This section deals with loading Prolog source-files. A Prolog source file is a plain text file containing a Prolog program or part thereof. Prolog source files come in three flavours:
Prolog source-files are located using absolute_file_name/3 with the following options:
locate_prolog_file(Spec, Path) :- absolute_file_name(Spec, [ file_type(prolog), access(read) ], Path).
file_type(prolog) option is used to determine the
extension of the file using prolog_file_type/2.
The default extension is
.pl. Spec allows for the path-alias
construct defined by absolute_file_name/3.
The most commonly used path-alias is
The example below loads the library file
(containing predicates for manipulating ordered sets).
SWI-Prolog recognises grammar rules (DCG) as defined in Clocksin & Melish, 1987. The user may define additional compilation of the source file by defining the dynamic predicates term_expansion/2 and goal_expansion/2. Transformations by term_expansion/2 overrule the systems grammar rule transformations. It is not allowed to use assert/1, retract/1 or any other database predicate in term_expansion/2 other than for local computational purposes.22It does work for normal loading, but not for qcompile/1.
Directives may be placed anywhere in a source file, invoking any predicate. They are executed when encountered. If the directive fails, a warning is printed. Directives are specified by :-/1 or ?-/1. There is no difference between the two.
SWI-Prolog does not have a separate reconsult/1 predicate. Reconsulting is implied automatically by the fact that a file is consulted which is already loaded.
The following options are currently supported:
false), indicate this load is a demand load. This implies that, depending on the setting of the Prolog flag verbose_autoload the load-action is printed at level
silent. See also print_message/2 and current_prolog_flag/2.
true, run the filenames through expand_file_name/2 and load the returned files. Default is
false, except for consult/1 which is intended for interactive use. Flexible location of files is defined by file_search_path/2.
stream(Stream)option. Default is
source, loading Prolog source text. If
qlf, load QLF data (see qcompile/1).
trueloads the file unconditionally,
changedloads the file if it was not loaded before, or has been modified since it was loaded the last time,
not_loadedloads the file if it was not loaded before.
all. Import is passed from the second argument of use_module/2. Traditionally it is a list of predicate indicators to import. As part of the SWI-Prolog/YAP integration, we also support Pred as Name to import a predicate under another name. Finally, Import can be a term
except(Exceptions), where Exceptions is a list of predicate indicators that specify predicates that are not imported or Pred as Name terms to denote renamed predicates. See also reexport/2 and use_module/2.bugName/Arity as NewName is currently implemented using a link clause. This harms efficiency and does not allow for querying the relation through predicate_property/2.
true, raise an error if the file is not a module file. Used by use_module/[1,2].
true, the contents of the argument files are included in the
.qlffile instead of the loading directive.
truere-export the imported predicate. Used by reexport/1 and reexport/2.
true, load the file without printing a message. The specified value is the default for all files loaded as a result of loading the specified files. This option writes the Prolog flag verbose_load with the negation of Bool.
This option is added to allow compiling from non-file locations such
as databases, the web, the user (see consult/1)
or other servers. It can be combined with
load QLF data from a stream.
predicate can be hooked to load other data or data from other objects
than files. See prolog_load_file/2
for a description and
library(http_load) for an example.
$<var>. File may also be
library(Name), in which case the libraries are searched for a file with the specified name. See also library_directory/1 and file_search_path/2. consult/1 may be abbreviated by just typing a number of file names in a list. Examples:
|% consult |
|% load Quintus compatibility library|
The predicate consult/1
is equivalent to load_files(Files, ), except for handling the special
user, which reads clauses from the terminal. See also
stream(Input) option of load_files/2.
With the semantics, we hope to get as closely possible to the clear semantics without the presence of a module system. Applications using modules should consider using use_module/[1,2].
Equivalent to load_files(Files, [if(not_loaded)]).23On older versions the condition used to be if(changed). Poor time management on some machines or due to copying often caused problems. The make/0 predicate deals with updating the running system after changing the source code.
:- include(File)appears. The include construct is only honoured if it appears as a directive in a source-file. Normally File contains a sequence of directives.
The implementation normally first verifies whether the predicate is already defined. If not, it will search the libraries and load the required library.
SWI-Prolog, having autoloading, does not load the library. Instead it creates a procedure header for the predicate if it does not exist. This will flag the predicate as `undefined'. See also check/0 and autoload/0.
pl -c ...and files loaded using consult or one of its derivatives. The predicate make/0 is called after edit/1, automatically reloading all modified files. If the user uses an external editor (in a separate window), make/0 is normally used to update the program after editing. In addition, make/0 updates the autoload indices (see section 2.13) and runs list_undefined/0 from the
library(check)library to report on undefined predicates.
/lib/prologand the system's library (in this order) are defined. The user may add library directories using assert/1, asserta/1 or remove system defaults using retract/1.
the file specification
demo(myfile) will be expanded to
/usr/lib/prolog/demo/myfile. The second argument of
may be another alias.
Below is the initial definition of the file search path. This path
swi(<Path>) refers to a file in
the SWI-Prolog home directory. The alias
is intended for storing shared libraries (
files). See also
user:file_search_path(library, X) :- library_directory(X). user:file_search_path(swi, Home) :- current_prolog_flag(home, Home). user:file_search_path(foreign, swi(ArchLib)) :- current_prolog_flag(arch, Arch), atom_concat('lib/', Arch, ArchLib). user:file_search_path(foreign, swi(lib)).
The file_search_path/2 expansion is used by all loading predicates as well as by absolute_file_name/[2,3].
The Prolog flag verbose_file_search
can be set to
true to help debugging Prolog's search for
userdetermines the extensions considered by file_search_path/2. Extension is the filename extension without the leading dot, Type denotes the type as used by the
file_type(Type)option of file_search_path/2. Here is the initial definition of prolog_file_type/2:
user:prolog_file_type(pl, prolog). user:prolog_file_type(Ext, prolog) :- current_prolog_flag(associate, Ext), Ext \== pl. user:prolog_file_type(qlf, qlf). user:prolog_file_type(Ext, executable) :- current_prolog_flag(shared_object_extension, Ext).
Users may wish to change the extension used for Prolog source files
to avoid conflicts (for example with perl) as well as to be
compatible with some specific implementation. The preferred alternative
|Module into which file is loaded|
|File loaded. Returns the
original Prolog file when loading a |
|Currently equivalent to |
|Stream identifier (see current_input/1)|
|Directory in which |
|Compatibility mode. See expects_dialect/1.|
|Position of last term read. Term of the form '$stream_position'(0,<Line>,0,0,0). See also stream_position_data/3.|
useror a string), unify File with an absolute path to the file and Line with the line-number in the file. New code should use prolog_load_context/2.
ISO Prolog defines no way for program transformations such as macro expansion or conditional compilation. Expansion through term_expansion/2 and expand_term/2 can be seen as part of the de-facto standard. This mechanism can do arbitrary translation between valid Prolog terms read from the source file to Prolog terms handed to the compiler. As term_expansion/2 can return a list, the transformation does not need to be term-to-term.
Various Prolog dialects provide the analogous goal_expansion/2 and expand_goal/2, that allow for translation of individual body terms, freeing the user of the task to disassemble each clause.
When compiling a module (see chapter
5 and the directive module/2),
will first try term_expansion/2
in the module being compiled to allow for term-expansion rules that are
local to a module. If there is no local definition, or the local
definition fails to translate the term, expand_term/2
will try term_expansion/2
user. For compatibility with SICStus and Quintus Prolog,
this feature should not be used. See also expand_term/2, goal_expansion/2
The predicate goal_expansion/2
is first called in the module that is being compiled, and then on the
module. If Goal is of the form Module:Goal
where Module is instantiated,
is called on Goal using rules from module
Module followed by
Only goals appearing in the body of clauses when reading a source-file are expanded using this mechanism, and only if they appear literally in the clause, or as an argument to the meta-predicates not/1, call/1, once/1, ignore/1, findall/3, bagof/3, setof/3 or forall/2. A real predicate definition is required to deal with dynamically constructed calls.
%f' is replaced by the name of the file to be loaded. The standard output of resulting command is loaded. To use the Unix C preprocessor one should define:
?- preprocessor(Old, '/lib/cpp -C -P %f'), consult(...). Old = none
Using cpp for Prolog preprocessing is not ideal as the tokenization rules for comment and quoted strings differ between C and Prolog. Another problem is availability and compatibility with regard to option processing of cpp.
Conditional compilation builds on the same principle as term_expansion/2, goal_expansion/2 and the expansion of grammar rules to compile sections of the source-code conditionally. One of the reasons for introducing conditional compilation is to simplify writing portable code. See section C for more information. Here is a simple example:
:- if(\+source_exports(library(lists), suffix/2)). suffix(Suffix, List) :- append(_, Suffix, List). :- endif.
Note that these directives can only appear as separate terms in the input. Typical usage scenarios include:
:- if(test1). section_1. :- elif(test2). section_2. :- elif(test3). section_3. :- else. section_else. :- endif.
Traditionally, Prolog environments allow for reloading files holding currently active code. In particular, the following sequence is valid use of the development environment:
Goals running during the reload keep running on the old definition, while new goals use the reloaded definition, which is why the retry must be used after the reload. This implies that clauses of predicates that are active during the reload cannot be reclaimed. Normally a small amount of dead clauses should not be an issue during development. Such clauses can be reclaimed with garbage_collect_clauses/0.
As of version 5.5.30, there is basic thread-safety for reloading source files while other threads are executing code defined in these source files. Reloading a file freezes all threads after marking the active predicates originating from the file being reloaded. The threads are resumed after the file has been loaded. In addition, after completing loading the outermost file, the system runs garbage_collect_clauses/0.
What does that mean? Unfortunately it does not mean we can `hot-swap' modules. Consider the case where thread A is executing the recursive predicate P. We `fix' P and reload. The already running goals for P continue to run the old definition, but new recursive calls will use the new definition! Many similar cases can be constructed with dependent predicates.
It provides some basic security for reloading files in multi-threaded applications during development. In the above scenarios the system does not crash uncontrolled, but behaves like any broken program: it may return the wrong bindings, wrong truth value or raise an exception.
Future versions may have an `update now' facility. Such a facility can be implemented on top of the logical update view. It would allow threads to do a controlled update between processing independent jobs.
SWI-Prolog supports compilation of individual or multiple Prolog
source files into `Quick Load Files'. A `Quick Load Files' (
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
or one of the other file-loading predicates described in
section 4.3. If consult is
given the explicit
.pl file, it will load the Prolog
source. When given the
.qlf file, it will load the file.
When no extension is specified, it will load the
.qlf file when present and the
library(LibFile)and, in addition to the normal compilation, creates a Quick Load File from File. The file-extension of this file is
.qlf. The base name of the Quick Load File is the same as the input file.
If the file contains `
:- consult(+File)', `
[qcompile(true), ...]) 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
For term_expansion/2, the same rules as described in section 2.10 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.