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\chapter{Using Modules}            \label{sec:modules}
\section{Why Using Modules?}
In traditional Prolog systems the predicate space was flat.  This
approach is not very suitable for the development of large
applications, certainly not if these applications are developed by
more than one programmer.  In many cases, the definition of a Prolog
predicate requires sub-predicates that are intented only to complete
the definition of the main predicate.  With a flat and global
predicate space these support predicates will be visible from the
entire program.
For this reason, it is desirable that each source module has it's own
predicate space.  A module consists of a declaration for it's name,
it's {\em public} predicates and the predicates themselves.  This
approach allow the programmer to use short (local) names for support
predicates without worrying about name conflicts with the support
predicates of other modules.  The module declaration also makes
explicit which predicates are meant for public usage and which for
private purposes.  Finally, using the module information, cross
reference programs can indicate possible problems much better.
\section{Name-based versus Predicate-based Modules}
Two approaches to realise a module system are commonly used in Prolog
and other languages.  The first one is the {\em name based} module
system.  In these systems, each atom read is tagged (normally
prefixed) with the module name, with the exception of those atoms that
are defined {\em public}.  In the second approach, each module
actually implements its own predicate space.
A critical problem with using modules in Prolog is introduced by the
meta-predicates that transform between Prolog data and Prolog
predicates.  Consider the case where we write:
\begin{boxed}\begin{code}
:- module(extend, [add_extension/3]).
add_extension(Extension, Plain, Extended) :-
    maplist(extend_atom(Extension), Plain, Extended).
extend_atom(Extension, Plain, Extended) :-
    concat(Plain, Extension, Extended).
\end{code}\end{boxed}
In this case we would like maplist to call extend_atom/3 in the module
{\tt extend}.  A name based module system will do this correctly. It
will tag the atom {\tt extend_atom} with the module and maplist will
use this to construct the tagged term extend_atom/3.  A name based
module however, will not only tag the atoms that will eventually be
used to refer to a predicate, but {\em all} atoms that are not declared
public.  So, with a name based module system also data is local to the
module.  This introduces another serious problem:
\begin{boxed}\begin{code}
:- module(action, [action/3]).
action(Object, sleep, Arg) :- ....
action(Object, awake, Arg) :- ....
:- module(process, [awake_process/2]).
awake_process(Process, Arg) :-
    action(Process, awake, Arg).
\end{code}\end{boxed}
This code uses a simple object-oriented implementation technique
were atoms are used as method selectors.  Using a name based module
system, this code will not work, unless we declare the selectors
puclic atoms in all modules that use them.  Predicate based module
systems do not require particular precautions for handling this case.
It appears we have to choose either to have local data, or to have
trouble with meta-predicates.  Probably it is best to choose for
the predicate based approach as novice users will not often write
generic meta-predicates that have to be used across multiple modules,
but are likely to write programs that pass data around across
modules.  Experienced Prolog programmers should be able to deal with
the complexities of meta-predicates in a predicate based module
system.
\section{Defining a Module}
Modules normally are created by loading a {\em module file}.  A module
file is a file holding a module/2 directive as its first term.  The
module/2 directive declares the name and the public (i.e.\
externally visible) predicates of the module.  The rest of the file
is loaded into the module.  Below is an example of a module file,
defining reverse/2.
\begin{boxed}\begin{code}
:- module(reverse, [reverse/2]).
reverse(List1, List2) :-
    rev(List1, [], List2).
rev([], List, List).
rev([Head|List1], List2, List3) :-
    rev(List1, [Head|List2], List3).
\end{code}\end{boxed}
\section{Importing Predicates into a Module}
As explained before, in the predicate based approach ad<apted by
SWI-Prolog, each module has it's own predicate space.  In SWI-Prolog,
a module initially is completely empty.  Predicates can be added
to a module by loading a module file as demonstrated in the previous
section, using assert or by {\em importing} them from another module.
Two mechanisms for importing predicates explicitely from another module
exist.  The use_module/[1,2] predicates load a module file and import
(part of the) public predicates of the file.  The import/1 predicate
imports any predicate from any module.
.C use_module 1 +File
Load the file(s) specified with {\em File} just like ensure_loaded/1.
The files should all be module files.  All exported predicates from the
loaded files are imported into the context module.  The difference
between this predicate and ensure_loaded/1 becomes apparent if the file
is already loaded into another module.  In this case ensure_loaded/1
does nothing; use_module will import all public predicates of the module
into the current context module.
.C use_module 2 +File, +ImportList
Load the file specified with {\em File} (only one file is accepted).
{\em File} should be a module file. {\em ImportList} is a list of
name/arity pairs specifying the predicates that should be imported from
the loaded module.  If a predicate is specified that is not exported from
the loaded module a warning will be printed.  The predicate will
nevertheless be imported to simplify debugging.
.C import 1 +Head
Import predicate {\em Head} into the current context module. {\em Head}
should specify the source module using the {\em $<$module$>$:$<$term$>$}
construct. Note that predicates are normally imported using one of the
directives use_module/[1,2]. import/1 is meant for handling imports
into dynamically created modules.
It would be rather inconvient to have to import each predicate
refered to by the module, including the system predicates.  For this
reason each module is assigned a {\em default} module.  All predicates
in the default module are available without extra declarations.  Their
definition however can be overruled in the local module.  This schedule
is implemented by the exception handling mechanism of SWI-Prolog:  if
an undefined predicate exception is raised for a predicate in some
module, the exception handler first tries to import the predicate from
the module's default module.  On success, normal execution is resumed.
\subsection{Reserved Modules}
SWI-Prolog contains two special modules.  The first one is the module
{\tt system}.  This module contains all built-in predicates described
in this manual.  Module {\tt system} has no default module assigned to
it.  The second special module is the module {\tt user}.  This module
forms the initial working space of the user.  Initially it is empty.
The default module of module {\tt user} is {\tt system}, making all
built-in predicate definitions available as defaults.  Built-in
predicates thus can be overruled by defining them in module {\tt user}
before they are used.
All other modules default to module {\tt user}.  This implies they can
use all predicates imported into {\tt user} without explicitely
importing them.
\section{Using the Module System}
The current structure of the module system has been designed with some
specific organisations for large programs in mind.  Many large
programs define a basic library layer on top of which the actual
program itself is defined.  The module {\em user}, acting as the
default module for all other modules of the program can be used to
distribute these definitions over all program module without
introducing the need to import this common layer each time
explicitely.  It can also be used to redefine built-in predicates
if this is required to maintain compatibility to some other Prolog
implementation.  Typically, the loadfile of a large application
looks like this:
\begin{boxed}\begin{code}
:- use_module(compatibility).    % load XYZ prolog compatibility
:- use_module(            % load generic parts
    [ error            % errors and warnings
    , goodies        % general goodies (library extensions)
    , debug            % application specific debugging
    , virtual_machine    % virtual machine of application
    , ...            % more generic stuff
:- ensure_loaded(
    [ ...            % the application itself
\end{code}\end{boxed}
The `use_module' declarations will import the public predicates from
the generic modules into the {\tt user} module.  The `ensure_loaded'
directive loads the modules that constitute the actual application.
It is assumed these modules import predicates from each other using
use_module/[1,2] as far as necessary.
In combination with the object-oriented schema described below it is
possible to define a neat modular architecture.  The generic code
defines general utilities and the message passing predicates (invoke/3
in the example below).  The application modules define classes that
communicate using the message passint predicates.
\subsection{Object Oriented Programming}
Another typical way to use the module system is for defining classes
within an object oriented paradigm.  The class structure and the
methods of a class can be defined in a module and the explicit
module-boundary  overruling describes in section~\ref{sec:overrule}
can by used by the message passing code to invoke the behaviour.  An
outline of this mechanism is given below.
\begin{boxed}\begin{code}
%    Define class point
:- module(point, []).        % class point, no exports
%     name        type,        default    access
%                    value
variable(x,        integer,    0,    both).
variable(y,        integer,    0,    both).
%      method name    predicate name    arguments
behaviour(mirror,    mirror,        []).
mirror(P) :-
    fetch(P, x, X),
    fetch(P, y, Y),
    store(P, y, X),
    store(P, x, Y).
\end{code}\end{boxed}
The predicates fetch/3 and store/3 are predicates that change instance
variables of instances.  The figure below indicates how message
passing can easily be implemented:
\begin{boxed}\begin{code}
%    invoke(+Instance, +Selector, ?ArgumentList)
%    send a message to an instance
invoke(I, S, Args) :-
    class_of_instance(I, Class),
    Class:behaviour(S, P, ArgCheck), !,
    convert_arguments(ArgCheck, Args, ConvArgs),
    Goal =.. [P|ConvArgs],
    Class:Goal.
\end{code}\end{boxed}
The construct {\em `Module:Goal'} explicitely calls {\em Goal} in
module {\em Module}.  It is discussed in more detail in
section~\ref{sec:metacall}.
\section{Meta-Predicates in Modules}
As indicated in the introduction, the problem with a predicate based
module system lies in the difficulty to find the correct predicate
from a Prolog term.  The predicate `solution(Solution)' can exist in
more than one module, but `assert(solution(4))' in some module is
supposed to refer to the correct version of solution/1.
Various approaches are possible to solve this problem.  One is to add
an extra argument to all predicates (e.g.\ `assert(Module, Term)').
Another is to tag the term somehow to indicate which module is desired
(e.g.\ `assert(Module:Term)').  Both approaches are not very
attractive as they make the user responsible for chosing the correct
module, inviting unclear programming by asserting in other modules.
The predicate assert/1 is supposed to assert in the module it is
called from and should do so without being told explicitely.  For
this reason, the notion {\em context module} has been introduced.
\subsection{Definition and Context Module}
Each predicate of the program is assigned a module, called it's {\em
definition module}.  The definition module of a predicate is always the
module in which the predicate was originally defined.  Each active goal
in the Prolog system has a {\em context module} assigned to it.
The context module is used to find predicates from a Prolog term.  By
default, this module is the definition module of the predicate running
the goal.  For meta-predicates however, this is the context module of
the goal that invoked them.  We call this {\em module_transparent} in
SWI-Prolog.  In the `using maplist' example above, the predicate
maplist/3 is declared module_transparent. This implies the context
module remains {\tt extend}, the context module of add_extension/3.
This way maplist/3 can decide to call extend_atom in module {\tt
extend} rather than in it's own definition module.
All built-in predicates that refer to predicates via a Prolog term are
declared module_transparent.  Below is the code defining maplist.
\begin{boxed}\begin{code}
:- module(maplist, maplist/3).
:- module_transparent maplist/3.
%    maplist(+Goal, +List1, ?List2)
%    True if Goal can succesfully be applied to all succesive pairs
%    of elements of List1 and List2.
maplist(_, [], []).
maplist(Goal, [Elem1|Tail1], [Elem2|Tail2]) :-
    apply(Goal, [Elem1, Elem2]),
    maplist(Goal, Tail1, Tail2).
\end{code}\end{boxed}
\subsection{Overruling Module Boundaries}
The mechanism above is sufficient to create an acceptable module
system.  There are however cases in which we would like to be able to
overrule this schema and explicitely call a predicate in some module
or assert explicitly in some module.  The first is useful to invoke
goals in some module from the user's toplevel or to implement a
object-oriented system (see above).  The latter is useful to create
and modify {\em dynamic} modules (see section~\ref{sec:dynamic-modules}).
For this purpose, the reserved term \verb$:/2$ has been introduced.
All built-in predicates that transform a term into a predicate
reference will check whether this term is of the form
\mbox{`$<$Module$>$:$<$Term$>$'}.  If so, the predicate is searched
for in {\em Module} instead of the goal's context module.  The
\verb$:/2$ operator may be nested, in which case the inner-most module
is used.
The special calling construct \mbox{$<$Module$>$:$<$Goal$>$} pretends
{\em Goal} is called from {\em Module} instead of the context module.
Examples:
\begin{code}
?- assert(world:done).    % asserts done/0 into module world
?- world:assert(done).    % the same
?- world:done.        % calls done/0 in module world
\end{code}
\section{Dynamic Modules}        \label{sec:dynamic-modules}
Sofar, we discussed modules that were created by loading a
module-file.  These modules have been introduced on facilitate the
development of large applications.  The modules are fully defined at
load-time of the application and normally will not change during
execution.  Having the notion of a set of predicates as a
self-contained world can be attractive for other purposes as well.
For example, assume an application that can reason about multiple
worlds.  It is attractive to store the data of a particular world in a
module, so we extract information from a world simply by invoking
goals in this world.
Dynamic modules can easily be created.  Any built-in predicate that
tries to locate a predicate in a specific module will create this
module as a side-effect if it did not yet exist.  Example:
\begin{code}
?- assert(world_1, consistent),
   world_1:unkown(_, fail).
\end{code}
These calls create a module called {\tt world_1} and make the call
`world_1:consistent' succeed.  Undefined predicates will not start the
tracer or autoloader for this module (see unknown/2).
Import and export from dynamically created world is arranged via the
predicates import/1 and export/1:
\begin{code}
?- world_5:export(solve(_,_)).        % exports solve/2 from world_5
?- world_3:import(world_5:solve(_,_)).    % and import it to world_3
\end{code}
\section{Module Handling Predicates}
This section gives the predicate definitions for the remaining
built-in predicates that handle modules.
.D module 2 +Module, +PublicList
This directive can only be used as the first term of a source file. It
declares the file to be a {\em module file}, defining {\em Module} and
exporting the predicates of {\em PublicList}. {\em PublicList} is a
list of name/arity pairs.
.P module_transparent +Name/+Arity, ...
{\em Preds} is a comma separated list of name/arity pairs (like
dynamic/1).  Each goal associated with a transparent declared predicate
will inherit the {\em context module} from its parent goal.
.C context_module 1 -Module
Unify {\em Module} with the context module of the current goal.
context_module/1 itself is transparent.
.C export 1 +Head
Add a predicate to the public list of the context module.  This implies
the predicate will be imported into another module if this module is
imported with use_module/[1,2].  Note that predicates are normally
exported using the directive module/2. export/1 is meant to handle
export from dynamically created modules.
.C export_list 2 +Module, -Exports
Unifies {\em Exports} with a list of terms.  Each term has the name and
arity of a public predicate of {\em Module}.  The order of the terms in
{\em Exports} is not defined.  See also predicate_property/2.
.S Compatibility of the Module System
The principles behind the module system of SWI-Prolog differ in a
number of aspects from the Quintus Prolog module system.
\begin{itemize}
    \item
The SWI-Prolog module system allows the user to redefine system
predicates.
    \item
All predicates that are available in the {\em system} and {\em user}
modules are visible in all other modules as well.
    \item
Quintus has the `meta_predicate/1' declaration were SWI-Prolog has the
module_transparent/1 declaration.
\end{itemize}
The meta_predicate/1 declaration causes the compiler to tag arguments
that pass module sensitive information with the module using the
\verb$:/2$ operator.  This approach has some disadvantages:
\begin{itemize}
    \item
Changing a meta_predicate declaration implies all predicates {\em
calling} the predicate need to be reloaded. This can cause serious
consistency problems.
    \item
It does not help for dynamically defined predicates calling module
sensitive predicates.
    \item
It slows down the compiler (at least in the SWI-Prolog architecture).
    \item
At least within the SWI-Prolog architecture the run-time overhead is
larger than the overhead introduced by the transparent mechanism.
\end{itemize}
Unfortunately the transparent predicate approach also has some
disadvantages. If a predicate {\em A} passes module sensitive information
to a predicate {\em B}, passing the same information to a module
sensitive system predicate both {\em A} and {\em B} should be declared
transparent. Using the Quintus approach only {\em A} needs to be
treated special (i.e. declared with meta_predicate/1)%
    \footnote{Although this would make it impossible to call {\it B}
          directly.}.
A second problem arises if the body of a transparent predicate uses
module sensitive predicates for which it wants to refer to its own
module. Suppose we want to define findall/3 using assert/1 and
retract/1%
    \footnote{The system version uses recordz/2 and recorded/3.}.
The example in figure~\ref{fig:findall} gives the solution.
\begin{figure}
\begin{boxed}
\begin{code}
:- module(findall, [findall/3]).
:- dynamic
    solution/1.
:- module_transparent
    findall/3,
    store/2.
findall(Var, Goal, Bag) :-
    assert(findall:solution('$mark')),
    store(Var, Goal),
    collect(Bag).
store(Var, Goal) :-
    Goal,             % refers to context module of
                % caller of findall/3
    assert(findall:solution(Var)),
    fail.
store(_, _).
collect(Bag) :-
    ...,
\end{code}
\end{boxed}
    \caption{Findall using modules}
    \label{fig:findall}
\end{figure}
The Quintus meta_predicate/1 directive can in many cases be replaced by
the transparent declaration.  Figure~\ref{fig:meta} gives a definition
of meta_predicate/1 as available from the `quintus' library package.
\begin{figure}
\begin{boxed}
\begin{code}
:- op(1150, fx, (meta_predicate)).
meta_predicate((Head, More)) :- !,
    meta_predicate1(Head),
    meta_predicate(More).
meta_predicate(Head) :-
    meta_predicate1(Head).
meta_predicate1(Head) :-
    Head =.. [Name|Arguments],
    member(Arg, Arguments),
    module_expansion_argument(Arg), !,
    functor(Head, Name, Arity),
    module_transparent(Name/Arity).
meta_predicate1(_).        % just a mode declaration
module_expansion_argument(:).
module_expansion_argument(N) :- integer(N).
\end{code}
\end{boxed}
    \caption{Definition of meta_predicate/1}
    \label{fig:meta}
\end{figure}
The discussion above about the problems with the transparent mechanism
show the two cases in which this simple transformation does not work.