/usr/lib/swi-prolog/doc/Manual/intlibs.html

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<H2><A NAME="sec:B.5"><SPAN class="sec-nr">B.5</SPAN> <SPAN class="sec-title">Hooks 
for integrating libraries</SPAN></A></H2>

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

<P>Some libraries realise an entirely new programming paradigm on top of 
Prolog. An example is XPCE which adds an object-system to Prolog as well 
as an extensive set of graphical primitives. SWI-Prolog provides several 
hooks to improve the integration of such libraries. See also
<A class="sec" href="listing.html">section 4.4</A> for editing hooks and <A class="sec" href="exception.html">section 
4.9.3</A> for hooking into the message system.

<DL class="latex">
<DT class="pubdef"><A NAME="prolog_list_goal/1"><STRONG>prolog_list_goal</STRONG>(<VAR>:Goal</VAR>)</A></DT>
<DD class="defbody">
Hook, normally not defined. This hook is called by the 'L' command of 
the tracer in the module <CODE>user</CODE> to list the currently called 
predicate. This hook may be defined to list only relevant clauses of the 
indicated <VAR>Goal</VAR> and/or show the actual source-code in an 
editor. See also <A NAME="idx:portray1:1660"></A><A class="pred" href="termrw.html#portray/1">portray/1</A> 
and <A NAME="idx:multifile1:1661"></A><A class="pred" href="dynamic.html#multifile/1">multifile/1</A>.</DD>
<DT class="pubdef"><A NAME="prolog:debug_control_hook/1"><STRONG>prolog:debug_control_hook</STRONG>(<VAR>:Action</VAR>)</A></DT>
<DD class="defbody">
Hook for the debugger-control predicates that allows the creator of more 
high-level programming languages to use the common front-end predicates 
to control de debugger. For example, XPCE uses these hooks to allow for 
spying methods rather then predicates. <VAR>Action</VAR> is one of:

<DL class="latex">
<DT><STRONG>spy</STRONG>(<VAR>Spec</VAR>)</DT>
<DD class="defbody">
Hook in <A NAME="idx:spy1:1662"></A><A class="pred" href="debugger.html#spy/1">spy/1</A>. 
If the hook succeeds <A NAME="idx:spy1:1663"></A><A class="pred" href="debugger.html#spy/1">spy/1</A> 
takes no further action.
</DD>
<DT><STRONG>nospy</STRONG>(<VAR>Spec</VAR>)</DT>
<DD class="defbody">
Hook in <A NAME="idx:nospy1:1664"></A><A class="pred" href="debugger.html#nospy/1">nospy/1</A>. 
If the hook succeeds <A NAME="idx:spy1:1665"></A><A class="pred" href="debugger.html#spy/1">spy/1</A> 
takes no further action. If <A NAME="idx:spy1:1666"></A><A class="pred" href="debugger.html#spy/1">spy/1</A> 
is hooked, it is advised to place a complementary hook for
<A NAME="idx:nospy1:1667"></A><A class="pred" href="debugger.html#nospy/1">nospy/1</A>.
</DD>
<DT><STRONG>nospyall</STRONG></DT>
<DD class="defbody">
Hook in <A NAME="idx:nospyall0:1668"></A><A class="pred" href="debugger.html#nospyall/0">nospyall/0</A>. 
Should remove all spy-points. This hook is called in a failure-driven 
loop.
</DD>
<DT><STRONG>debugging</STRONG></DT>
<DD class="defbody">
Hook in <A NAME="idx:debugging0:1669"></A><A class="pred" href="debugger.html#debugging/0">debugging/0</A>. 
It can be used in two ways. It can report the status of the additional 
debug-points controlled by the above hooks and fail to let the system 
report the others or it succeed, overruling the entire behaviour of <A NAME="idx:debugging0:1670"></A><A class="pred" href="debugger.html#debugging/0">debugging/0</A>.
</DD>
</DL>

</DD>
<DT class="pubdef"><A NAME="prolog:help_hook/1"><STRONG>prolog:help_hook</STRONG>(<VAR>+Action</VAR>)</A></DT>
<DD class="defbody">
Hook into <A NAME="idx:help0:1671"></A><A class="pred" href="help.html#help/0">help/0</A> 
and <A NAME="idx:help1:1672"></A><A class="pred" href="help.html#help/1">help/1</A>. 
If the hook succeeds, the built-in actions are not executed. For 
example, <CODE>?- help(picture).</CODE> is caught by the XPCE help-hook 
to give help on the class <EM>picture</EM>. Defined actions are:

<DL class="latex">
<DT><STRONG>help</STRONG></DT>
<DD class="defbody">
User entered plain <A NAME="idx:help0:1673"></A><A class="pred" href="help.html#help/0">help/0</A> 
to give default help. The default performs
<CODE>help(<A NAME="idx:help1:1674"></A><A class="pred" href="help.html#help/1">help/1</A>)</CODE>, 
giving help on help.
</DD>
<DT><STRONG>help</STRONG>(<VAR>What</VAR>)</DT>
<DD class="defbody">
Hook in <A NAME="idx:help1:1675"></A><A class="pred" href="help.html#help/1">help/1</A> 
on the topic <VAR>What</VAR>.
</DD>
<DT><STRONG>apropos</STRONG>(<VAR>What</VAR>)</DT>
<DD class="defbody">
Hook in <A NAME="idx:apropos1:1676"></A><A class="pred" href="help.html#apropos/1">apropos/1</A> 
on the topic <VAR>What</VAR>.
</DD>
</DL>

</DD>
</DL>

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