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).
The 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 library(LibraryFile).
The example below loads the library file ordsets.pl
(containing predicates for manipulating ordered sets).
:- use_module(library(ordsets)).
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 system's 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.33It
does work for normal loading, but not for qcompile/1.
Code that needs to create additional clauses must use
compile_aux_clauses/1.
See library(library(apply_macros)) for an example.
A directive is an instruction to the compiler. Directives
are used to set (predicate) properties (see section
4.14), set flags (see set_prolog_flag/2)
and load files (this section). Directives are terms of the form :- <term>.
. Here are some examples:
:- use_module(library(lists)).
:- dynamic
store/2. % Name, Value
The directive initialization/1 can be used to run arbitrary Prolog goals. The specified goal is started after loading the file in which it appears has completed.
SWI-Prolog compiles code as it is read from the file and directives
are executed as goals. This implies that directives may call
any predicate that has been defined before the point where the directive
appears. It also accepts ?- <term>.
as a synonym.
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.
Advanced topics are handled in subsequent sections: mutually dependent files (section 4.3.2.1), multi-threaded loading (section 4.3.2.2) and reloading running code (section 4.3.2.3).
The core of the family of loading predicates is load_files/2. The predicates consult/1, ensure_loaded/1, use_module/1, use_module/2 and reexport/1 pass the file argument directly to load_files/2 and pass additional options as expressed in the table 4:
| Predicate | if | must_be_module | import |
| consult/1 | true | false | all |
| ensure_loaded/1 | not_loaded | false | all |
| use_module/1 | not_loaded | true | all |
| use_module/2 | not_loaded | true | specified |
| reexport/1 | not_loaded | true | all |
| reexport/2 | not_loaded | true | specified |
| Table 4 : Properties of the file-loading predicates. The import column specifies what is imported if the loaded file is a module file. |
The following options are currently supported:
true (default false), indicate that 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 informational or 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).true loads the file unconditionally, changed
loads the file if it was not loaded before or has been modified since it
was loaded the last time, and not_loaded loads 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 the 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.
If Import equals all, all operators are
imported as well. Otherwise, operators are not imported.
Operators can be imported selectively by adding terms op(Pri,Assoc,Name)
to the
Import list. If such a term is encountered, all exported
operators that unify with this term are imported. Typically, this
construct will be used with all arguments unbound to import all
operators or with only Name bound to import a particular
operator.
stream(Input) option, for example after extracting the
time from an HTTP server or database.true, raise an error if the file is not a module file.
Used by
use_module/[1,2]..qlf
file instead of the loading directive.
If this option is not present, it uses the value of the Prolog flag qcompile as default.
false
(default), which prints an error and refuses to load the file, or
true, which uses unload_file/1
on the old file and then proceeds loading the new file. Finally, there
is ask, which starts interaction with the user. ask
is only provided if user_input is associated with a terminal.true re-export the imported predicate. Used by reexport/1
and reexport/2.false, do not register the load location and options.
This option is used by make/0
and load_hotfixes/1
to avoid polluting the load-context database. See source_file_property/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 format(qlf) to
load QLF data from a stream.
The load_files/2
predicate can be hooked to load other data or data from objects other
than files. See prolog_load_file/2
for a description and
library(http_load) for an example.
?- consult(load). | % consult load
or load.pl |
?- [library(lists)]. | % load library lists |
?- [user]. | % Type program on the terminal |
The predicate consult/1
is equivalent to load_files(File, []), except for handling
the special file user, which reads clauses from the
terminal. See also the stream(Input) option of load_files/2.
With this semantics, we hope to get as close as 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)]).34On
older versions the condition used to be if(changed). Poor
time management on some machines or 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. Textual include (similar to C/C++ #include) is obviously
useful for sharing declarations such as dynamic/1
or
multifile/1
by including a file with directives from multiple files that use these
predicates.
Textual including files that contain clauses is less obvious. Normally, in SWI-Prolog, clauses are owned by the file in which they are defined. This information is used to replace the old definition after the file has beeen modified and is reloaded by, e.g., make/0. As we understand it, include/1 is intended to include the same file multiple times. Including a file holding clauses multiple times into the same module is rather meaningless as it just duplicates the same clauses. Including a file holding clauses in multiple modules does not suffer from this problem, but leads to multiple equivalent copies of predicates. Using use_module/1 can achieve the same result, while sharing the predicates.
Despite these observations, various projects seem to be using include/1 to load files holding clauses, typically loading them only once. Such usage would allow replacement by, e.g., consult/1. Unfortunately, the same project might use include/1 to share directives. Another example of a limitation of mapping to consult/1 is that if the clauses of a predicate are distributed over two included files, discontiguous/1 is appropriate, while if they are distributed over two consulted files, one must use multifile/1.
To accommodate included files holding clauses, SWI-Prolog distinguishes between the source location of a clause (in this case the included file) and the owner of a clause (the file that includes the file holding the clause). The source location is used by, e.g., edit/1, the graphical tracer, etc., while the owner is used to determine which clauses are removed if the file is modified. Relevant information is found with the following predicates:
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/1
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, /lib/prolog and the system's library
(in this order) are defined. The user may add library directories using
assertz/1, asserta/1
or remove system defaults using retract/1.
Deprecated. New code should use file_search_path/2.file_search_path(demo, '/usr/lib/prolog/demo').
the file specification demo(myfile) will be expanded to
/usr/lib/prolog/demo/myfile. The second argument of
file_search_path/2
may be another alias.
Below is the initial definition of the file search path. This path
implies swi(<Path>) and refers to a file
in the SWI-Prolog home directory. The alias foreign(<Path>)
is intended for storing shared libraries (.so or .DLL
files). See also
load_foreign_library/[1,2].
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)).
user:file_search_path(path, Dir) :-
getenv('PATH', Path),
( current_prolog_flag(windows, true)
-> atomic_list_concat(Dirs, (;), Path)
; atomic_list_concat(Dirs, :, Path)
),
member(Dir, Dirs).
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
files.
user
determines 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 can add extensions for Prolog source files to avoid conflicts
(for example with perl) as well as to be compatible with another
Prolog implementation. We suggest using .pro for avoiding
conflicts with perl. Overriding the system definitions can stop
the system from finding libraries.
derived_from(Original) option to load_files/2.includes property.user if the file was loaded from the terminal or another
unknown source. Options are the options passed to load_files/2.
Note that all predicates to load files are mapped to load_files/2,
using the option argument to specify the exact behaviour.This predicare should be used with care. The multi-threaded nature of SWI-Prolog makes removing static code unsafe. Attempts to do this should be reserved to development or situations where the application can guarantee that none of the clauses associated to File are active.
| Key | Description |
module | Module into which file is loaded |
source | File being loaded. If
the system is processing an included file, the value is the main
file. Returns the original Prolog file when loading a
.qlf file. |
file | Similar to source,
but returns the file being included when called while an include file is
being processed. |
stream | Stream identifier (see current_input/1) |
directory | Directory in which source
lives. |
dialect | Compatibility mode. See expects_dialect/1. |
term_position | Position of last term read. Term of the form '$stream_position'(0,<Line>,0,0,0). See also stream_position_data/3. |
script | Boolean that indicates whether the file is loaded as a script file (see -s). |
The directory is commonly used to add rules to file_search_path/2,
setting up a search path for finding files with absolute_file_name/3.
E.g.:
:- dynamic user:file_search_path/2.
:- multifile user:file_search_path/2.
:- prolog_load_context(directory, Dir),
asserta(user:file_search_path(my_program_home, Dir)).
...
absolute_file_name(my_program_home('README.TXT'), ReadMe,
[ access(read) ]),
...
user or 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.The ISO standard only allows for using :- Term if Term
is a
directive. This means that arbitrary goals can only be called
from a directive by means of the initialization/1
directive. SWI-Prolog does not enforce this rule.
The initialization/1 directive must be used to do program initialization in saved states (see qsave_program/1). A saved state contains the predicates, Prolog flags and operators present at the moment the state was created. Other resources (records, foreign resources, etc.) must be recreated using initialization/1 directives or from the entry goal of the saved state.
Up to SWI-Prolog 5.7.11, Goal was executed immediately
rather than after loading the program text in which the directive
appears as dictated by the ISO standard. In many cases the exact moment
of execution is irrelevant, but there are exceptions. For example,
load_foreign_library/1
must be executed immediately to make the loaded foreign predicates
available for exporting. SWI-Prolog now provides the directive use_foreign_library/1
to ensure immediate loading as well as loading after restoring a saved
state. If the system encounters a directive :-
initialization(load_foreign_library(...)), it will load the
foreign library immediately and issue a warning to update your code.
This behaviour can be extended by providing clauses for the multifile
hook predicate prolog:initialize_now(Term, Advice), where Advice
is an atom that gives advice how to resolve the compatibility issue.
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.
?- Goal. or :- Goal. Goal
is then treated as a directive. If Term2 is a list, all terms
of the list are stored in the database or called (for directives). If
Term2 is of the form below, the system will assert Clause
and record the indicated source location with it.
'$source_location'(<File>, <Line>):<Clause>
When compiling a module (see chapter
5 and the directive module/2),
expand_term/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
in module
user. For compatibility with SICStus and Quintus Prolog,
this feature should not be used. See also expand_term/2, goal_expansion/2
and
expand_goal/2.
[]
if we are in a `false branch' of the conditional compilation. See section
4.3.1.1.
user.
The predicate goal_expansion/2
is first called in the module that is being compiled, and then on the user
module. If Goal is of the form Module:Goal
where Module is instantiated,
goal_expansion/2
is called on Goal using rules from module
Module followed by user.
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 a defined meta-predicate that is annotated using `0' (see meta_predicate/1). Other cases need a real predicate definition.
Head-->Body
into a normal Prolog clause. Normally this functionality should be
accessed using expand_term/2.%f' is replaced by the name of the file to be loaded. The
standard output of the 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:
:- else. :-if(Goal). ... :- endif.
In a sequence as below, the section below the first matching elif
is processed. If no test succeeds the else branch is processed.
:- 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 a 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.
Large programs are generally split into multiple files. If file A
accesses predicates from file B which accesses predicates
from file
A, we consider this a mutual or circular dependency. If
traditional load predicates (e.g., consult/1)
are used to include file B from A and A
from B, loading either file results in a loop. This is
because
consult/1
is mapped to load_files/2
using the option if(true)(if(true)) Such programs are
typically loaded using a load file that consults all required
(non-module) files. If modules are used, the dependencies are made
explicit using use_module/1
statements. The
use_module/1
predicate, however, maps to load_files/2
with the option
if(not_loaded)(if(not_loaded)) A use_module/1
on an already loaded file merely makes the public predicates of the used
module available.
Summarizing, mutual dependency of source files is fully supported with no precautions when using modules. Modules can use each other in an arbitrary dependency graph. When using consult/1, predicate dependencies between loaded files can still be arbitrary, but the consult relations between files must be a proper tree.
This section discusses compiling files for the first time. For re-loading see section 4.3.2.3.
In older versions, compilation was thread-safe due to a global lock in load_files/2 and the code dealing with autoloading (see section 2.13). Besides unnecessary stalling when multiple threads trap unrelated undefined predicates, this easily leads to deadlocks, notably if threads are started from an initialization/1 directive.35Although such goals are started after loading the file in which they appear, the calling thread is still likely to hold the `load' lock because it is compiling the file from which the file holding the directive is loaded.
Starting with version 5.11.27, the autoloader is no longer locked and multiple threads can compile files concurrently. This requires special precautions only if multiple threads wish to load the same file at the same time. Therefore, load_files/2 checks automatically whether some other thread is already loading the file. If not, it starts loading the file. If another thread is already loading the file, the thread blocks until the other thread finishes loading the file. After waiting, and if the file is a module file, it will make the public predicates available.
Note that this schema does not prevent deadlocks under all situations. Consider two mutually dependent (see section 4.3.2.1) module files A and B, where thread 1 starts loading A and thread 2 starts loading B at the same time. Both threads will deadlock when trying to load the used module.
The current implementation does not detect such cases and the involved threads will freeze. This problem can be avoided if a mutually dependent collection of files is always loaded from the same start file.
This section discusses not re-loading of code. Initial loading of code is discussed in section 4.3.2.2.
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 scenario 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 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 the .pl file
otherwise.
.qlf.
The base name 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 2.10 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.