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<H2><A NAME="sec:7.4"><SPAN class="sec-nr">7.4</SPAN> <SPAN class="sec-title">Handling 
default arguments</SPAN></A></H2>

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

<P>The predicate <A NAME="idx:default3:237"></A><A class="pred" href="udcpreds.html#default/3">default/3</A> 
provides a comfortable way to specify the meaning of default arguments. 
Future versions may incorporate the default value in the <A class="" href="summary.html#class:type">type</A> 
object itself.

<DL>
<DT class="pubdef"><A NAME="default/3"><STRONG>default</STRONG>(<VAR>+Argument, 
+Default, -Value</VAR>)</A></DT>
<DD class="defbody">
Used to specify and compute defaults for arguments. <EM>Argument</EM> is 
the actual argument passed to the method implementation, <EM>Default</EM> 
is any valid <font size=-1>XPCE</font> object description (reference, 
integer, real, atom or compound ground term describing an object, see
<A NAME="idx:send212:238"></A><A class="pred" href="sec-7.1.html#send/[2-12]">send/[2-12]</A>). <EM>Default</EM> 
can also be the term

<PRE class="code">
resource(&lt;Object&gt;, &lt;Name&gt;)
</PRE>

<P>In which case the <B><CODE>&lt;-</CODE>resource_value</B>: &lt;<VAR>Name</VAR>&gt; 
from &lt;<VAR>Object</VAR>&gt; will be used as default value. <EM>Value</EM> 
is unified with <EM>Argument</EM> if <EM>Argument</EM> is not <A NAME="idx:default:239"></A><B>@default</B> 
and with <EM>Default</EM> otherwise.

<P>The following is an example that sets the volume to a specified value 
or the value of the resource `volume' if <A NAME="idx:default:240"></A><B>@default</B> 
is passed as an argument.

<PRE class="code">
resource(volume, 0..130,  75, "Volume in decibels").

volume(X, Vol:[0..130]) :-&gt;
        default(Vol, resource(X, volume), V),
        &lt;set the volume here&gt;.
</PRE>

<P></DD>
</DL>

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