If ---at runtime--- an undefined predicate is trapped the system will
first try to import the predicate from the module's default module. If
this fails the auto loader is activated. On first activation an
index to all library files in all library directories is loaded in core
(see library_directory/1).
If the undefined predicate can be located in the one of the libraries
that library file is automatically loaded and the call to the
(previously undefined) predicate is resumed. By default this mechanism
loads the file silently. The feature/2
verbose_autoload is provided to get verbose loading. The
feature
autoload can be used to enable/disable the entire auto load
system.
The auto-loader only works if the unknown flag (see unknown/2)
is set to
trace (default). A more appropriate interaction with this
flag will be considered.
Autoloading only handles (library) source files that use the module
mechanism described in chapter 4. The files are
loaded with use_module/2
and only the trapped undefined predicate will be imported to the module
where the undefined predicate was called. Each library directory must
hold a file INDEX.pl that contains an index to all library
files in the directory. This file consists of lines of the following
format:
index(Name, Arity, Module, File). |
The predicate make/0
scans the autoload libraries and updates the index if it exists, is
writable and out-of-date. It is advised to create an empty file called INDEX.pl
in a library directory meant for auto loading before doing anything
else. This index file can then be updated by running the prolog make_library_index/1
(`%' is the Unix prompt):
% mkdir ~/lib/prolog % cd !$ % pl -g true -t 'make_library_index(.)' |
If there are more than one library files containing the desired predicate the following search schema is followed: