3.1.1 File Names and Locations

3.1.1.1 File Name Extensions

The first consideration is what extension to use for the source files. Tradition calls for .pl, but conflicts with Perl force the use of another extension on systems where extensions have global meaning, such as MS-Windows. On such systems .pro is the common alternative. On MS-Windows, the alternative extension is stored in the registry key HKEY_CURRENT_USER/Software/SWI/Prolog/fileExtension or HKEY_LOCAL_MACHINE/Software/SWI/Prolog/fileExtension. All versions of SWI-Prolog load files with the extension .pl as well as with the registered alternative extension without explicitly specifying the extension. For portability reasons we propose the following convention:

If there is no conflict

because you do not use a conflicting application or the system does not force a unique relation between extension and application, use .pl.

With a conflict

choose .pro and use this extension for the files you want to load through your file manager. Use .pl for all other files for maximal portability.

3.1.1.2 Project Directories

Large projects are generally composed of sub-projects, each using its own directory or directory structure. If nobody else will ever touch your files and you use only one computer, there is little to worry about, but this is rarely the case with a large project.

To improve portability, SWI-Prolog uses the POSIX notation for filenames, which uses the forward slash (/) to separate directories. Just before reaching the file system, SWI-Prolog uses prolog_to_os_filename/2 to convert the filename to the conventions used by the hosting operating system. It is strongly advised to write paths using the /, especially on systems using the \ for this purpose (MS-Windows). Using \ violates the portability rules and requires you to double the \ due to the Prolog quoted-atom escape rules.

Portable code should use prolog_to_os_filename/2 to convert computed paths into system paths when constructing commands for shell/1 and friends.

3.1.1.3 Sub-projects using search paths

Thanks to Quintus, Prolog adapted an extensible mechanism for searching files using file_search_path/2. This mechanism allows for comfortable and readable specifications.

Suppose you have extensive library packages on graph algorithms, set operations and GUI primitives. These sub-projects are likely candidates for re-use in future projects. A good choice is to create a directory with sub-directories for each of these sub-projects.

Next, there are three options. One is to add the sub-projects to the directory hierarchy of the current project. Another is to use a completely dislocated directory. Third, the sub-project can be added to the SWI-Prolog hierarchy. Using local installation, a typical file_search_path/2 is:

:- prolog_load_context(directory, Dir),
   asserta(user:file_search_path(myapp, Dir)).

user:file_search_path(graph, myapp(graph)).
user:file_search_path(ui,    myapp(ui)).

When using sub-projects in the SWI-Prolog hierarchy, one should use the path alias swi as basis. For a system-wide installation, use an absolute path.

Extensive sub-projects with a small well-defined API should define a load file with calls to use_module/1 to import the various library components and export the API.