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

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<H1><A NAME="sec:6"><SPAN class="sec-nr">6</SPAN> <SPAN class="sec-title">Special 
Variables and Coroutining</SPAN></A></H1>

<A NAME="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 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">section 
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 uninstantated 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 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 to 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 NAME="idx:assert1:1294"></A><A class="pred" href="db.html#assert/1">assert/1</A>, <A NAME="idx:restract2:1295"></A><SPAN class="pred-ext">restract/2</SPAN>, <A NAME="idx:findall3:1296"></A><A class="pred" href="allsolutions.html#findall/3">findall/3</A>,
<A NAME="idx:copyterm2:1297"></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>
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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 5.10.4-3ubuntu1 / usr / lib / swi-prolog / doc / Manual / extvar.html
