3.1 The project source-files

Organisation of source-files depends largely on the size of your project. If you are doing exercises for a Prolog course you'll normally use one file for each exercise. If you have a small project you'll work work with one directory holding a couple of files and some files to link it all together. Even bigger projects will be organised in sub-projects each using their own directory.

3.1.1 File Names and Locations 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.13On 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. Project Directories

Large projects are generally composed of sub-projects, each using their 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 hitting the file-system it 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. 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 and finally 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)).

For 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 using use_module/1 calls to import the various library-components and export the API.

3.1.2 Project Special Files

There are a number of tasks you typically carry out on your project, such as loading it, creating a saved-state, debugging it, etc. Good practice on large projects is to define small files that hold the commands to execute such a task, name this file after the task and give it a file-extension that makes starting easy (see section The task load is generally central to these tasks. Here is a tentative list.

3.1.3 International source files

As discussed in section 2.17, SWI-Prolog supports international character handling. Its internal encoding is UNICODE. I/O streams convert to/from this internal format. This sections discusses the options for source-files not in US-ASCII.

SWI-Prolog can read files in any of the encodings described in section 2.17. Two encodings are of particular interest. The text encoding deals with the current locale, the default used by this computer for representing text files. The encodings utf8, unicode_le and unicode_be are UNICODE encodings: they can represent---in the same file---characters of virtually any known language. In addition, they do so unambiguously.

If one wants to represent non US-ASCII text as Prolog terms in a source-file there are several options: