built-in predicate
logtalk_library_path/2
Description
logtalk_library_path(Library, Path)
Dynamic and multifile user-defined predicate, allowing the declaration
of aliases to library paths. Library aliases may also be used on
the second argument (using the notation alias(path)). Paths must always
end with the path directory separator character ('/').
Warning
Library aliases should be unique. The logtalk_make/1
built-in predicate can be used to detect and report duplicated library
aliases using the check target.
Clauses for this predicate should preferably be facts. Defining rules to
dynamically compute at runtime both library alias names and their paths,
although sometimes handy, can inadvertently result in endless loops when
those rules also attempt to expand library paths. When asserting facts
for this predicate, use preferably asserta/1 instead of assertz/1
to help ensure the facts will be used before any rules.
Relative paths (e.g. '../' or './') should only be used within
the alias(path)) notation so that library paths can always be expanded
to absolute paths independently of the (usually unpredictable) current
directory at the time the logtalk_library_path/2 predicate is
called.
When working with a relocatable application, the actual application
installation directory can be retrieved by calling the
logtalk_load_context/2 predicate with the directory
key and using the returned value to define the logtalk_library_path/2
predicate. On a settings file file or a loader file file,
simply use an initialization/1 directive to wrap the
call to the logtalk_load_context/2 predicate and the assert of the
logtalk_library_path/2 fact.
This predicate may be used to override the default scratch directory
by defining the library alias scratch_directory in a backend Prolog
initialization file (assumed to be loaded prior to Logtalk loading). This
allows e.g. Logtalk to be installed in a read-only directory by setting
this alias to the operating-system directory for temporary files. It also
allows several Logtalk instances to run concurrently without conflict by
using a unique scratch directory per instance (e.g. using a process ID or
a UUID generator).
This predicate may be used to override the default location used by the
packs tool to store registries and packs by defining
the logtalk_packs library alias in settings file or in a backend
Prolog initialization file (assumed to be loaded prior to Logtalk loading).
The logtalk built-in object provides an expand_library_path/2 predicate that can be used to expand library aliases and files expressed using library notation.
Modes and number of proofs
logtalk_library_path(?atom, -atom) - zero_or_more
logtalk_library_path(?atom, -compound) - zero_or_more
Errors
(none)
Examples
:- initialization((
logtalk_load_context(directory, Directory),
asserta(logtalk_library_path(my_application_root, Directory))
)).
| ?- logtalk_library_path(viewpoints, Path).
Path = examples('viewpoints/')
yes
| ?- logtalk_library_path(Library, Path).
Library = home,
Path = '$HOME/' ;
Library = logtalk_home,
Path = '$LOGTALKHOME/' ;
Library = logtalk_user
Path = '$LOGTALKUSER/' ;
Library = examples
Path = logtalk_user('examples/') ;
Library = library
Path = logtalk_user('library/') ;
Library = viewpoints
Path = examples('viewpoints/')
yes
| ?- logtalk::expand_library_path(viewpoints, Path).
Path = '/Users/pmoura/logtalk/examples/viewpoints/'.
yes
| ?- logtalk::expand_library_path(viewpoints('loader.lgt'), Path).
Path = '/Users/pmoura/logtalk/examples/viewpoints/loader.lgt'.
yes