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<h2 id="sec:exception3"><a id="sec:B.7"><span class="sec-nr">B.7</span> <span class="sec-title">Hooks 
using the exception predicate</span></a></h2>

<a id="sec:exception3"></a>

<p>This section describes the predicate <a id="idx:exception3:1871"></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 id="idx:catch3:1872"></a><a class="pred" href="exception.html#catch/3">catch/3</a> 
and <a id="idx:throw1:1873"></a><a class="pred" href="exception.html#throw/1">throw/1</a>.

<p>The predicate <a id="idx:exception3:1874"></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. The mechanism allows for
<em>lazy</em> creation of objects such as predicates.

<dl class="latex">
<dt class="pubdef"><a id="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 id="idx:catch3:1875"></a><a class="pred" href="exception.html#catch/3">catch/3</a> 
and <a id="idx:throw1:1876"></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 class="latex">
<dt><strong>undefined_predicate</strong></dt>
<dd class="defbody">
<var>Context</var> is instantiated to a predicate indicator ([module]:&lt;<var>name</var>&gt;/&lt;<var>arity</var>&gt;). 
If the predicate fails, Prolog will generate an <code>existence_error</code> 
exception. The hook is intended to implement alternatives to the 
built-in autoloader, such as autoloading code from a database. Do <em>not</em> 
use this hook to suppress existence errors on predicates. See also <a class="flag" href="flags.html#flag:unknown">unknown</a> 
and <a class="sec" href="autoload.html">section 2.13</a>.</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 id="idx:nbsetval2:1877"></a><a class="pred" href="gvar.html#nb_setval/2">nb_setval/2</a> 
or <a id="idx:bsetval2:1878"></a><a class="pred" href="gvar.html#b_setval/2">b_setval/2</a> 
before returning with the action <code>retry</code>.
</dd>
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swi-prolog-nox 6.6.4-2ubuntu1 / usr / lib / swi-prolog / doc / Manual / exception3.html
