The structured data objects of the language are the compound
terms. A compound term comprises a functor (called the
principal functor of the term) and a sequence of one or more
terms called arguments. A functor is characterized by
its name, which is an atom, and its arity or number of
arguments. For example the compound term whose
functor is named `point`

of arity 3, with
arguments `X`

, `Y`

and `Z`

, is written

point(X, Y, Z)

Note that an atom is considered to be a functor of arity 0.

Functors are generally analogous to common nouns in natural language. One may think of a functor as a record type and the arguments of a compound term as the fields of a record. Compound terms are usefully pictured as trees. For example, the term

s(np(john),vp(v(likes),np(mary)))

would be pictured as the compound term

s / \ np vp | / \ john v np | | likes mary

Sometimes it is convenient to write certain functors as operators—2-ary functors may be declared as infix operators and 1-ary functors as prefix or postfix operators. Thus it is possible to write e.g.:

X+Y (P;Q) X<Y +X P;

as optional alternatives to

+(X,Y) ;(P,Q) <(X,Y) +(X) ;(P)

The use of operators is described fully below (see Operators).

Lists form an important class of data structures in Prolog. They
are essentially the same as the lists of LISP: a list either
is the atom `[]`

representing the empty list, or is a
compound term with functor `.`

and two arguments,
which are respectively the head and tail of the list.
Thus a list of the first three natural numbers is the
compound term

. / \ 1 . / \ 2 . / \ 3 []

which could be written, using the standard syntax, as

.(1,.(2,.(3,[])))

but which is normally written, in a special list notation, as

[1,2,3]

The special list notation in the case when the tail of a list is a variable is exemplified by

[X|L] [a,b|L]

representing

. . / \ / \ X L a . / \ b L

respectively.

Note that this notation does not add any new power to the language; it simply makes it more readable. e.g. the above examples could equally be written

.(X,L) .(a,.(b,L))

For convenience, a further notational variant is allowed for lists of integers that correspond to character codes or one-char atoms. Lists written in this notation are called strings. E.g.:

"SICStus"

which, by default, denotes exactly the same list as

[83,73,67,83,116,117,115]

The Prolog flag `double_quotes`

can be used to change the way
strings are interpreted. The default value of the flag is `codes`

,
which implies the above interpretation. If the flag is set to
`chars`

, a string is transformed to a char-list. E.g. with
this setting the above string represents the list:

['S','I','C','S',t,u,s]

Finally if `double_quotes`

has the value `atom`

, then the
string is made equivalent to the atom formed from its characters:
the above sample string is then the same as the atom
`'SICStus'`

.

Unless character escapes have been switched off, backslashes in the
sequence denote escape sequences (see Escape Sequences). As
for quoted atoms, if a double quote character is included in the
sequence it must be escaped, e.g. `"can\"t"`

.