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<H2><A NAME="sec:4.11"><SPAN class="sec-nr">4.11</SPAN> <SPAN class="sec-title">The 
`block' control-structure</SPAN></A></H2>

<A NAME="sec:block3"></A>

<P>The <A NAME="idx:block3:595"></A><A class="pred" href="block3.html#block/3">block/3</A> 
predicate and friends have been introduced before ISO compatible <A NAME="idx:catch3:596"></A><A class="pred" href="exception.html#catch/3">catch/3</A> 
exception handling for compatibility with some Prolog implementation. 
The only feature not covered by <A NAME="idx:catch3:597"></A><A class="pred" href="exception.html#catch/3">catch/3</A> 
and <A NAME="idx:throw1:598"></A><A class="pred" href="exception.html#throw/1">throw/1</A> 
is the possibility to execute global cuts. New code should use <A NAME="idx:catch3:599"></A><A class="pred" href="exception.html#catch/3">catch/3</A> 
and <A NAME="idx:throw1:600"></A><A class="pred" href="exception.html#throw/1">throw/1</A> 
to deal with exceptions.

<DL>
<DT class="pubdef"><A NAME="block/3"><STRONG>block</STRONG>(<VAR>+Label, 
:Goal, -ExitValue</VAR>)</A></DT>
<DD class="defbody">
Execute <VAR>Goal</VAR> in a <EM>block</EM>. <VAR>Label</VAR> is the 
name of the block. <VAR>Label</VAR> is normally an atom, but the system 
imposes no type constraints and may even be a variable. <VAR>ExitValue</VAR> 
is normally unified to the second argument of an <A NAME="idx:exit2:601"></A><A class="pred" href="block3.html#exit/2">exit/2</A> 
call invoked by <VAR>Goal</VAR>.
</DD>
<DT class="pubdef"><A NAME="exit/2"><STRONG>exit</STRONG>(<VAR>+Label, 
+Value</VAR>)</A></DT>
<DD class="defbody">
Calling <A NAME="idx:exit2:602"></A><A class="pred" href="block3.html#exit/2">exit/2</A> 
makes the innermost <VAR>block</VAR> which <VAR>Label</VAR> unifies 
exit. The block's <VAR>ExitValue</VAR> is unified with <VAR>Value</VAR>. 
If this unification fails the block fails.
</DD>
<DT class="pubdef"><A NAME="fail/1"><STRONG>fail</STRONG>(<VAR>+Label</VAR>)</A></DT>
<DD class="defbody">
Calling <A NAME="idx:fail1:603"></A><A class="pred" href="block3.html#fail/1">fail/1</A> 
makes the innermost <VAR>block</VAR> which <VAR>Label</VAR> unifies fail 
immediately. Implemented as

<PRE class="code">
fail(Label) :- !(Label), fail.
</PRE>

</DD>
<DT class="pubdef"><A NAME="!/1"><STRONG>!</STRONG>(<VAR>+Label</VAR>)</A></DT>
<DD class="defbody">
Cut all choice-points created since the entry of the innermost
<VAR>block</VAR> which <VAR>Label</VAR> unifies.
</DD>
</DL>

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