12.3.1 Resource manipulation predicates

resource(+Name, +Class, +FileSpec)

This predicate is defined as a dynamic predicate in the module user. Clauses for it may be defined in any module, including the user module. Name is the name of the resource (an atom). A resource name may contain any character, except for $ and :, which are reserved for internal usage by the resource library. Class describes the kind of object stored in the resource. In the current implementation, it is just an atom. FileSpec is a file specification that may exploit file_search_path/2 (see absolute_file_name/2).

Normally, resources are defined as unit clauses (facts), but the definition of this predicate also allows for rules. For proper generation of the saved state, it must be possible to enumerate the available resources by calling this predicate with all its arguments unbound.

Dynamic rules are useful to turn all files in a certain directory into resources, without specifying a resource for each file. For example, assume the file_search_path/2 icons refers to the resource directory containing icon files. The following definition makes all these images available as resources:

resource(Name, image, icons(XpmName)) :-
        atom(Name), !,
        file_name_extension(Name, xpm, XpmName).
resource(Name, image, XpmFile) :-
        var(Name),
        absolute_file_name(icons(.), [type(directory)], Dir)
        concat(Dir, '/*.xpm', Pattern),
        expand_file_name(Pattern, XpmFiles),
        member(XpmFile, XpmFiles).

open_resource(+Name, ?Class, -Stream)

Opens the resource specified by Name and Class. If the latter is a variable, it will be unified to the class of the first resource found that has the specified Name. If successful, Stream becomes a handle to a binary input stream, providing access to the content of the resource.

The predicate open_resource/3 first checks resource/3. When successful it will open the returned resource source file. Otherwise it will look in the program's resource database. When creating a saved state, the system normally saves the resource contents into the resource archive, but does not save the resource clauses.

This way, the development environment uses the files (and modifications) to the resource/3 declarations and/or files containing resource info, thus immediately affecting the running environment, while the runtime system quickly accesses the system resources.