>person_friends(zelda,[tom,jim])</code>.

Special cases of compound terms:
* A ''List'' is an ordered collection of terms. It is denoted by square brackets with the terms separated by commas, or in the case of the empty list, by <code>[]</code>. For example, <code>[1,2,3,4]</code> or <code>[red,green,blue]</code>.
* ''Strings'': A sequence of characters surrounded by quotes is equivalent to either a list of (numeric) character codes, a list of characters (atoms of length 1), or an atom depending on the value of the Prolog flag <code>double_quotes</code>. For example, <code>"to be, or not to be"</code>.<ref name="ISO 13211-1 6.3.7">ISO/IEC 13211-1:1995 Prolog, 6.3.7 Terms - double quoted list notation. [[International Organization for Standardization]], Geneva.</ref>