Consulting Prolog Source files

SWI-Prolog source files normally have a suffix `.pl '. Specifying the suffix is optional. All predicates that handle source files first check whether a file with suffix `.pl ' exists. If not the plain file name is checked for existence. Library files are specified by embedding the file name using the functor library/1. Thus `foo ' refers to `foo.pl ' or `foo ' in the current directory, `library(foo) ' refers to `foo.pl ' or `foo ' in one of the library directories specified by the dynamic predicate library_directory/1.

SWI-Prolog recognises grammar rules as defined in [Clocksin & Mellish, 1981]. The user may define additional compilation of the source file by defining the dynamic predicate term_expansion/2. Transformations by this predicate 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.

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.

consult( +File)

  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 csh(1) 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:

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 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].

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 this does not exist. This will flag the predicate as `undefined'. See also check/0 and autoload/0.

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 or one of its derivates. make/0 is normally invoked by the edit/[0,1] and ed/[0,1] predicates. make/0 can be combined with the compiler to speed up the development of large packages. In this case compile the package using

sun% pl -g make -o my_program -c file ...

If `my_program' is started it will first reconsult all source files that have changed since the compilation.

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 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, '~/demo').

the file specification demo(myfile) will be expanded to ' /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].

file_search_path(library, X) :-
	library_directory(X).
user:file_search_path(swi, Home) :-
	feature(home, Home).
user:file_search_path(foreign, swi(ArchLib)) :-
	feature(arch, Arch),
	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.

source_file( -File)

  Succeeds if File was loaded using consult/1 or ensure_loaded/1. File refers to the full path name of the file (see expand_file_name/2). S ource_file/1 backtracks over all loaded source files.

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 expand_file_name/2). Can be used with any instantiation pattern, but the database only maintains the source file for each predicate. Predicates declared multifile (see multifile/1) cannot be found this way.

prolog_load_context( ?Key, ?Value)

  Determine loading context. The following keys are defined:

Quintus compatibility predicate. See also source_location/2.

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.

term_expansion( +Term1, -Term2)

  Dynamic predicate, normally not defined. When defined by the user all terms read during consulting that 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 a the form `?- Goal' or `:- Goal'. Goal is then treated as a directive. Term2 may also be a list, in which case all terms of the list are stored in the database or called (for directives). See also expand_term/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.

at_initialization( +Goal)

  Register Goal to be ran 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 ran in the order they were registered. A warning message is issued if Goal fails, but execution continues. See also at_halt/1

at_halt( +Goal)

  Register Goal to be ran when the system halts. The hooks are ran in the order they were registered. Success or failure executing a hook is ignored. These hooks may not call halt/[0,1].

initialization( +Goal)

  Call Goal and register it using at_initialization/1. Directives that do other things that 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].

compiling

  Succeeds if the system is compiling source files with the -c

option into an intermediate code file. Can be used to perform code optimisations in expand_term/2 under this condition.

preprocessor( -Old, +New)

  Read the input file via a Unix 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 resulting atom is called as a Unix command and the standard output of this command is loaded. To use the Unix C preprocessor one should define:

?- preprocessor(Old, '/lib/cpp -C -P %f'), consult(...).

Old = none

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