predicate_options.pl -- Access and analyse predicate options

This module provides the developers interface for the directive predicate_options/3. This directive allows us to specify that, e.g., open/4 processes options using the 4th argument and supports the option type using the values text and binary. Declaring options that are processed allows for more reliable handling of predicate options and simplifies porting applications. This library provides the following functionality:

Query supported options through current_predicate_option/3 or current_predicate_options/3. This is intended to support conditional compilation and an IDE.
Derive additional declarations through dataflow analysis using derive_predicate_options/0.
Perform a compile-time analysis of the entire loaded program using check_predicate_options/0.

Below, we describe some use-cases.

Quick check of a program

This scenario is useful as an occasional check or to assess problems with option-handling for porting an application to SWI-Prolog. It consists of three steps: loading the program (1 and 2), deriving option handling for application predicates (3) and running the checker (4).

1 ?- [load].
2 ?- autoload.
3 ?- derive_predicate_options.
4 ?- check_predicate_options.

Add declarations to your program

Adding declarations about option processes improves the quality of the checking. The analysis of derive_predicate_options/0 may miss options and does not derive the types for options that are processed in Prolog code. The process is similar to the above. In steps 4 and further, the inferred declarations are listed, inspected and added to the source code of the module.

1 ?- [load].
2 ?- autoload.
3 ?- derive_predicate_options.
4 ?- derived_predicate_options(module_1).
5 ?- derived_predicate_options(module_2).
6 ?- ...

Declare option processing requirements

If an application requires that open/4 needs to support lock(write), it may do so using the directive below. This directive raises an exception when loaded on a Prolog implementation that does not support this option.

:- current_predicate_option(open/4, 4, lock(write)).

See also: - library(option) for accessing options in Prolog code.

predicate_options(:PI, +Arg, +Options) is det

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.

Undocumented predicates

The following predicates are exported, but not or incorrectly documented.