4.15 Declaring predicate properties

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.

Notably with the introduction of tabling (see section 7) it is common that a set of predicates require multiple options to be set. SWI-Prolog offers two mechanisms to cope with this. The predicate dynamic/2 can be used to make a list of predicates dynamic and set additional options. In addition and for compatibility with XSB,^{91Note that as is in XSB a high-priority operator and in SWI a low-priority and therefore both the sets of predicate indicators as multiple options require parenthesis.} all the predicates below accept a term as((:PredicateIndicator, ... ), (+Options)), where Options is a comma-list of one of more of the following options:

incremental

Include a dynamic predicate into the incremental tabling dependency graph. See section 7.7.

opaque

Opposite of incremental. For XSB compatibility.^{92In XSB, opaque is distinct from the default in the sense that dynamic switching between opaque and incremental is allowed.}

abstract(Level)

Used together with incremental to reduce the dependency graph. See section 7.7.

volatile

Do not save this predicate. See volatile/1.

multifile

Predicate may have clauses in multiple clauses. See multifile/1.

discontiguous

Predicate clauses may not be contiguous in the file. See discontiguous/1.

shared

Dynamic predicate is shared between all threads. This is currently the default.

local

private

Dynamic predicate has distinct set of clauses in each thread. See thread_local/1.

Below are some examples, where the last two are semantically identical.

:- dynamic person/2 as incremental.
:- dynamic (person/2,organization/2) as (incremental, abstract(0)).
:- dynamic([ person/2,
             organization/2
           ],
           [ incremental(true),
             abstract(0)
           ]).

[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 multithreaded 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.

dynamic(:ListOfPredicateIndicators, +Options)

As dynamic/1, but allows for setting additional properties. This predicate allows for setting multiple properties on multiple predicates in a single call. SWI-Prolog also offers the XSB compatible :- dynamic (p/1) as (incremental,abstract(0)). syntax. See the introduction of section 4.15. Defined Options are:

incremental(+Boolean)

Make the dynamic predicate signal depending tables. See section 7.7.

abstract(0)

This option must be used together with incremental. The only supported value is 0. With this option a call to the incremental dynamic predicate is recorded as the most generic term for the predicate rather than the specific variant.

thread(+Local)

Local is one of shared (default) or local. See also thread_local/1.

multifile(+Boolean)

discontiguous(+Boolean)

volatile(+Boolean)

Set the corresponding property. See multifile/1, discontiguous/1 and volatile/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.^{93The specification of this predicate is from Richard O'Keefe. The implementation is allowed to optimise the predicate. This is not yet implemented. In multithreaded 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.^{94This 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.

non_terminal :PredicateIndicator, ...

Sets the non_terminal property on the predicate. This indicates that the predicate implements a grammar rule. See predicate_property/2. The non_terminal property is set for predicates exported as Name//Arity as well as predicates that have at least one clause written using the -->/2 notation.