## 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 (see module/1). They are normally loaded using consult/1 or ensure_loaded/1. Currently, a non-module file can only be loaded into a single module.33This limitation may be lifted in the future. Existing limitations in SWI-Prolog's source code administration make this non-trivial.
A module
  Prolog source file starts with a module declaration. The subsequent Prolog code is loaded into the specified module, and only the exported predicates are made available to the context loading the module. Module files are normally loaded with use_module/[1,2]. See chapter 5 for details.
An include
  Prolog source file is loaded using the include/1 directive, textually including Prolog text into another Prolog source. A file may be included into multiple source files and is typically used to share declarations such as multifile or dynamic between source files.

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.34It 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), multithreaded 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.

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 OptionName(OptionValue).

  The following options are currently supported:

  autoload(Bool)
    If `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.
  derived_from(File)
    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.
  dialect(+Dialect)
    Load Files with enhanced compatibility with the target Prolog system identified by Dialect. See expects_dialect/1 and section C for details.
  encoding(Encoding)
    Specify the way characters are encoded in the file. Default is taken from the Prolog flag encoding. See section 2.18.1 for details.
  expand(Bool)
    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.
  format(+Format)
    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).
  if(Condition)
    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, and `not_loaded` loads the file if it was not loaded before.
  imports(Import)
    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 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.

  modified(TimeStamp)
    Claim that the source was loaded at TimeStamp without checking the source. This option is intended to be used together with the `stream(Input)` option, for example after extracting the time from an HTTP server or database.
  must_be_module(Bool)
    If `true`, raise an error if the file is not a module file. Used by use_module/[1,2].
  qcompile(Atom)
    How to deal with quick-load-file compilation by qcompile/1. Values are:
    never
      Default. Do not use qcompile unless called explicitly.
    auto
      Use qcompile for all writeable files. See comment below.
    large
      Use qcompile if the file is `large'. Currently, files larger than 100 Kbytes are considered large.
    part
      If this load_file/2 appears in a directive of a file that is compiled into Quick Load Format using qcompile/1, the contents of the argument files are included in the `.qlf` file instead of the loading directive.

    If this option is not present, it uses the value of the Prolog flag qcompile as default.

  redefine_module(+Action)
    Defines what to do if a file is loaded that provides a module that is already loaded from another file. Action is one of `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.
  reexport(Bool)
    If `true` re-export the imported predicate. Used by reexport/1 and reexport/2.
  register(Bool)
    If `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.
  sandboxed(Bool)
    Load the file in sandboxed mode. This option controls the flag sandboxed_load. The only meaningful value for Bool is `true`. Using `false` while the Prolog flag is set to `true` raises a permission error.
  silent(Bool)
    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.
  stream(Input)
    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 to remove all clauses if the data is reconsulted.

    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.8.

consult(:File)
  Read File as a Prolog source file. Calls to consult/1 may be abbreviated by just typing a number of filenames in a list. Examples:

  `?- 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.

ensure_loaded(:File)
  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 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)]).`35On 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.

[ISO]include(+File)
  Textually include the content of File in the file in which the directive `:- 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:

  - source_file/2 describes the owner relation.
  - predicate_property/2 describes the source location (of the first clause).
  - clause_property/2 provides access to both source and ownership.
  - source_file_property/2 can be used to query include relationships between files.
require(+ListOfNameAndArity)
  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.

encoding(+Encoding)
  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 2.18.1.
make
  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/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.
library_directory(?Atom)
  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 assertz/1, asserta/1 or remove system defaults using retract/1. Deprecated. New code should use file_search_path/2.
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>)` 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.

expand_file_search_path(+Spec, -Path)
  Unifies Path with all possible expansions of the filename 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, 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.

source_file(?File)
  True if File is a loaded Prolog source file. File is the absolute and canonical path to the source file.
source_file(?Pred, ?File)
  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. Note that the relation between files and predicates is more complicated if include/1 is used. The predicate describes the owner of the predicate. See include/1 for details.
source_file_property(?File, ?Property)
  True when Property is a property of the loaded file File. If File is non-var, it can be a file specification that is valid for load_files/2. Defined properties are:
  derived_from(Original, OriginalModified)
    File was generated from the file Original, which was last modified at time OriginalModified at the time it was loaded. This property is available if File was loaded using the `derived_from(Original)` option to load_files/2.
  includes(IncludedFile, IncludedFileModified)
    File used include/1 to include IncludedFile. The last modified time of IncludedFile was IncludedFileModified at the time it was included.
  included_in(MasterFile, Line)
    File was included into MasterFile from line Line. This is the inverse of the `includes` property.
  load_context(Module, Location, Options)
    Module is the module into which the file was loaded. If File is a module, this is the module into which the exports are imported. Otherwise it is the module into which the clauses of the non-module file are loaded. Location describes the file location from which the file was loaded. It is either a term <file>:<line> or the atom `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.
  modified(Stamp)
    File modification time when File was loaded. This is used by make/0 to find files whose modification time is different from when it was loaded.
  module(Module)
    File is a module file that declares the module Module.
unload_file(+File)
  Remove all clauses loaded from File. If File loaded a module, clear the module's export list and disassociate it from the file. File is a canonical filename or a file indicator that is valid for load_files/2.

  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.

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 and if/1. The following keys are defined:

  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. 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) ]),
          ...

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.
at_halt(:Goal)
  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.
[ISO]:- initialization(:Goal)
  Call Goal after loading the source file in which this directive appears has been completed. In addition, Goal is executed if a saved state created using qsave_program/1 is restored.

  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.

initialization(:Goal, +When)
  Similar to initialization/1, but allows for specifying when Goal is executed while loading the program text:
  now
    Execute Goal immediately.
  after_load
    Execute Goal after loading program text. This is the same as initialization/1.
  restore
    Do not execute Goal while loading the program, but only when restoring a state.
compiling
  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 than 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 on terms read from the input to perform preprocessing. It consists of four steps, where each step processes the output of the previous step.

  1. Test conditional compilation directives and translate all input to `[]` if we are in a `false branch' of the conditional compilation. See section 4.3.1.1.

  2. Call term_expansion/2. This predicate is first tried in the module that is being compiled and then in the module `user`.

  3. Call DCG expansion (dcg_translate_rule/2).

  4. Call expand_goal/2 on each body term that appears in the output of the previous steps.
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 a defined meta-predicate that is annotated using `0' (see meta_predicate/1). Other cases need a real predicate definition.

expand_goal(+Goal1, -Goal2)
  This predicate is normally called by the compiler to perform preprocessing using goal_expansion/2. The predicate computes a fixed-point by applying transformations until there are no more changes. If optimisation is enabled (see -O and optimise), expand_goal/2 simplifies the result by removing unneeded calls to true/0 and fail/0 as well as unreachable branches.
compile_aux_clauses(+Clauses)
  Compile clauses on behalf of goal_expansion/2. This predicate compiles the argument clauses into static predicates, associating the predicates with the current file but avoids changing the notion of current predicate and therefore discontiguous warnings.
dcg_translate_rule(+In, -Out)
  This predicate performs the translation of a term `Head-->Body` into a normal Prolog clause. Normally this functionality should be accessed using expand_term/2.

#### 4.3.1.1 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:

- Load different libraries on different dialects.
- Define a predicate if it is missing as a system predicate.
- Realise totally different implementations for a particular part of the code due to different capabilities.
- Realise different configuration options for your software.

:- 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).
      section_1.
      :- elif(test2).
      section_2.
      :- elif(test3).
      section_3.
      :- else.
      section_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 a valid use of the development environment:

- Trace a goal
- Find unexpected behaviour of a predicate
- Enter a break using the b command
- Fix the sources and reload them using make/0
- Exit the break, retry using the r command

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.

garbage_collect_clauses
  Clean up 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. Predicates are, like atoms, shared resources and therefore all threads are suspended during the execution of this predicate.

#### 4.3.2.1 Compilation of mutually dependent code

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.

#### 4.3.2.2 Compilation with multiple threads

This section discusses compiling files for the first time. For reloading, 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.36Although 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.

#### 4.3.2.3 Reloading running code

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 multithreaded 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.

### 4.3.3 Quick load files

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.

qcompile(:File)
  Takes a file specification as consult/1, etc., and, in addition to the normal compilation, creates a Quick Load File from File. The file extension of this file is `.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 2.10 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.

qcompile(:File, +Options)
  As qcompile/1, but processes additional options as defined by load_files/2.bugOption processing is currently incomplete.