This manual describes version 7.1.21 of SWI-Prolog. SWI-Prolog is widely considered to be a robust and scalable implementation of the Prolog language. It is widely used in education and research. In addition, it is in use for 24 × 7 mission critical commercial server processes. The site http://www.swi-prolog.org is hosted using the SWI-Prolog HTTP server infrastructure. It receives approximately 2.3 million hits and serves approximately 300 Gbytes on manual data and downloads each month. SWI-Prolog applications range from student assignments to commercial applications that count more than one million lines of Prolog code.
SWI-Prolog has two development tracks. Stable releases have an even minor version number (e.g., 6.2.1) and are released as a branch from the development version when the development version is considered stable and there is sufficient new functionality to justify a stable release. Stable releases often get a few patch updates to deal with installation issues or major flaws. A new Development version is typically released every couple of weeks as a snapshot of the public git repository. `Extra editions' of the development version may be released after problems that severely hindered the user in their progress have been fixed.
Known bugs that are not likely to be fixed soon are described as footnotes in this manual.