## A.52 library(settings): Setting management

author
  Jan Wielemaker
See also
  `library(config)` distributed with XPCE provides an alternative aimed at graphical applications.

This library allows management of configuration settings for Prolog applications. Applications define settings in one or multiple files using the directive setting/4 as illustrated below:

    :- use_module(library(settings)).

    :- setting(version, atom,   '1.0', 'Current version').
    :- setting(timeout, number,    20, 'Timeout in seconds').

The directive is subject to term_expansion/2, which guarantees proper synchronisation of the database if source-files are reloaded. This implies it is not possible to call setting/4 as a predicate.

Settings are local to a module. This implies they are defined in a two-level namespace. Managing settings per module greatly simplifies assembling large applications from multiple modules that configuration through settings. This settings management library ensures proper access, loading and saving of settings.

[det]setting(:Name, +Type, +Default, +Comment)
  Define a setting. Name denotes the name of the setting, Type its type. Default is the value before it is modified. Default can refer to environment variables and can use arithmetic expressions as defined by eval_default/4.

  If a second declaration for a setting is encountered, it is ignored if Type and Default are the same. Otherwise a permission_error is raised.

  Name | Name of the setting (an atom)
  Type | Type for setting. One of `any` or a type defined by must_be/2.
  Default | Default value for the setting.
  Comment | Atom containing a (short) descriptive note.

[nondet]setting(:Name, ?Value)
  True when Name is a currently defined setting with Value. Note that `setting(Name, Value)` only enumerates the settings of the current module. All settings can be enumerated using `setting(Module:Name, Value)`. This predicate is `det` if Name is ground.
  Errors
    `existence_error(setting, Name)`
[det]env(+Name:atom, -Value:number)
[det]env(+Name:atom, +Default:number, -Value:number)
  Evaluate environment variables on behalf of arithmetic expressions.
[det]set_setting(:Name, +Value)
  Change a setting. Performs existence and type-checking for the setting. If the effective value of the setting is changed it broadcasts the event below.

      settings(changed(Module:Name, Old, New))

  Note that modified settings are not automatically persistent. The application should call save_settings/0 to persist the changes.

  Errors
    - `existence_error(setting, Name)`
    - `type_error(Type, Value)`
[det]restore_setting(:Name)
  Restore the value of setting Name to its default. Broadcast a change like set_setting/2 if the current value is not the default.
[det]set_setting_default(:Name, +Default)
  Change the default for a setting. The effect is the same as set_setting/2, but the new value is considered the default when saving and restoring a setting. It is intended to change application defaults in a particular context.
[det]load_settings(File)
[det]load_settings(File, +Options)
  Load local settings from File. Succeeds if File does not exist, setting the default save-file to File. Options are:
  undefined(+Action)
    Define how to handle settings that are not defined. When `error`, an error is printed and the setting is ignored. when `load`, the setting is loaded anyway, waiting for a definition.

  If possibly changed settings need to be persistent, the application must call save_settings/0 as part of its shutdown. In simple cases calling `at_halt(save_settings)` is sufficient.

[semidet]save_settings
[semidet]save_settings(+File)
  Save modified settings to File. Fails silently if the settings file cannot be written. The save_settings/0 only attempts to save the settings file if some setting was modified using set_setting/2.
  Errors
    `context_error(settings, no_default_file)` for save_settings/0 if no default location is known.
[nondet]current_setting(?Setting)
  True if Setting is a currently defined setting
[det]setting_property(+Setting, +Property)
[nondet]setting_property(?Setting, ?Property)
  Query currently defined settings. Property is one of
  comment(-Atom)
  type(-Type)
    Type of the setting.
  default(-Default)
    Default value. If this is an expression, it is evaluated.
  source(-File:-Line)
    Location where the setting is defined.
[det]list_settings
[det]list_settings(+Module)
  List settings to `current_output`. The second form only lists settings on the matching module.
  To be done
    Compute the required column widths
convert_setting_text(+Type, +Text, -Value)
  Converts from textual form to Prolog Value. Used to convert values obtained from the environment. Public to provide support in user-interfaces to this library.
  Errors
    `type_error(Type, Value)`