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

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<h2 id="sec:topvars"><a id="sec:2.8"><span class="sec-nr">2.8</span> <span class="sec-title">Reuse 
of top-level bindings</span></a></h2>

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

<p>Bindings resulting from the successful execution of a top-level goal 
are asserted in a database <em>if they are not too large</em>. These 
values may be reused in further top-level queries as $Var. If the same 
variable name is used in a subsequent query the system associates the 
variable with the latest binding. Example:

<pre class="code">
1 ?- maplist(plus(1), "hello", X).
X = [105,102,109,109,112].

2 ?- format('~s~n', [$X]).
ifmmp
true.

3 ?-
</pre>

<div class="caption"><b>Figure 1 : </b>Reusing top-level bindings</div>
<a id="fig:topevelvars"></a>

<p>Note that variables may be set by executing <a class="pred" href="compare.html#=/2">=/2</a>:

<pre class="code">
6 ?- X = statistics.
X = statistics.

7 ?- $X.
28.00 seconds cpu time for 183,128 inferences
4,016 atoms, 1,904 functors, 2,042 predicates, 52 modules
55,915 byte codes; 11,239 external references

                      Limit    Allocated       In use
Heap         :                                624,820 Bytes
Local  stack :    2,048,000        8,192          404 Bytes
Global stack :    4,096,000       16,384          968 Bytes
Trail  stack :    4,096,000        8,192          432 Bytes
true.
</pre>

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swi-prolog-nox 6.6.4-2ubuntu1 / usr / lib / swi-prolog / doc / Manual / topvars.html
