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:
:- 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.
- - Jan Wielemaker
- See also
library(config) distributed with XPCE provides an alternative
aimed at graphical applications.
- setting(:Name, +Type, +Default, +Comment) is det
- 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
|Default||- Default value for the setting.|
|Comment||- Atom containing a (short) descriptive note.|
- setting(:Name, ?Value) is nondet
- True when Name is a currently defined setting with Value. Note
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
- env(+Name:atom, -Value:number) is det
- env(+Name:atom, +Default:number, -Value:number) is det
- Evaluate environment variables on behalf of arithmetic
- set_setting(:Name, +Value) is det
- 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.
- restore_setting(:Name) is det
- Restore the value of setting Name to its default. Broadcast a
change like set_setting/2 if the current value is not the
- set_setting_default(:Name, +Default) is det
- 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.
- load_settings(File) is det
- load_settings(File, +Options) is det
- Load local settings from File. Succeeds if File does not exist,
setting the default save-file to File. Options are:
- Define how to handle settings that are not defined. When
error, an error is printed and the setting is ignored.
load, the setting is loaded anyway, waiting for a
If possibly changed settings need to be persistent, the application
must call save_settings/0 as part of its shutdown. In simple cases
at_halt(save_settings) is sufficient.
- save_settings is semidet
- save_settings(+File) is semidet
- 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.
context_error(settings, no_default_file) for save_settings/0
if no default location is known.
- current_setting(?Setting) is nondet
- True if Setting is a currently defined setting
- setting_property(+Setting, +Property) is det
- setting_property(?Setting, ?Property) is nondet
- Query currently defined settings. Property is one of
- Type of the setting.
- Default value. If this is an expression, it is
- source((-File :- Line))
- Location where the setting is defined.
- list_settings is det
- list_settings(+Module) is det
- 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.
The following predicates are exported, but not or incorrectly documented.
- load_settings(Arg1, Arg2)