predicate_options:predicate

Documentation
- Reference manual
  - The SWI-Prolog library
    - library(predicate_options): Declare option-processing of predicates
      - Improving on the current situation
        
        Reflective access to options
        
        predicate_options/3
        
        assert_predicate_options/4
        
        current_predicate_option/3
        
        check_predicate_option/3
        
        current_option_arg/2
        
        current_predicate_options/3
        
        check_predicate_options/0
        
        derive_predicate_options/0
        
        retractall_predicate_options/0
        
        derived_predicate_options/3
        
        derived_predicate_options/1
- Packages

Availability::- use_module(library(predicate_options)).(can be autoloaded)

[det]predicate_options(:PI, +Arg, +Options)

Declare that the predicate PI processes options on Arg. Options is a list of options processed. Each element is one of:

Option(ModeAndType) PI processes Option. The option-value must comply to ModeAndType. Mode is one of + or - and Type is a type as accepted by must_be/2.
pass_to(:PI,Arg) The option-list is passed to the indicated predicate.

Below is an example that processes the option header(boolean) and passes all options to open/4:

:- predicate_options(write_xml_file/3, 3,
                     [ header(boolean),
                       pass_to(open/4, 4)
                     ]).

write_xml_file(File, XMLTerm, Options) :-
    open(File, write, Out, Options),
    (   option(header(true), Options, true)
    ->  write_xml_header(Out)
    ;   true
    ),
    ...

This predicate may only be used as a directive and is processed by expand_term/2. Option processing can be specified at runtime using assert_predicate_options/3, which is intended to support program analysis.