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Literate Programming
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<title editable="nocomment">Chap11b</title>
<h1>Chap11b: retract</h1>
<p>The database manipulations we have been making have
changed the meaning of the predicate happy/1 . More generally,
database manipulation commands give us the ability to change
the meaning of predicates while we are running programs.
Predicates whose definitions change during run-time are
called dynamic predicates, as opposed to the static predicates
that we have previously dealt with. Most Prolog interpreters
insist that we explicitly declare the predicates that we wish
to be dynamic. We will soon examine an example involving dynamic
predicates, but let’s first complete our discussion of the
database manipulation commands.</p>
<p>Now that we know how to assert new information into the database,
we should also learn how to remove information when we no longer
need it. There is an inverse predicate to assertz/1 , namely retract/1 .
For example, if we carry straight on from the previous example by
giving the command:</p>
<table style="width: 100%">
<tr>
<td class="input" style="width: 100%"
contenteditable="true" id="text0" spellcheck="false"
>:- dynamic happy/1.
happy(mia).
happy(vincent).
happy(marcellus).
happy(butch).
happy(vincent).
?- retract(happy(marcellus)).
</td>
<td rowspan="2" valign="top">
<button class="run" id="launch0"
>⏵</button>
</td>
</tr>
<tr>
<td class="result" id="demo0"></td>
</tr>
</table>
<p>and then list the database, we get:</p>
<table style="width: 100%">
<tr>
<td class="input" style="width: 100%"
contenteditable="true" id="text1" spellcheck="false"
>?- listing.
</td>
<td rowspan="2" valign="top">
<button class="run" id="launch1"
>⏵</button>
</td>
</tr>
<tr>
<td class="result" id="demo1"></td>
</tr>
</table>
<p>That is, the fact happy(marcellus) has been removed.</p>
<p>Suppose we go on further, and say</p>
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contenteditable="true" id="text2" spellcheck="false"
>?- once(retract(happy(vincent))).
</td>
<td rowspan="2" valign="top">
<button class="run" id="launch2"
>⏵</button>
</td>
</tr>
<tr>
<td class="result" id="demo2"></td>
</tr>
</table>
<p>and then ask for a listing. We get:</p>
<table style="width: 100%">
<tr>
<td class="input" style="width: 100%"
contenteditable="true" id="text3" spellcheck="false"
>?- listing.
</td>
<td rowspan="2" valign="top">
<button class="run" id="launch3"
>⏵</button>
</td>
</tr>
<tr>
<td class="result" id="demo3"></td>
</tr>
</table>
<p>Note that the first occurrence of happy(vincent) , and only the
first occurrence, was removed.</p>
<p>To remove all of our assertions contributing to the definition of
the predicate happy/1 we can use a variable:</p>
<table style="width: 100%">
<tr>
<td class="input" style="width: 100%"
contenteditable="true" id="text4" spellcheck="false"
>?- retract(happy(X)).
</td>
<td rowspan="2" valign="top">
<button class="run" id="launch4"
>⏵</button>
</td>
</tr>
<tr>
<td class="result" id="demo4"></td>
</tr>
</table>
<p>A listing reveals that the database is now empty.</p>
<table style="width: 100%">
<tr>
<td class="input" style="width: 100%"
contenteditable="true" id="text5" spellcheck="false"
>?- listing.</td>
<td rowspan="2" valign="top">
<button class="run" id="launch5"
>⏵</button>
</td>
</tr>
<tr>
<td class="result" id="demo5"></td>
</tr>
</table>
<script type="module">
import {setup_engine, link_cells_async
} from "./engine.mjs";
setup_engine();
link_cells_async();
</script>
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