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# library(module_options)

Utility library to manage options within a library module [more]

## Predicates

> get_options(+OptionList, -OptionStruct)
>   Create a structure OptionStruct from OptionList and the context module's default settings
> print_default_options(+Stream)
>   Print the valid options and their current default values to Stream
> set_default_option(+OptionName, +OptionValue)
>   Permanently set the default value for the given option in the context module

## Description

This library provides utilities to manage option settings on behalf of other library modules. The basic idea is that each client module can define what it considers to be valid option names and values, plus the structure in terms of which the set of all options will be stored and returned.

For each client library, global default option settings are maintained which can be modified (in a non-backtrackable fashion) using set_default_option/2. Whenever the settings are retrieved, the defaults can be individually overridden using a user-supplied option list.

Every client module has to define 3 local predicates:

valid_option_field(?Name, -FieldIndex)
  defines the option names (atoms) and the position (integer) of the option value within an option structure.
valid_option_value(+Name, +Value)
  defines what constitues a valid option value, by provides a type/range check for Value. The predicate should fail if Value is not a valid value for option Name.
default_options(-OptionStructure)
  this should be a single fact which should return a structure. The structure arguments define the initial default settings for each option field. The structure's functor defines the skeleton in terms of which option settings will be returned by get_options/2. Note that the structure can have extra fields which are not defined as valid options, are not user-modifiable, and will therefore always be returned unchanged by get_options/2.

A typical application would in addition define a toplevel predicate that accepts a user-supplied option list per invocation, and possibly a predicate to modify the global default settings. Sample usage:

    	:- module(my_module).

    	:- lib(module_options).

    	valid_option_field(a, 1).
    	valid_option_field(b, 2).
    	valid_option_field(c, 3).

    	valid_option_value(a, Value) :- integer(Value).
    	valid_option_value(b, Value) :- atom(Value).
    	valid_option_value(c, Value) :- atom(Value).

    	default_options(options(23,hello,world,there)).

    	:- export my_set_default_option/2.
    	my_set_default_option(Name, Value) :-
    	    set_default_option(Name, Value).

    	:- export my_predicate/2.
    	my_predicate(Arguments, OptionList) :-
    	    ( get_options(OptionList, OptionStruct) ->
    		...
    	    ;
    		printf(error, "Invalid option list: %w%n", [OptionList]),
    		print_default_options(error),
    		abort
    	    ).

In practice, it is recommended to use structure notation for the option structure for better readability and maintainability, i.e.

    	:- module(my_module).

    	:- lib(module_options).

    	:- local struct(options(a,b,c,d)).

    	valid_option_field(a, a of options).
    	valid_option_field(b, b of options).
    	valid_option_field(c, c of options).

    	valid_option_value(a, Value) :- integer(Value).
    	valid_option_value(b, Value) :- atom(Value).
    	valid_option_value(c, Value) :- atom(Value).

    	default_options(options{a:23,b:hello,c:world,d:there}).

    	:- export my_set_default_option/2.
    	my_set_default_option(Name, Value) :-
    	    set_default_option(Name, Value).

    	:- export my_predicate/2.
    	my_predicate(Arguments, OptionList) :-
    	    ( get_options(OptionList, OptionStruct) ->
    		...
    	    ;
    		printf(error, "Invalid option list: %w%n", [OptionList]),
    		print_default_options(error),
    		abort
    	    ).

It is not absulotely necessary to define a predicate like my_set_default_option/2 since `my_set_default_option(opt,val)` is equivalent to `set_default_option(opt,val)@my_module`.

## About

- Author: Joachim Schimpf
- Copyright © Cisco Systems, Inc
- Date: $Date: 2009/07/16 09:11:24 $

Generated from module_options.eci on 2022-09-03 14:26