4.2.3 Control Structures

As we have seen, the goals in the body of a sentence are linked by the operator ‘,’, which can be interpreted as conjunction (and). The Prolog language provides a number of other operators, known as control structures, for building complex goals. Apart from being built-in predicates, these control structures play a special role in certain language features, namely Grammar Rules (see ref-gru), and when code is loaded or asserted in the context of modules (see ref-mod). The set of control structures is described in this section, and consists of:

:P,:Q           ISO

prove P and Q

:P;:Q           ISO

prove P or Q

+M::P           ISO

call P in module M

:P->:Q;:R           ISO

if P succeeds, then prove Q; if not, then prove R

:P->:Q           ISO

if P succeeds, then prove Q; if not, then fail

!           ISO

cut any choices taken in the current procedure

\+ :P           ISO

goal P is not provable

?X ^ :P

there exists an X such that P is provable (used in setof/3 and bagof/3)

+Iterators do :Body

executes Body iteratively according to Iterators

if(:P,:Q,:R)

for each solution of P succeeds, prove Q; if none, then prove R

once(:P)           ISO

Find the first solution, if any, of goal P.