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

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<H2><A NAME="sec:2.8"><SPAN class="sec-nr">2.8</SPAN> <SPAN class="sec-title">Reuse 
of top-level bindings</SPAN></A></H2>

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

<P>Bindings resulting from the successful execution of a top-level goal 
are asserted in a database. These values may be reused in further 
top-level queries as $Var. Only the latest binding is available. 
Example:

<PRE class="code">
1 ?- maplist(plus(1), "hello", X).

X = [105,102,109,109,112]

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

Yes
3 ?-
</PRE>

<TABLE ALIGN=center WIDTH="75%"><TR><TD>
<B>Figure 1 : </B>Reusing top-level bindings</TABLE>

<A NAME="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

Yes
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

Yes
8 ?-
</PRE>

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swi-prolog-nox 5.10.4-3ubuntu1 / usr / lib / swi-prolog / doc / Manual / topvars.html
