4.3.3 Quick load files

SWI-Prolog supports compilation of individual or multiple Prolog source files into ‘Quick Load Files’. A ‘Quick Load File’ (.qlf file) stores the contents of the file in a precompiled format.

These files load considerably faster than source files and are normally more compact. They are machine-independent and may thus be loaded on any implementation of SWI-Prolog. Note, however, that clauses are stored as virtual machine instructions. Changes to the compiler will generally make old compiled files unusable.

Quick Load Files are created using qcompile/1. They are loaded using consult/1 or one of the other file-loading predicates described in section 4.3. If consult/1 is given an explicit .pl file, it will load the Prolog source. When given a .qlf file, it will load the file. When no extension is specified, it will load the .qlf file when present and up-to-date, and the .pl file otherwise.

A .qlf file is up-to-date if the source it was compiled from has not changed since. Whether it changed is decided by the content of the source, a hash of which is stored in the .qlf file, and not by its modification time: a tree that arrives by checkout, copy, unpack or install carries times of its own, in either direction. A source that is newer but unchanged is thus still loaded from its .qlf file. If the source did change, the .qlf file is recompiled when it can be written and the source is loaded without it otherwise. Set the source flag to ignore .qlf files altogether.

qcompile(:File)

Takes a file specification as consult/1, etc., and, in addition to the normal compilation, creates a Quick Load File from File. The file extension of this file is .qlf. The basename of the Quick Load File is the same as the input file.

If the file contains ‘:- consult(+File)’, ‘:- [+File]’ or ‘:- load_files(+File, [qcompile(part), ...])’ statements, the referred files are compiled into the same .qlf file. Other directives will be stored in the .qlf file and executed in the same fashion as when loading the .pl file.

For term_expansion/2, the same rules as described in section 2.11 apply.

Conditional execution or optimisation may test the predicate compiling/0.

Source references (source_file/2) in the Quick Load File refer to the Prolog source file from which the compiled code originates.

qcompile(:File, +Options)

As qcompile/1, but processes additional options as defined by load_files/2. Options are passed to load_files/2. In addition the following options are processed:

include(+Include)

What to include into the QLF file. Currently accepts only a single value: the atom user. When specified, files loaded indirectly from File that to not come from the Prolog library are included into the .qlf file. This may be used to generate a single file from an application. The result is comparable to a save state (see qsave_program/2) with the following differences:

• Only your application code is included. The Prolog libraries and boot files are not.
• Only Prolog code is included, .qlf files cannot include arbitrary resources.
• The file can be loaded into a running Prolog process, while a saved state can only be loaded into a virgin Prolog virtual machine.