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.

 assert_predicate_options(:PI, +Arg, +Options, ?New) is semidet

As predicate_options(:PI, +Arg, +Options). New is a boolean indicating whether the declarations have changed. If New is provided and false, the predicate becomes semidet and fails without modifications if modifications are required.

 current_option_arg(:PI, ?Arg) is nondet

True when Arg of PI processes predicate options. Which options are processed can be accessed using current_predicate_option/3.

 current_predicate_option(:PI, ?Arg, ?Option) is nondet

True when Arg of PI processes Option. For example, the following is true:

?- current_predicate_option(open/4, 4, type(text)).
true.

This predicate is intended to support conditional compilation using if/1 ... endif/0. The predicate current_predicate_options/3 can be used to access the full capabilities of a predicate.

 check_predicate_option(:PI, +Arg, +Option) is det

Verify predicate options at runtime. Similar to current_predicate_option/3, but intended to support runtime checking.

Errors

- existence_error(option, OptionName) if the option is not supported by PI.

- type_error(Type, Value) if the option is supported but the value does not match the option type. See must_be/2.

 current_predicate_options(:PI, ?Arg, ?Options) is nondet

True when Options is the current active option declaration for PI on Arg. See predicate_options/3 for the argument descriptions. If PI is ground and refers to an undefined predicate, the autoloader is used to obtain a definition of the predicate.

 derived_predicate_options(:PI, ?Arg, ?Options) is nondet

Derive option arguments using static analysis. True when Options is the current derived active option declaration for PI on Arg.

 derived_predicate_options(+Module) is det

Derive predicate option declarations for a module. The derived options are printed to the current_output stream.

 retractall_predicate_options is det

Remove all dynamically (derived) predicate options.

 check_predicate_options is det

Analyse loaded program for erroneous options. This predicate decompiles the current program and searches for calls to predicates that process options. For each option list, it validates whether the provided options are supported and validates the argument type. This predicate performs partial dataflow analysis to track option-lists inside a clause.

See also

- derive_predicate_options/0 can be used to derive declarations for predicates that pass options. This predicate should normally be called before check_predicate_options/0.

 derive_predicate_options is det

Derive new predicate option declarations. This predicate analyses the loaded program to find clauses that process options using one of the predicates from library(option) or passes options to other predicates that are known to process options. The process is repeated until no new declarations are retrieved.

See also

- autoload/0 may be used to complete the loaded program.

 check_predicate_options(:PredicateIndicator) is det

Verify calls to predicates that have options in all clauses of the predicate indicated by PredicateIndicator.