- 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)