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 multifile predicates
term_expansion/2, term_expansion/4, goal_expansion/2
and
goal_expansion/4.
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.52It
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
4.15), set flags (see set_prolog_flag/2)
and load files (this section). Directives are terms of the form :-
:- 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 ?-
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 4.3.2.2), multithreaded loading (section 4.3.2.3) and reloading running code (section 4.3.2).
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 |
load_files(Files,[]). Same as consult/1,
See load_files/2
for supported options.
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.false (default true), do not check the
first character to be #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.:- 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]..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 the stream 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. Using false while the
Prolog flag is set to true raises a permission error.true. The system and user initialization files (see -f
and
-F) are loading with scope_settings(false).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/http_load) for an example. All hooks for load_files/2
are documented in section
B.9.
?- 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.
Abbreviation using ?- [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
directives.
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)]).53On
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.
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 require/1 directive provides less control over the exact nature and location of the predicate. As autoload/2, it prevents a local definition of this predicate. As SWI-Prolog guarantees that the set of built-in predicates and predicates available for autoloading is unambiguous (i.e., has no duplicates) the specification is unambiguous. It provides four advantages over autoload/2: (1) the user does not have to remember the exact library, (2) the directive can be supported in other Prolog systems54SICStus provides it, providing compatibility despite differences in library and built-in predicate organization, (3) it is robust against changes to the SWI-Prolog libraries and (4) it is less typing.
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.app_config(lib) (see file_search_path/2)
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.Alias(Name), e.g., library(lists). This
feature is best described using an example. Given the definition:
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
use_foreign_library/1.
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).
user:file_search_path(user_app_data, Dir) :-
'$xdg_prolog_directory'(data, Dir).
user:file_search_path(common_app_data, Dir) :-
'$xdg_prolog_directory'(common_data, Dir).
user:file_search_path(user_app_config, Dir) :-
'$xdg_prolog_directory'(config, Dir).
user:file_search_path(common_app_config, Dir) :-
'$xdg_prolog_directory'(common_config, Dir).
user:file_search_path(app_data, user_app_data('.')).
user:file_search_path(app_data, common_app_data('.')).
user:file_search_path(app_config, user_app_config('.')).
user:file_search_path(app_config, common_app_config('.')).
The '$xdg_prolog_directory'/2 uses either the XDG Base Directory or win_folder/2 on Windows. On Windows, user config is mapped to roaming appdata (CSIDL_APPDATA), user data to the non-roaming (CSIDL_LOCAL_APPDATA) and common data to (CSIDL_COMMON_APPDATA).
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,
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.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.file if the source was loaded from a file,
resource if the source was loaded from a resource or
state if the file was included in the saved state.:- if(exists_source(library(error))). :- use_module_library(error). :- endif.
The implementation uses absolute_file_name/3
using
file_type(prolog).
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.
| Key | Description |
directory | Directory in which source
lives |
dialect | Compatibility mode. See expects_dialect/1. |
file | Similar to source, but
returns the file being included when called while an include file is
being processed |
module | Module into which file is loaded |
reload | true if the file is
being
reloaded. Not present on first load |
script | Boolean that indicates whether the file is loaded as a script file (see -s) |
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. |
stream | Stream identifier (see current_input/1) |
term_position | Start position of last term
read. See also
stream_property/2
(position property and
stream_position_data/3.56Up
to version 7.1.22, the position term carried fake data except for the line_count
and had five arguments, where the position property of a stream
only has four. |
term | Term being expanded by expand_term/2. |
variable_names | A list of‘Name = Var’of the last term read. See read_term/2 for details. |
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.
For example:
:- 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 on how to resolve the compatibility issue.
restore. restore is 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 prolog
causes the application to start the normal interactive toplevel after
completing
Goal. See also the Prolog flag toplevel_goal
and
section 2.11.2.1.
initialization(Goal, program). Raises an error
initialization_error(Reason, Goal, File:Line) if Goal
fails or raises an exception. Reason is failed
or 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.
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
6 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.
It is possible to act on the beginning and end of a file by expanding
the terms begin_of_file and end_of_file. The
latter is supported by most Prolog systems that support term expansion
as
read_term/3
returns end_of_file on reaching the end of the input.
Expanding 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.
The current macro-expansion mechanism originates from Prolog systems in the 1980s and 1990s. It has several flaws, (1) the hooks act globally (except for definitions in a module), (2) it is hard to deal with interactions between transformations, (3) macros can not be reused between modules using the normal module export/import protocol and (4) it is hard to make source code aware tools such as the graphical debugger act properly in the context of macro expansion. Several Prolog implementations have tried to implement better expansion mechanisms. None of these solve all problems and all are largely incompatible with our current macro expansion. Future versions may provide a new mechanism to solve these issues.
Controlled interaction is provided between macro expansion defined in
a module and the user and system modules.
Here, SWI-Prolog uses a pipeline where the result of local
module expansion is the input for the expansion in user,
which is the input for the expansion in system. See also section
6.10.
Scoping, i.e., make a rule defined in a module only active
if this module is imported into the module being compiled, can be
emulated by defining the macro globally in the user module
and using
prolog_load_context/2
and some logic to verify the macro expansion should apply. If (goal)
expansion effectively defined inlining it is good practice to
also define the predicate and have the macro expansion check that the
predicate is in scope. Here is an example.
:- module(m1, [double/2]).
double(X, D) :- D is X*2.
user:goal_expansion(double(X,D), D is X*2) :-
prolog_load_context(module, M),
predicate_property(M:double(_,_), imported_from(m1)).
For term expansion that is not related to a specific predicate we can define a sentinel predicate rather than using the goal predicate and check it is imported into the current module to verify that the module that defines the expansion is imported into the current compilation context.
[]
if we are in a‘false branch' of the conditional compilation. See section
4.3.1.2.
user
and finally in system. Library modules inherit directly
from system and can thus not be re-interpreted by term
expansion rules in user.
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 user and 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.58After 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 goal expansion should not bind any variables in the clause. Doing so may impact the semantics of the clause if the variable is also used elsewhere. In the general case this is not verified. It is verified for \+/1 and ;/2, resulting in an exception.
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-->Body
into a normal Prolog clause. Normally this functionality should be
accessed using expand_term/2.true if the variable is
guaranteed to be unbound at entry of the goal, otherwise its value is false.
This implies that the variable first appears in this goal or a previous
appearance was in a negation (\+/1)
or a different branch of a disjunction.true if the variable is a
syntactic singleton in the term it appears in. Note that this
tests that the variable appears exactly once in the term being expanded
without making any claim on the syntax of the variable. Variables that
appear only once in multiple branches are not singletons
according to this property. Future implementations may improve on that.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:
subterm_positions of read_term/2.
The output layout should be a variable if no layout information can be
computed for the expansion; a sub-term can also be a variable to
indicate “don't know''.
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:
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/0.59As 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.
dynamic or meta_predicate
are in part applied immediately and in part during the fixup process
after the file completes loading. Currently, dynamic and
thread_local are 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
or term_expansion/2
are defined or goals are executed using
directives.60Note that initialization/1
directives are executed after loading the file. SWI-Prolog
allows for directives that are executed while loading the file
using :- 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.61This feature was
implemented by Keri Harris.
Errors and warnings reported while compiling a file are reported using print_message/2. Typical errors are syntax errors, errors during macro expansion by term_expansion/2 and goal_expansion/2, compiler errors such as illegal clauses or an attempt to redefine a system predicate and errors caused by executing directives, notably using initialization/1 and initialization/2.
Merely reporting error messages and warnings is typically desirable for interactive usage. Non-interactive applications often require to be notified of such issues, typically using the exit code of the process. We can distinguish two types of errors and warnings: (1) those resulting from loading an invalid program and (2) messages that result from running the program. A typical example is user code that wishes to try something and in case of an error report this and continue.
...,
E = error(_,_),
catch(do_something, E,
print_message(error, E)),
...
User code may be (and often is) started from directives, while running user code may involve compilation due to autoloading, loading of data files, etc. As a result, it is unclear whether an error message should merely be printed, should result in a non-zero exit status at the end or should immediately terminate the process.
The default behaviour is defined by the Prolog flags
on_error and on_warning.
It can be fine tuned by defining the hook predicate message_hook/3.
The compiler calls print_message/2
using the level silent and the message below if errors or
warnings where printed during the execution of
load_files/2.
’myfile.pl' or library(lists), Errors
is the number of errors printed while loading and Warnings is
the number of warnings printed while loading. Note that these counts
include messages from (initialization) directives.
This allows the user to fine tune the behaviour on errors and, for example, halt the process on a non-zero error count right after loading the file wth errors using the code below.
:- multifile user:message_hook/3.
user:message_hook(load_file_errors(_File, Errors, _Warnings),
_Level, _Lines) :-
Errors > 0,
halt(1).
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)(.) 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)(.) 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 reloading, see section 4.3.2.
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 whether some other thread is already loading the file. If not, it starts loading the file. If a thread detects that 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 imports the exported predicates and operators from the module.
Note that this schema does not prevent deadlocks under all situations. Consider two mutually dependent (see section 4.3.2.2) 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' (.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 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.