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|Loading Prolog source files|
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
Melish, 1987. The user may define additional compilation of
the source file by defining the dynamic multifile predicates
term_expansion/2, term_expansion/4, goal_expansion/2
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.48It
does work for normal loading, but not for qcompile/1.
Code that needs to create additional clauses must use compile_aux_clauses/1.
library(library(apply_macros)) for an example.
A directive is an instruction to the compiler. Directives
are used to set (predicate) properties (see section
4.15), set flags (see set_prolog_flag/2)
and load files (this section). Directives are terms of the form
. 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
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.
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:
load_files(Files,). Same as consult/1, See load_files/2 for supported options.
The following options are currently supported:
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
silent. See also print_message/2 and current_prolog_flag/2.
true), do not check the first character to be
and skip the first line when found.
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, and
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 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
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
stream(Input)option, for example after extracting the time from an HTTP server or database.
:- module(Name, ...)directive. This currently serves two purposes: (1) allow loading two module files that specify the same module into the same process and force and (2): force loading source code in a specific module, even if the code provides its own module name. Experimental.
true, raise an error if the file is not a module file. Used by use_module/[1,2].
.qlffile 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.
askis only provided if the stream
user_inputis associated with a terminal.
truere-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.
falsewhile the Prolog flag is set to
trueraises a permission error.
true. The system and user initialization files (see -f and -F) are loading with
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 objects other
than files. See prolog_load_file/2
for a description and
library(http/http_load) for an example. All hooks for load_files/2
are documented in section
|% consult |
|% load library lists|
|% 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
?- [file1,file2]. does
not work for the empty list (
). This facility is
implemented by defining the list as a predicate. Applications may only
rely on using the list abbreviation at the Prolog toplevel and in
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].
older versions the condition used to be
time management on some machines or copying often caused problems. The make/0
predicate deals with updating the running system after changing the
:- 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.
Textually 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 been 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.
If include/1 is used to load files holding clauses, and if these files are loaded only once, then these include/1 directives can be replaced by other predicates (such as consult/1). However, there are several cases where either include/1 has no alternative, or using any alternative also requires other changes. An example of the former is using include/1 to share directives. An example of the latter are cases where clauses of different predicates are distributed over multiple files: If these files are loaded with include/1, the directive discontiguous/1 is appropriate, whereas if they are consulted, one must use the directive 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 2.14) 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 assertz/1, asserta/1 or remove system defaults using retract/1. Deprecated. New code should use file_search_path/2.
library(lists). This feature is best described using an example. Given the definition:
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>) and 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)). 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 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, and 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.
userif 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.
fileif the source was loaded from a file,
resourceif the source was loaded from a resource or
stateif the file was included in the saved state.
This predicate should be used with care. The multithreaded nature of SWI-Prolog makes removing static code unsafe. Attempts to do this should be reserved for development or situations where the application can guarantee that none of the clauses associated to File are active.
|Directory in which |
|Compatibility mode. See expects_dialect/1.|
|Similar to |
|Module into which file is loaded|
|Boolean that indicates whether the file is loaded as a script file (see -s)|
|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
|Stream identifier (see current_input/1)|
|Start position of last term
read. See also
|Term being expanded by expand_term/2.|
|A list of `Name = Var' of the last term read. See read_term/2 for details.|
:- 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) ]), ...
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.
The ISO standard only allows for using
:- Term if Term
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,
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
prolog:initialize_now(Term, Advice), where Advice
is an atom that gives advice on how to resolve the compatibility issue.
restoreis still accepted for backward compatibility.
initialization(Goal, main)is executed as main goal. If Goal fails or raises an exception, the process terminates with non-zero exit code. If not explicitly specified using the -t the toplevel goal is set to halt/0, causing the process to exit with status 0. An explicitly specified toplevel is executed normally. This implies that
-t prologcauses the application to start the normal interactive toplevel after completing Goal. See also the Prolog flag toplevel_goal and section 22.214.171.124.
initialization(Goal, program). Raises an error
initialization_error(Reason, Goal, File:Line)if Goal fails or raises an exception. Reason is
failedor the exception raised.
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.
:- 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:
When compiling a module (see chapter
6 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
It is possible to act on the beginning and end of a file by expanding
latter is supported by most Prolog systems that support term expansion
end_of_file on reaching the end of the input.
begin_of_file may be used to initialise the
compilation, for example base on the file name extension. It was added
in SWI-Prolog 8.1.1.
if we are in a `false branch' of the conditional compilation. See section 126.96.36.199.
userand finally in
system. Library modules inherit directly from
systemand can thus not be re-interpreted by term expansion rules in
The predicate goal_expansion/2
is first called in the module that is being compiled, and then follows
the module inheritance path as defined by default_module/2,
i.e., by default
system. 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 default modules for Module.
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.
The expansion hook can use prolog_load_context/2 to obtain information about the context in which the goal is expanded such as the module, variable names or the encapsulating term.
If goal_expansion/2 wraps a goal as in the example below the system still reaches fixed-point as it prevents re-expanding the expanded term while recursing. It does re-enable expansion on the arguments of the expanded goal as illustrated in t2/1 in the example.53After discussion with Peter Ludemann and Paulo Moura on the forum.
:- meta_predicate run(0). may_not_fail(test(_)). may_not_fail(run(_)). goal_expansion(G, (G *-> true ; error(goal_failed(G),_))) :- may_not_fail(G). t1(X) :- test(X). t2(X) :- run(run(X)).
Is expanded into
t1(X) :- ( test(X) *-> true ; error(goal_failed(test(X)), _) ). t2(X) :- ( run((run(X)*->true;error(goal_failed(run(X)), _))) *-> true ; error(goal_failed(run(run(X))), _) ).
Note that in some cases multiple expansions of similar goals can
share the same compiled auxiliary predicate. In such cases, the
implementation of goal_expansion/2
can use predicate_property/2
using the property
defined to test whether the predicate is already defined in
the current context.
Head-->Bodyinto a normal Prolog clause. Normally this functionality should be accessed using expand_term/2.
This sections documents extended versions of the program transformation predicates that also transform the source layout information. Extended layout information is currently processed, but unused. Future versions will use for the following enhancements:
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
elifis 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:
Reloading a previously loaded file is safe, both in the debug scenario above and when the code is being executed by another thread. Executing threads switch atomically to the new definition of modified predicates, while clauses that belong to the old definition are (eventually) reclaimed by garbage_collect_clauses/0.54As of version 7.3.12. Older versions wipe all clauses originating from the file before loading the new clauses. This causes threads that executes the code to (typically) die with an undefined predicate exception. Below we describe the steps taken for reloading a file to help understanding the limitations of the process.
meta_predicateare in part applied immediately and in part during the fixup process after the file completes loading. Currently,
thread_localare applied immediately.
The above generally ensures that changes to the content of source files can typically be activated safely using make/0. Global changes such as operator changes, changes of module names, changes to multi-file predicates, etc. sometimes require a restart. In almost all cases, the need for restart is indicated by permission or syntax errors during the reload or existence errors while running the program.
In some cases the content of a source file refers `to itself'. This
is notably the case if local rules for goal_expansion/2
are defined or goals are executed using
directives.55Note that initialization/1
directives are executed after loading the file. SWI-Prolog
allows for directives that are executed while loading the file
:- Goal. or initialization/2.
Up to version 7.5.12 it was typically needed to reload the file twice,
once for updating the code that was used for compiling the remainder of
the file and once to effectuate this. As of version 7.5.13, conventional transaction
semantics apply. This implies that for the thread performing the
reload the file's content is first wiped and gradually rebuilt, while
other threads see an atomic update from the old file content to
the new.56This feature was
implemented by Keri Harris.
Large programs are generally split into multiple files. If file A
accesses predicates from file B which accesses predicates
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
is mapped to load_files/2
using the option
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
predicate, however, maps to load_files/2
with the option
if(not_loaded)(.) A use_module/1
on an already loaded file merely makes the public predicates of the used
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 reloading, see section 4.3.2.
In older versions, compilation was thread-safe due to a global lock in load_files/2 and the code dealing with autoloading (see section 2.14). 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.57Although 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 188.8.131.52) 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.
SWI-Prolog supports compilation of individual or multiple Prolog
source files into `Quick Load Files'. A `Quick Load File' (
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/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
.qlf. The basename of the Quick Load File is the same as the input file.
If the file contains `
:- consult(+File)', `
[+File]' or `
[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
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.