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

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<h1 id="sec:extvar"><a id="sec:6"><span class="sec-nr">6</span> <span class="sec-title">Special 
Variables and Coroutining</span></a></h1>

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

<p>This chapter deals with extensions primarily designed to support 
constraint logic programming (CLP). The low-level attributed variable 
interface defined in <a class="sec" href="attvar.html">section 6.1</a> 
is not intended for the typical Prolog programmer. Instead, the typical 
Prolog programmer should use the coroutining predicates and the various 
constraint solvers built on top of attributed variables. CHR (<a class="sec" href="chr.html">chapter 
7</a>) provides a general purpose constraint handling language.

<p>As a rule of thumb, constraint programming reduces the search space 
by reordering goals and joining goals based on domain knowledge. A 
typical example is constraint reasoning over integer domains. Plain 
Prolog has no efficient means to deal with (integer) <var>X &gt; 0</var> 
and <var>X &lt; 3</var>. At best it could translate <var>X &gt; 0</var> 
with uninstantiated X to <code>between(1, infinite, X)</code> and a 
similar primitive for <var>X &lt; 3</var>. If the two are combined it 
has no choice but to generate and test over this infinite 
two-dimensional space. Instead, a constraint system will <em>delay</em> 
an uninstantiated goal to <var>X &gt; 0</var>. If, later, it finds a 
value for
<var>X</var> it will execute the test. If it finds <var>X &lt; 3</var> 
it will combine this knowledge to infer that X is in 1..2 (see below). 
If it never finds a concrete value for <var>X</var> it can be asked to <em>label</em> <var>X</var> 
and produce 1 and 2 on backtracking. See <a class="sec" href="clpfd.html">section 
A.7</a>.

<pre class="code">
1 ?- [library(clpfd)].
...
true.

2 ?- X #&gt; 0, X #&lt; 3.
X in 1..2.
</pre>

<p>Using constraints generally makes your program more
<em>declarative</em>. There are some caveats though:

<p>
<ul class="latex">
<li>Constraints and cuts do not merge well. A cut after a goal that is 
delayed prunes the search space before the condition is true.
<li>Term-copying operations (<a id="idx:assert1:1489"></a><a class="pred" href="db.html#assert/1">assert/1</a>, <a id="idx:retract2:1490"></a><span class="pred-ext">retract/2</span>, <a id="idx:findall3:1491"></a><a class="pred" href="allsolutions.html#findall/3">findall/3</a>,
<a id="idx:copyterm2:1492"></a><a class="pred" href="manipterm.html#copy_term/2">copy_term/2</a>, 
etc.) generally also copy constraints. The effect varies from ok, silent 
copying of huge constraint networks to violations of the internal 
consistency of constraint networks. As a rule of thumb, copying terms 
holding attributes must be deprecated.
</ul>

<p>
<hr>
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<h2>Section Index</h2>

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<hr>
<div class="toc">
<div class="toc-h2"><a class="sec" href="attvar.html"><span class="sec-nr">6.1</span> <span class="sec-title">Attributed 
variables</span></a></div>
<div class="toc-h3"><a class="sec" href="attvar.html#sec:6.1.1"><span class="sec-nr">6.1.1</span> <span class="sec-title">Attribute 
manipulation predicates</span></a></div>
<div class="toc-h3"><a class="sec" href="attvar.html#sec:6.1.2"><span class="sec-nr">6.1.2</span> <span class="sec-title">Attributed 
variable hooks</span></a></div>
<div class="toc-h3"><a class="sec" href="attvar.html#sec:6.1.3"><span class="sec-nr">6.1.3</span> <span class="sec-title">Operations 
on terms with attributed variables</span></a></div>
<div class="toc-h3"><a class="sec" href="attvar.html#sec:6.1.4"><span class="sec-nr">6.1.4</span> <span class="sec-title">Special 
purpose predicates for attributes</span></a></div>
<div class="toc-h2"><a class="sec" href="coroutining.html"><span class="sec-nr">6.2</span> <span class="sec-title">Coroutining</span></a></div>
<div class="toc-h2"><a class="sec" href="gvar.html"><span class="sec-nr">6.3</span> <span class="sec-title">Global 
variables</span></a></div>
<div class="toc-h3"><a class="sec" href="gvar.html#sec:6.3.1"><span class="sec-nr">6.3.1</span> <span class="sec-title">Compatibility 
of SWI-Prolog Global Variables</span></a></div>
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swi-prolog-nox 6.6.4-2ubuntu1 / usr / lib / swi-prolog / doc / Manual / extvar.html
