/usr/share/doc/swi-prolog-doc/UserGuide/namedref.html

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<H2><A NAME="sec:10.3"><SPAN class="sec-nr">10.3</SPAN> <SPAN class="sec-title">Defining 
global named objects</SPAN></A></H2>

<A NAME="sec:namedref"></A>

<P>As explained before, named references should be restricted to 
debugging and reusable objects. A couple of problems related to their 
creation and usage can be identified:

<P>
<UL>
<LI><I>Creating</I><BR>
Objects need to be created before they can be used. Reusable objects 
generally are used from various places in the application. How are they 
best declared and when are they best created?
<LI><I>Versioning</I><BR>
Symbolic programming languages generally allow the programmer to change 
the program when it is running. This property makes them suitable for 
rapid-prototyping. Global objects are created from the Prolog system. It 
is desirable that the global object changes if the source-code that 
declares it has been changed.
</UL>

<P>Various alternatives to creating global objects have been tried. We 
will present some of the straight-forward approaches below and describe 
the (dis)advantages. <A class="sec" href="namedref.html">Section 10.3.3</A> 
describes the <A NAME="idx:pceglobal2:333"></A><A class="pred" href="sec-D.1.html#pce_global/2">pce_global/2</A> 
directive to solve this problem. We will use a particular fill-pattern 
(image) as an example.

<H3><A NAME="sec:10.3.1"><SPAN class="sec-nr">10.3.1</SPAN> <SPAN class="sec-title">Using 
directives</SPAN></A></H3>

<P>Using a directive to create a reusable global object appears to be a 
logical way of dealing with them. This leads to the following code:

<PRE class="code">
:- new(@stones_image, image(stones)).

        ...,
        send(Box, fill_pattern, @stones_image),
        ...
</PRE>

<P>This code has a serious disadvantage. Whenever this file is reloaded 
after the Prolog code inside it has changed, the directive will be 
executed as well. The predicate <A NAME="idx:new2:334"></A><A class="pred" href="sec-D.1.html#new/2">new/2</A> 
will generate a warning on an attempt to create an object with an 
existing name.

<H3><A NAME="sec:10.3.2"><SPAN class="sec-nr">10.3.2</SPAN> <SPAN class="sec-title">Inline 
testing</SPAN></A></H3>

<P>Another common approach is to test inline. For the example, this 
would look like:

<PRE class="code">
        ...,
        (   object(@stones_image)
        -&gt;  true
        ;   new(@stones_image, image(stones))
        ),
        send(Box, fill_pattern, @stones_image),
        ...
</PRE>

<P>This approach is bad programming style. If <A NAME="idx:stonesbitmap:335"></A><B>@stones_bitmap</B> 
is required at various places in the source files this construct needs 
to be repeated at various places.

<H3><A NAME="sec:10.3.3"><SPAN class="sec-nr">10.3.3</SPAN> <SPAN class="sec-title">The 
`pce_global' directive</SPAN></A></H3>

<A NAME="sec:global"></A>

<P>This approach is based on <A NAME="idx:exceptionhandling:336">exception-handling</A>. 
If PCE translates a named reference into an internal reference and the 
named reference does not exist it will raise an exception. The <A NAME="idx:pceglobal2:337"></A><A class="pred" href="sec-D.1.html#pce_global/2">pce_global/2</A> 
directive installs an exception handler dealing with a specific global 
reference. The example becomes:

<PRE class="code">
:- pce_global(@stones_image, new(image(stones))).

        ...,
        send(Box, fill_pattern, @stones_image),
        ...
</PRE>

<P><A NAME="idx:globalobjectswithreconsult:338"></A>This directive 
applies some heuristics to take care of redefinitions when the file is 
reconsulted: if the definition is still the same it will not complain. 
If the definition has modified and the object is already created it will 
rename the object using `object
<B><CODE>-&gt;</CODE>name_reference</B>'. A later reference to the 
object will trap the exception handler again, creating a new object 
according to the current specification. The directive prints diagnostics 
messages on redefinitions and other possible problems during 
compilation. See
<A class="sec" href="interface.html">appendix D</A> for details on <A NAME="idx:pceglobal2:339"></A><A class="pred" href="sec-D.1.html#pce_global/2">pce_global/2</A>.

<H3><A NAME="sec:10.3.4"><SPAN class="sec-nr">10.3.4</SPAN> <SPAN class="sec-title">Global 
objects for recognisers</SPAN></A></H3>

<P>Recogniser objects (see <A class="sec" href="recogniser.html">section 
5.5</A>) that make graphical objects sensitive to mouse events are often 
created with a global reference. Suppose an application requires box 
objects that can be moved, resized and that have a popup menu. The 
recogniser may be defined as:

<PRE class="code">
:- pce_global(@mybox_recogniser, make_mybox_recogniser).

make_mybox_recogniser(R) :-
        Gr = @arg1,
        new(P, popup),
        send_list(P, append,
                  [ menu_item(cut, message(Gr, free))
                    ...
                  ]),
        new(R, handler_group(new(resize_gesture),
                             new(move_gesture),
                             popup_gesture(P))).</PRE>

<P>This recogniser object may be attached to the graphical objects 
either using <A NAME="idx:graphicalsendrecogniser:340"></A>`<B>graphical<CODE>-&gt;</CODE>recogniser</B>' 
or by redefining the <A NAME="idx:graphicalsendevent:341"></A>`<B>graphical<CODE>-&gt;</CODE>event</B>' 
method. In the first case, the recogniser is an integral part of the 
graphical object and cannot be changed easily during the lifetime of the 
object. In the second case, the reference to the gesture is through the 
Prolog implementation of the method and replacing the global object will 
make all members of the class behave according to the new definition 
immediately.

<P>If the latter approach is taken and the recogniser is under 
development, you may wish to use <A NAME="idx:free1:342"></A><A class="pred" href="sec-D.1.html#free/1">free/1</A> 
to make sure the new definition is created:

<PRE class="code">
:- free(@mybox_recogniser).
:- pce_global(@mybox_recogniser, make_mybox_recogniser).
</PRE>

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