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<H2><A NAME="sec:B.4"><SPAN class="sec-nr">B.4</SPAN> <SPAN class="sec-title">Hooks 
using the exception predicate</SPAN></A></H2>

<A NAME="sec:exception3"></A>

<P>This section describes the predicate <A NAME="idx:exception3:1557"></A><A class="pred" href="exception3.html#exception/3">exception/3</A>, 
which can be defined by the user in the module <CODE>user</CODE> as a 
multifile predicate. Unlike the name suggests, this is actually a <EM>hook</EM> 
predicate that has no relation to Prolog exceptions as defined by the 
ISO predicates
<A NAME="idx:catch3:1558"></A><A class="pred" href="exception.html#catch/3">catch/3</A> 
and <A NAME="idx:throw1:1559"></A><A class="pred" href="exception.html#throw/1">throw/1</A>.

<P>The predicate <A NAME="idx:exception3:1560"></A><A class="pred" href="exception3.html#exception/3">exception/3</A> 
is called by the kernel on a couple of events, allowing the user to 
`fix' errors just in time events. The mechanism allows for <EM>lazy</EM> 
creation of objects such as predicates.

<DL>
<DT class="pubdef"><A NAME="exception/3"><STRONG>exception</STRONG>(<VAR>+Exception, 
+Context, -Action</VAR>)</A></DT>
<DD class="defbody">
Dynamic predicate, normally not defined. Called by the Prolog system on 
run-time exceptions that can be repaired `just in time'. The values for <VAR>Exception</VAR> 
are described below. See also <A NAME="idx:catch3:1561"></A><A class="pred" href="exception.html#catch/3">catch/3</A> 
and <A NAME="idx:throw1:1562"></A><A class="pred" href="exception.html#throw/1">throw/1</A>.

<P>If this hook predicate succeeds it must instantiate the <VAR>Action</VAR> 
argument to the atom <CODE>fail</CODE> to make the operation fail 
silently,
<CODE>retry</CODE> to tell Prolog to retry the operation or <CODE>error</CODE> 
to make the system generate an exception. The action <CODE>retry</CODE> 
only makes sense if this hook modified the environment such that the 
operation can now succeed without error.

<DL>
<DT><STRONG>undefined_predicate</STRONG></DT>
<DD class="defbody">
<VAR>Context</VAR> is instantiated to a term <VAR>Name</VAR>/<VAR>Arity</VAR>.
<VAR>Name</VAR> refers to the name and <VAR>Arity</VAR> to the arity of 
the undefined predicate. If the definition module of the predicate is 
not
<VAR>user</VAR>, <VAR>Context</VAR> will be of the form <TT>&lt;Module&gt;:&lt;<VAR>Name</VAR>&gt;/&lt;<VAR>Arity</VAR>&gt;</TT>. 
If the predicate fails Prolog will generate an
<CODE>existence_error</CODE> exception.</DD>
<DT><STRONG>undefined_global_variable</STRONG></DT>
<DD class="defbody">
<VAR>Context</VAR> is instantiated to the name of the missing global 
variable. The hook must call <A NAME="idx:nbsetval2:1563"></A><A class="pred" href="gvar.html#nb_setval/2">nb_setval/2</A> 
or <A NAME="idx:bsetval2:1564"></A><A class="pred" href="gvar.html#b_setval/2">b_setval/2</A> 
before returning with the action <CODE>retry</CODE>.
</DD>
</DL>

</DD>
</DL>

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