## 4.16 Examining the program

current_atom(-Atom)
  Successively unifies Atom with all atoms known to the system. Note that current_atom/1 always succeeds if Atom is instantiated to an atom.
current_blob(?Blob, ?Type)
  Examine the type or enumerate blobs of the given Type. Typed blobs are supported through the foreign language interface for storing arbitrary BLOBs (Binary Large Object) or handles to external entities. See section 12.4.10 for details.
current_functor(?Name, ?Arity)
  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
          ).

current_flag(-FlagKey)
  Successively unifies FlagKey with all keys used for flags (see flag/3).
current_key(-Key)
  Successively unifies Key with all keys used for records (see recorda/3, etc.).
[ISO]current_predicate(:PredicateIndicator)
  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`.

current_predicate(?Name, :Head)
  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.

predicate_property(:Head, ?Property)
  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.
dwim_predicate(+Term, -Dwim)
  ‘Do What I Mean’(`dwim’) support predicate. Term is a term, whose name and arity are used as a predicate specification. Dwim is instantiated with the most general term built from Name and the arity of a defined predicate that matches the predicate specified by Term in the‘Do What I Mean’sense. See dwim_match/2 for‘Do What I Mean’string matching. Internal system predicates are not generated, unless the access level is `system` (see access_level). Backtracking provides all alternative matches.
[ISO]clause(:Head, ?Body)
  True if Head can be unified with a clause head and Body with the corresponding clause body. Gives alternative clauses on backtracking. For facts, Body is unified with the atom true. Note that SWI-Prolog allows clause/2 to work on both dynamic and static code.96Using clause/2 is disallowed if either the flag iso or protect_static_code is `true`. Note that clause/2 decompiles the actual clause and may return a clause that is different from the source or asserted clause, i.e., clause/2 only promises semantic equivalence.
clause(:Head, ?Body, ?Reference)
  Equivalent to clause/2, but unifies Reference with a unique reference to the clause (see also assert/2, erase/1). If Reference is instantiated to a reference the clause's head and body will be unified with Head and Body.
nth_clause(?Pred, ?Index, ?Reference)
  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).

clause_property(+ClauseRef, -Property)
  Queries properties of a clause. ClauseRef is a reference to a clause as produced by clause/3, nth_clause/3 or prolog_frame_attribute/3. Unlike most other predicates that access clause references, clause_property/2 may be used to get information about erased clauses that have not yet been reclaimed. Property is one of the following:
  file(FileName)
    Unify FileName with the name of the file from which the clause is loaded. Fails if the clause was not created by loading a file (e.g., clauses added using assertz/1). See also `source`.
  line_count(LineNumber)
    Unify LineNumber with the line number of the clause. Fails if the clause is not associated to a file.
  size(SizeInBytes)
    True when SizeInBytes is the size that the clause uses in memory in bytes. The size required by a predicate also includes the predicate data record, a linked list of clauses, clause selection instructions and optionally one or more clause indexes.
  source(FileName)
    Unify FileName with the name of the source file that created the clause. This is the same as the `file` property, unless the file is loaded from a file that is textually included into source using include/1. In this scenario, `file` is the included file, while the `source` property refers to the main file.
  fact
    True if the clause has no body.
  erased
    True if the clause has been erased, but not yet reclaimed because it is referenced.
  predicate(PredicateIndicator)
    PredicateIndicator denotes the predicate to which this clause belongs. This is needed to obtain information on erased clauses because the usual way to obtain this information using clause/3 fails for erased clauses.
  module(Module)
    Module is the context module used to execute the body of the clause. For normal clauses, this is the same as the module in which the predicate is defined. However, if a clause is compiled with a module qualified head, the clause belongs to the predicate with the qualified head, while the body is executed in the context of the module in which the clause was defined.