Predicate information
Author: Daniel Diaz
Keywords: GNU Prolog, manual, Prolog, compiler, constraints, finite domains

### 8.8 Predicate information

- current_predicate/1
- predicate_property/2

#### 8.8.1 current_predicate/1

Templates

  current_predicate(?predicate_indicator)

Description

current_predicate(Pred) succeeds if there exists a predicate indicator of a defined procedure that unifies with Pred. All user defined procedures are found, whether static or dynamic. Internal system procedures whose name begins with ’$’ are not found. A user-defined procedure is found even when it has no clauses. A user-defined procedure is not found if it has been abolished. To conform to the ISO reference, built-in predicates are not found except if the strict_iso Prolog flag is switched off (section 8.22.1). This predicate is re-executable on backtracking.

Errors

Pred is neither a variable nor a predicate indicator |  | type_error(predicate_indicator, Pred)
Pred is a term Name/Arity and Arity is neither a variable nor an integer |  | type_error(integer, Arity)
Pred is a term Name/Arity and Name is neither a variable nor an atom |  | type_error(atom, Name)
Pred is a term Name/Arity and Arity is an integer < 0 |  | domain_error(not_less_than_zero, Arity)
Pred is a term Name/Arity and Arity is an integer > max_arity flag (section 8.22.1) |  | representation_error(max_arity)

Portability

ISO predicate.

#### 8.8.2 predicate_property/2

Templates

  predicate_property(?callable, ?predicate_property)

Description

predicate_property(Head, Property) succeeds if Head refers to a predicate that has a property Property. All user defined procedures and built-in predicates are found. Internal system procedures whose name begins with ’$’ are not found. This predicate is re-executable on backtracking.

Since version 1.4.0, predicate_property/2 no longer accepts a predicate indicator. Control constructs are now returned. Properties built_in_fd and control_construct now imply the property built_in.

Predicate properties:

- static: if the procedure is static.
- dynamic: if the procedure is dynamic.
- private: if the procedure is private.
- public: if the procedure is public.
- monofile: if the procedure is monofile.
- multifile: if the procedure is multifile.
- user: if the procedure is a user-defined procedure.
- built_in: if the procedure is a built-in predicate or a control construct.
- built_in_fd: if the procedure is an FD built-in predicate.
- control_construct: if the procedure is a control construct (section 7.2).
- native_code: if the procedure is compiled in native code.
- prolog_file(File): source file from which the procedure has been read.
- prolog_line(Line): line number of the source file.
- meta_predicate(Head): if the procedure is a meta-predicate 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 argument specifier as follows:
  integer N
    the argument is a term that is used to reference a predicate with N more arguments than the given argument term (e.g. call(0)).
  :
    the argument is module sensitive, but does not directly refer to a predicate (e.g. consult(:)).
  -
    the argument is not module sensitive and unbound on entry.
  ?
    the argument is not module sensitive and the mode is unspecified.
  +
    the argument is not module sensitive and bound (i.e., nonvar) on entry.

Errors

Head is neither a variable nor a callable term |  | type_error(callable, Head)
Property is neither a variable nor a predicate property term |  | domain_error(predicate_property, Property)
Property = prolog_file(File) and File is neither a variable nor an atom |  | type_error(atom, File)
Property = prolog_line(Line) and Line is neither a variable nor an integer |  | type_error(integer, Line)

Portability

GNU Prolog predicate.

Copyright (C) 1999-2021 Daniel Diaz Verbatim copying and distribution of this entire article is permitted in any medium, provided this notice is preserved. More about the copyright