True when Name/Arity is a known functor. This
means that at some point in time a term with name Name and Arity
arguments was created. Functor objects are currently not subject to
garbage collection. Due to timing, t/2 below with instantiated
Name and Arity can theoretically fail, i.e., a
functor may be visible in instantiated mode while it is not yet visible
in unbound mode. Considering that the only practical value of current_functor/2
we are aware of is to analyse resource usage we accept this impure
behaviour.
t(Name, Arity) :-
( current_functor(Name, Arity)
-> current_functor(N, A), N == Name, A == Arity
; true
).
True if PredicateIndicator is a currently defined predicate.
A predicate is considered defined if it exists in the specified module,
is imported into the module or is defined in one of the modules from
which the predicate will be imported if it is called (see
section 6.10). Note
that current_predicate/1
does not succeed for predicates that can be autoloaded
unless they are imported using autoload/2.
See also current_predicate/2
and
predicate_property/2.
If PredicateIndicator is not fully specified, the
predicate only generates values that are defined in or already imported
into the target module. Generating all callable predicates therefore
requires enumerating modules using current_module/1.
Generating predicates callable in a given module requires enumerating
the import modules using import_module/2
and the autoloadable predicates using the
predicate_property/2 autoload.
Classical pre-ISO implementation of current_predicate/1,
where the predicate is represented by the head term. The advantage is
that this can be used for checking the existence of a predicate before
calling it without the need for functor/3:
call_if_exists(G) :-
current_predicate(_, G),
call(G).
Because of this intended usage, current_predicate/2
also succeeds if the predicate can be autoloaded. Unfortunately,
checking the autoloader makes this predicate relatively slow, in
particular because a failed lookup of the autoloader will cause the
autoloader to verify that its index is up-to-date.
True when Head refers to a predicate that has property
Property. With sufficiently instantiated Head,
predicate_property/2
tries to resolve the predicate the same way as calling it would do: if
the predicate is not defined it scans the default modules (see default_module/2)
and finally tries the autoloader. Unlike calling, failure to find the
target predicate causes
predicate_property/2
to fail silently. If Head is not sufficiently bound, only
currently locally defined and already imported predicates are
enumerated. See current_predicate/1
for enumerating all predicates. A common issue concerns generating
all built-in predicates. This can be achieved using the code below:
generate_built_in(Name/Arity) :-
predicate_property(system:Head, built_in),
functor(Head, Name, Arity),
\+ sub_atom(Name, 0, _, _, $). % discard reserved names
The predicate predicate_property/2
is covered by part-II of the ISO standard (modules). Although we are not
aware of any Prolog system that implements part-II of the ISO standard, predicate_property/2
is available in most systems. There is little consensus on the
implemented properties though. SWI-Prolog's auto loading
feature further complicate this predicate.
Property is one of:
- autoload(File)
-
True if the predicate can be autoloaded from the file File.
Like
undefined, this property is not generated.
- built_in
-
True if the predicate is locked as a built-in predicate. This implies it
cannot be redefined in its definition module and it can normally not be
seen in the tracer.
- defined
-
True if the predicate is defined. This property is aware of sources
being reloaded, in which case it claims the predicate defined
only if it is defined in another source or it has seen a definition in
the current source. See compile_aux_clauses/1.
- det
-
The predicate is defined to be deterministic using det/1.
- discontiguous
-
True after discontiguous/1
was used to flag that the clauses of the predicates may not be
contiguous.
- dynamic
-
True if assert/1
and retract/1
may be used to modify the predicate. This property is set using dynamic/1.
- exported
-
True if the predicate is in the public list of the context module.
- imported_from(Module)
-
Is true if the predicate is imported into the context module from module Module.
- file(FileName)
-
Unify FileName with the name of the source file in which the
predicate is defined. See also source_file/2
and the property
line_count. Note that this reports the file of the first
clause of a predicate. A more robust interface can be achieved using nth_clause/3
and clause_property/2.
- foreign
-
True if the predicate is defined in the C language.
- implementation_module(-Module)
-
True when Module is the module in which Head is or
will be defined. Resolving this property goes through the same search
mechanism as when an undefined predicate is encountered, but does not
perform any loading. It searches (1) the module inheritance hierarchy
(see
default_module/2)
and (2) the autoload index if the unknown
flag is not set to
fail in the target module.
- indexed(Indexes)
-
Indexes is a list of additional (hash) indexes on the
predicate. Each element of the list is a term ArgSpec-Index.
ArgSpec denotes the indexed argument(s) and is one of
- single(Argument)
-
Hash on a single argument. Argument is the 1-based argument
number.
- multi(ArgumentList)
-
Hash on a combination of arguments.
- deep(Position)
-
Index on a sub-argument. Position is a list holding first the argument
of the predicate then the argument into the compound and recursively
into deeper compound terms.
Index is a term hash(Buckets, Speedup, Size, IsList).
Here
Buckets is the number of buckets in the hash and Speedup
is the expected speedup relative to trying all clauses linearly, Size
is the size of the index in memory in bytes and finally, IsList
indicates that a list is created for all clauses with the same key. This
is used to create deep indexes for the arguments of compound
terms.
Note: This predicate property should be used for analysis and
statistics only. The exact representation of Indexes may
change between versions. The utilities jiti_list/0 jiti_list/1
list the
jit indexes of matching predicates in a user friendly way.
- interpreted
-
True if the predicate is defined in Prolog. We return true on this
because, although the code is actually compiled, it is completely
transparent, just like interpreted code.
- iso
-
True if the predicate is covered by the ISO standard (ISO/IEC 13211-1).
- line_count(LineNumber)
-
Unify LineNumber with the line number of the first clause of
the predicate. Fails if the predicate is not associated with a file. See
also source_file/2.
See also the
file property above, notably the reference to clause_property/2.
- multifile
-
True if there may be multiple (or no) files providing clauses for the
predicate. This property is set using multifile/1.
- meta_predicate(Head)
-
If the predicate is declared as a meta-predicate using meta_predicate/1,
unify Head with the head-pattern. The head-pattern is a
compound term with the same name and arity as the predicate where each
argument of the term is a meta-predicate specifier. See meta_predicate/1
for details.
- monotonic
-
True if the predicate is tabled or dynamic using monotonic propagation.
See section 7.8.
- nodebug
-
Details of the predicate are not shown by the debugger. This is the
default for built-in predicates. User predicates can be compiled this
way using the Prolog flag generate_debug_info.
- non_terminal
-
True if the predicate implements a grammar rule. See
non_terminal/1.
- notrace
-
Do not show ports of this predicate in the debugger.
- number_of_clauses(ClauseCount)
-
Unify ClauseCount to the number of clauses associated with
the predicate. Fails for foreign predicates. This property respects the
logical update view and counts visible clauses at the moment
the predicate was started.
- number_of_rules(RuleCount)
-
Similar to
number_of_clauses(ClauseCount), but only counts
rules. A rule is defined as a clauses that has a body
that is not just true (i.e., a fact).
- last_modified_generation(Generation)
-
Database generation at which the predicate was modified for the last
time. Intended to quickly assesses the validity of caches.
- opaque
-
This property applies to dynamic and tabled predicates. For dynamic
predicates it (temporary) stops propagating updates to dependent
incrementally or monotonic tabled predicates. For tabled predicates it
is not an error for an opaque predicate to depend on incremental or
monotonic dynamic or tabled predicates.
- public
-
Predicate is declared public using public/1.
Note that without further definition, public predicates are considered
undefined and this property is not reported.
- quasi_quotation_syntax
-
The predicate (with arity 4) is declared to provide quasi quotation
syntax with quasi_quotation_syntax/1.
- size(Bytes)
-
Memory used for this predicate. This includes the memory of the
predicate header, the combined memory of all clauses including erased
but not yet garbage collected clauses (see garbage_collect_clauses/0
and
clause_property/2)
and the memory used by clause indexes (see the
indexed(Indexes) property. Excluded are lingering
data structures. These are garbage data structures that have been
detached from the predicate but cannot yet be reclaimed because they may
be in use by some thread.
- ssu
-
The predicate has been defined using single sided unification
rules. See section 5.6.
- static
-
The definition can not be modified using assertz/1
and friends. This property is the opposite from
dynamic,
i.e., for each defined predicate, either static or dynamic
is true but never both.
- tabled
-
True of the predicate is tabled. The
tabled(?Flag)
property can be used to obtain details about how the predicate is
tabled.
- tabled(?Flag)
-
True of the predicate is tabled and Flag applies.
Any tabled predicate has one of the mutually exclusive flags
variant
or subsumptive. In addition, tabled predicates may have one
or more of the following flags
- shared
-
The table is shared between threads. See section
7.9.
- incremental
-
The table is subject to
incremental tabling. See section
7.7
Use the tabled property to enumerate all tabled
predicates. See
table/1
for details.
- thread_local
-
If true (only possible on the multithreaded version) each thread has its
own clauses for the predicate. This property is set using
thread_local/1.
- transparent
-
True if the predicate is declared transparent using the
module_transparent/1
or meta_predicate/1
declaration. In the latter case the property
meta_predicate(Head)
is also provided. See chapter 6
for details.
- undefined
-
True if a procedure definition block for the predicate exists, but there
are no clauses for it and it is not declared dynamic or multifile. This
is true if the predicate occurs in the body of a loaded predicate, an
attempt to call it has been made via one of the meta-call predicates,
the predicate has been declared as e.g., a meta-predicate or the
predicate had a definition in the past. Originally used to find missing
predicate definitions. The current implementation of list_undefined/0
used cross-referencing. Deprecated.
- visible
-
True when predicate can be called without raising a predicate existence
error. This means that the predicate is (1) defined, (2) can be
inherited from one of the default modules (see default_module/2)
or (3) can be autoloaded. The behaviour is logically consistent iff the
property
visible is provided explicitly. If the property is left
unbound, only defined predicates are enumerated.
- volatile
-
If true, the clauses are not saved into a saved state by qsave_program/[1,2].
This property is set using volatile/1.
Provides access to the clauses of a predicate using their index number.
Counting starts at 1. If Reference is specified it unifies Pred
with the most general term with the same name/arity as the predicate and
Index with the index number of the clause. Otherwise the name
and arity of Pred are used to determine the predicate. If Index
is provided, Reference will be unified with the clause
reference. If Index is unbound, backtracking will yield both
the indexes and the references of all clauses of the predicate. The
following example finds the 2nd clause of append/3:
?- use_module(library(lists)).
...
?- nth_clause(append(_,_,_), 2, Ref), clause(Head, Body, Ref).
Ref = <clause>(0x994290),
Head = lists:append([_G23|_G24], _G21, [_G23|_G27]),
Body = append(_G24, _G21, _G27).
