Introduction -- library(directory)

For the most part, Prolog programs have little need to examine directories or to inquire about file properties. However, the need does occasionally arise. For example, an expert-system shell might offer the option of either loading a single file into its knowledge base, or of loading all the files in a directory having a particular extension. The Quintus Prolog library file library(directory) provides the tools you need to do this. For example, we might define

     kb_load(File) :-
         (  directory_property(File, searchable) ->
               forall(file_member_of_directory(File,'*.kb',_,Full),
                        kb_load(Full))
         ;  file_property(File, readable) ->
               kb_load_file(File)
         ;  format(user_error, '~N! cannot read ~w.~n', [File]),
            fail
         ).
     

The routines in this package were designed to be a complete toolkit for safely wandering around a UNIX-like file system. Although there are quite a few of them, they do actually fit together in a coherent group. For information on operations relating to individual files rather than to directories, see library(files) (lib-ofi).

The following principles have been observed:

• An absolute distinction is drawn between files and directories. The set of operations one can usefully perform on a directory is different from the set one can perform on a file: for example, having write permission to a directory allows the user to create new files in it, not to rewrite the entire directory! If any routine in this package tells you that a "file" exists, you can be sure that it means a "regular" file.
• The directory scanning routines do not actually open the files they find. Thus finer discriminations, such as that between source and object code, are not made.
• The predicate names are made up of complete English words in lowercase, separated by underscores, with no abbreviations.
• Every predicate acts like a genuine logical relation insofar as it possibly can.
• Like those in the library(unix) package, if anything goes wrong, the predicates in library(directory) raise an error exception. Any time that a predicate fails quietly, it should mean "this question is meaningful, but the answer is no"; any exception to this should be regarded as a bug.
• The directory scanning routines insist that the directory argument name a searchable directory. But the "property" routines are to be read as "there exists a thing of such a type with such a property", and quietly fail if there is no such file or directory.