Filed predicatesAuthor(s): Francisco Bueno.
This package allows using files as a “cache” for predicates defined by facts. This is useful for huge tables of facts that may push the memory limits of the system too far. Goals of a filed predicate are executed simply by reading from the corresponding file.
Anything in the DB file used for the predicate that is different from a fact for the corresponding predicate is ignored. Each call to a filed predicate forces opening the file, so the use of this package is subject to the limit on the number of open files that the system can support.
Dynamic modification of the filed predicates is also allowed during execution of the program. Thus filed predicates are regarded as dynamic, data predicates residing in a file. However, dynamic modifications to the predicates do not affect the file, unless the predicate is also declared persistent.
The package is compatible with persdb in the sense that a predicate can be made both filed and persistent. In this way, the predicate can be used in programs, but it will not be loaded (saving memory), can also be modified during execution, and modifications will persist in the file. Thus, the user interface to both packages is the same (so the DB file must be one for both filing and persistency).
Usage and interface
- Library usage:
This facility is used as a package, thus either including factsdb in the package list of the module, or by using the use_package/1 declaration. The facility predicates are defined in library module factsdb_rt.
Documentation on multifiles
Known bugs and planned improvements
- The DB files for persistent predicates have to be used as such from the beginning. Using a DB file for a filed predicate first, and then using it also when making the predicate persistent won't work. Nor the other way around: using a DB file for a persistent predicate first, and then using it also when making the predicate filed.