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<title>SWI-Prolog 7.3.6 Reference Manual: Section 7.2</title><link rel="home" href="index.html">
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<h2 id="sec:coroutining"><a id="sec:7.2"><span class="sec-nr">7.2</span> <span class="sec-title">Coroutining</span></a></h2>
<a id="sec:coroutining"></a>
<p>Coroutining deals with having Prolog goals scheduled for execution as
soon as some conditions are fulfilled. In Prolog the most commonly used
condition is the instantiation (binding) of a variable. Scheduling a
goal to execute immediately after a variable is bound can be used to
avoid instantiation errors for some built-in predicates (e.g. arithmetic),
do work <em>lazy</em>, prevent the binding of a variable to a particular
value, etc. Using <a id="idx:freeze2:1640"></a><a class="pred" href="coroutining.html#freeze/2">freeze/2</a>
for example we can define a variable that can only be assigned an even
number:
<pre class="code">
?- freeze(X, X mod 2 =:= 0), X = 3
No
</pre>
<dl class="latex">
<dt class="pubdef"><a id="freeze/2"><strong>freeze</strong>(<var>+Var,
:Goal</var>)</a></dt>
<dd class="defbody">
Delay the execution of <var>Goal</var> until <var>Var</var> is bound
(i.e. is not a variable or attributed variable). If <var>Var</var> is
bound on entry
<a id="idx:freeze2:1641"></a><a class="pred" href="coroutining.html#freeze/2">freeze/2</a>
is equivalent to <a id="idx:call1:1642"></a><a class="pred" href="metacall.html#call/1">call/1</a>.
The <a id="idx:freeze2:1643"></a><a class="pred" href="coroutining.html#freeze/2">freeze/2</a>
predicate is realised using an attributed variable associated with the
module <code>freeze</code>. Use <code>frozen(Var, Goal)</code> to find
out whether and which goals are delayed on <var>Var</var>.</dd>
<dt class="pubdef"><a id="frozen/2"><strong>frozen</strong>(<var>@Var,
-Goal</var>)</a></dt>
<dd class="defbody">
Unify <var>Goal</var> with the goal or conjunction of goals delayed on
<var>Var</var>. If no goals are frozen on <var>Var</var>, <var>Goal</var>
is unified to <code>true</code>.</dd>
<dt class="pubdef"><a id="when/2"><strong>when</strong>(<var>@Condition,
:Goal</var>)</a></dt>
<dd class="defbody">
Execute <var>Goal</var> when <var>Condition</var> becomes true. <var>Condition</var>
is one of <code>?=(X, Y)</code>, <code>nonvar(X)</code>, <code>ground(X)</code>,
<code><code>,</code>(Cond1, Cond2)</code> or <code><code>;</code>(Cond1,
Cond2)</code>. See also <a id="idx:freeze2:1644"></a><a class="pred" href="coroutining.html#freeze/2">freeze/2</a>
and <a id="idx:dif2:1645"></a><a class="pred" href="coroutining.html#dif/2">dif/2</a>.
The implementation can deal with cyclic terms in <var>X</var> and <var>Y</var>.
<p>The <a id="idx:when2:1646"></a><a class="pred" href="coroutining.html#when/2">when/2</a>
predicate is realised using attributed variables associated with the
module <code>when</code>. It is defined in the autoload library
<code>library(when)</code>.</dd>
<dt class="pubdef"><a id="dif/2"><strong>dif</strong>(<var>@A, @B</var>)</a></dt>
<dd class="defbody">
The <a id="idx:dif2:1647"></a><a class="pred" href="coroutining.html#dif/2">dif/2</a>
predicate provides a constraint stating that <var>A</var> and
<var>B</var> are different terms. If <var>A</var> and <var>B</var> can
never unify,
<a id="idx:dif2:1648"></a><a class="pred" href="coroutining.html#dif/2">dif/2</a>
succeeds deterministically. If <var>A</var> and <var>B</var> are
identical it fails immediately, and finally, if <var>A</var> and <var>B</var>
can unify, goals are delayed that prevent <var>A</var> and <var>B</var>
to become equal. The
<a id="idx:dif2:1649"></a><a class="pred" href="coroutining.html#dif/2">dif/2</a>
predicate behaves as if defined by
<code>dif(X, Y) :- when(?=(X, Y), X \== Y)</code>. See also <a class="pred" href="compare.html#?=/2">?=/2</a>.
The implementation can deal with cyclic terms.
<p>The <a id="idx:dif2:1650"></a><a class="pred" href="coroutining.html#dif/2">dif/2</a>
predicate is realised using attributed variables associated with the
module <code>dif</code>. It is defined in the autoload library
<code>library(dif)</code>.</dd>
<dt class="pubdef"><a id="call_residue_vars/2"><strong>call_residue_vars</strong>(<var>:Goal,
-Vars</var>)</a></dt>
<dd class="defbody">
Find residual attributed variables left by <var>Goal</var>. This
predicate is intended for debugging programs using coroutining or
constraints. Consider a program that poses contradicting constraints on
a variable. Such programs should fail, but sometimes succeed because the
constraint solver is too weak to detect the contradiction. Ideally,
delayed goals and constraints are all executed at the end of the
computation. The meta predicate <a id="idx:callresiduevars2:1651"></a><a class="pred" href="coroutining.html#call_residue_vars/2">call_residue_vars/2</a>
finds variables that are given attribute variables or whose attributes
are modified by <var>Goal</var>, regardless of whether or not these
variables are reachable from the arguments of
<var>Goal</var>.<sup class="fn">139<span class="fn-text">The
implementation of <a id="idx:callresiduevars2:1652"></a><a class="pred" href="coroutining.html#call_residue_vars/2">call_residue_vars/2</a>
is completely redone in version 7.3.2 (7.2.1) after discussion with Bart
Demoen. The current implementation no longer performs full scans of the
stacks. The overhead is proportional to the number of attributed
variables on the stack, dead or alive.</span></sup>.
</dd>
</dl>
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