This section describes directives which manipulate attributes of
predicate definitions. The functors dynamic/1, multifile/1,
discontiguous/1
and public/1
are operators of priority 1150 (see op/3),
which implies that the list of predicates they involve can just be a
comma-separated list:
:- dynamic
foo/0,
baz/2.
In SWI-Prolog all these directives are just predicates. This implies
they can also be called by a program. Do not rely on this feature if you
want to maintain portability to other Prolog implementations.
- [ISO]dynamic :PredicateIndicator,
...
-
Informs the interpreter that the definition of the predicate(s) may
change during execution (using assert/1
and/or retract/1).
In the multi-threaded version, the clauses of dynamic predicates are
shared between the threads. The directive thread_local/1
provides an alternative where each thread has its own clause list for
the predicate. Dynamic predicates can be turned into static ones using
compile_predicates/1.
- compile_predicates(:ListOfPredicateIndicators)
-
Compile a list of specified dynamic predicates (see dynamic/1
and
assert/1)
into normal static predicates. This call tells the Prolog environment
the definition will not change anymore and further calls to assert/1
or retract/1
on the named predicates raise a permission error. This predicate is
designed to deal with parts of the program that are generated at runtime
but do not change during the remainder of the program execution.54The
specification of this predicate is from Richard O'Keefe. The
implementation is allowed to optimise the predicate. This is not yet
implemented. In multi-threaded Prolog, however, static code runs faster
as it does not require synchronisation. This is particularly true on SMP
hardware.
- [ISO]multifile :PredicateIndicator,
...
-
Informs the system that the specified predicate(s) may be defined over
more than one file. This stops consult/1
from redefining a predicate when a new definition is found.
- [ISO]discontiguous :PredicateIndicator,
...
-
Informs the system that the clauses of the specified predicate(s) might
not be together in the source file. See also style_check/1.
- public :PredicateIndicator,
...
-
Instructs the cross-referencer that the predicate can be called. It has
no semantics.55This declaration is
compatible with SICStus. In YAP, public/1
instructs the compiler to keep the source. As the source is always
available in SWI-Prolog, our current interpretation also enhances the
compatibility with YAP. The public declaration can be
queried using predicate_property/2.
The public/1
directive does
not export the predicate (see module/1
and export/1).
The public directive is used for (1) direct calls into the module from,
e.g., foreign code, (2) direct calls into the module from other modules,
or (3) flag a predicate as being called if the call is generated by
meta-calling constructs that are not analysed by the cross-referencer.
