4.3 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:

A traditional
Prolog source file contains Prolog clauses and directives, but no module-declaration. They are normally loaded using consult/1 or ensure_loaded/1.
A module
Prolog source file starts with a module declaration. The subsequent Prolog code is loaded into the specified module and only the public predicates are made available to the context loading the module. Module files are normally loaded using use_module/[1,2]. See chapter 5 for details.
An include
Prolog source file is loaded using the include/1 directive and normally contains only directives.

Prolog source-files are located using absolute_file_name/3 with the following options:

locate_prolog_file(Spec, Path) :-
                           [ file_type(prolog),

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 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.

load_files(+Files, +Options)
The predicate load_files/2 is the parent of all the other loading predicates except for include/1. It currently supports a subset of the options of Quintus load_files/2. Files is either a single source-file, or a list of source-files. The specification for a source-file is handed to absolute_file_name/2. See this predicate for the supported expansions. Options is a list of options using the format

The following options are currently supported:

If true (default 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 informational or silent. See also print_message/2 and current_prolog_flag/2.
Indicate that the loaded file is derived from File. Used by make/0 to time-check and load the original file rather than the derived file.
Specify the way characters are encoded in the file. Default is taken from the Prolog flag encoding. See section 2.17.1 for details.
If 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.
Used to specify the file format if data is loaded from a stream using the stream(Stream) option. Default is source, loading Prolog source text. If qlf, load QLF data (see qcompile/1).
Load the file only if the specified condition is satisfied. The value 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, not_loaded loads the file if it was not loaded before.
Specify what to import from the loaded module. The default for use_module/1 is 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.
If true, raise an error if the file is not a module file. Used by use_module/[1,2].
If this call appears in a directive of a file that is compiled into Quick Load Format using qcompile/1 and this flag is true, the contents of the argument files are included in the .qlf file instead of the loading directive.
If true re-export the imported predicate. Used by reexport/1 and reexport/2.
If 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 SWI-Prolog extension compiles the data from the stream Input. If this option is used, Files must be a single atom which is used to identify the source-location of the loaded clauses as well as remove all clauses if the data is re-consulted.

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 other objects than files. See prolog_load_file/2 for a description and library(http_load) for an example.

Read File as a Prolog source file. File may be a list of files, in which case all members are consulted in turn. File may start with the Unix shell special sequences ,  <user> and $<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). % consult load or load.pl
?- [library(quintus)]. % load Quintus compatibility library
?- [user].

The predicate consult/1 is equivalent to load_files(Files, []), except for handling the special file user, which reads clauses from the terminal. See also the stream(Input) option of load_files/2.

If the file is not already loaded, this is equivalent to consult/1. Otherwise, if the file defines a module, import all public predicates. Finally, if the file is already loaded, is not a module file and the context module is not the global user module, ensure_loaded/1 will call consult/1.

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.

Pretend the terms in File are in the source-file in which :- 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.
Declare that this file/module requires the specified predicates to be defined ``with their commonly accepted definition''. This predicate originates from the Prolog portability layer for XPCE. It is intended to provide a portable mechanism for specifying that this module requires the specified 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.

This directive can appear anywhere in a source file to define how characters are encoded in the remainder of the file. It can be used in files that are encoded with a superset of US-ASCII, currently UTF-8 and ISO Latin-1. See also section 2.17.1.
Consult all source files that have been changed since they were consulted. It checks all loaded source files: files loaded into a compiled state using 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.
Dynamic predicate used to specify library directories. Default ./lib, /lib/prolog and 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.
file_search_path(+Alias, ?Path)
Dynamic predicate used to specify `path-aliases'. 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>) 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) :-
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 files.

expand_file_search_path(+Spec, -Path)
Unifies Path with all possible expansions of the file name specification Spec. See also absolute_file_name/3.
prolog_file_type(?Extension, ?Type)
This dynamic multifile predicate defined in module 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 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 extension is .pro.

True if File is a loaded Prolog source file. File is the absolute and canonical path to the source-file.
source_file(?Pred, ?File)
Is true if the predicate specified by Pred was loaded from file File, where File is an absolute path name (see absolute_file_name/2). Can be used with any instantiation pattern, but the database only maintains the source file for each predicate. See also clause_property/2.
prolog_load_context(?Key, ?Value)
Obtain context information during compilation. This predicate can be used from directives appearing in a source file to get information about the file being loaded. See also source_location/2. The following keys are defined:

module Module into which file is loaded
source File loaded. Returns the original Prolog file when loading a .qlf file. Compatible to SICStus Prolog.
file Currently equivalent to source. In future versions it may report a different values for files being loaded using include/1.
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.
source_location(-File, -Line)
If the last term has been read from a physical file (i.e., not from the file 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.
Register Goal to be run when the system initialises. Initialisation takes place after reloading a .qlf (formerly .wic) file as well as after reloading a saved-state. The hooks are run in the order they were registered. A warning message is issued if Goal fails, but execution continues. See also at_halt/1
Register Goal to be run from PL_cleanup(), which is called when the system halts. The hooks are run in the reverse order they were registered (FIFO). Success or failure executing a hook is ignored. If the hook raises an exception this is printed using print_message/2. An attempt to call halt/[0,1] from a hook is ignored.
Call Goal and register it using at_initialization/1. Directives that do other things than creating clauses, records, flags or setting predicate attributes should normally be written using this tag to ensure the initialisation is executed when a saved system starts. See also qsave_program/[1,2].
True if the system is compiling source files with the -c option or qcompile/1 into an intermediate code file. Can be used to perform conditional code optimisations in term_expansion/2 (see also the -O option) or to omit execution of directives during compilation.

4.3.1 Conditional compilation and program transformation

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.

term_expansion(+Term1, -Term2)
Dynamic and multifile predicate, normally not defined. When defined by the user all terms read during consulting are given to this predicate. If the predicate succeeds Prolog will assert Term2 in the database rather then the read term (Term1). Term2 may be a term of the form `?- 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.

expand_term(+Term1, -Term2)
This predicate is normally called by the compiler to perform preprocessing. First it calls term_expansion/2. If this predicate fails it performs a grammar-rule translation. If this fails it returns the first argument.
goal_expansion(+Goal1, -Goal2)
Like term_expansion/2, goal_expansion/2 provides for macro-expansion of Prolog source-code. Between expand_term/2 and the actual compilation, the body of clauses analysed and the goals are handed to expand_goal/2, which uses the goal_expansion/2 hook to do user-defined expansion.

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 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.

expand_goal(+Goal1, -Goal2)
This predicate is normally called by the compiler to perform preprocessing. First it calls goal_expansion/2. If this fails it returns the first argument.
Compile clauses on behalf of goal_expansion/2. This predicate compiled the argument clauses into static predicates, associating the predicates with the current file but avoid changing the notion of current predicate and therefore discontiguous warnings.
preprocessor(-Old, +New)
Read the input file via an external process that acts as preprocessor. A preprocessor is specified as an atom. The first occurrence of the string `%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

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(:Goal)
Compile subsequent code only if Goal succeeds. For enhanced portability, Goal is processed by expand_goal/2 before execution. If an error occurs, the error is printed and processing proceeds as if Goal has failed.
:- elif(:Goal)
Equivalent to :- 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).
:- elif(test2).
:- elif(test3).
:- else.
:- endif.
:- else
Start `else' branch.
:- endif
End of conditional compilation.

4.3.2 Loading files, active code and threads

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.

Cleanup all dirty predicates, where dirty predicates are defined to be predicates that have both old and new definitions due to reloading a source file while the predicate was active. Of course, predicates that are active using garbage_collect_clauses/0 cannot be reclaimed and remain dirty. Predicate are -like atoms- shared resources and therefore all threads are suspended during the execution of this predicate. Threads and reloading running code

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.

4.3.3 Quick load files

SWI-Prolog supports compilation of individual or multiple Prolog source files into `Quick Load Files'. A `Quick Load Files' (.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 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 .pl file otherwise.

Takes a single file specification like consult/1 (i.e., accepts constructs like 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)', `:- [+File]' or :- load_files(+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 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.