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<head><title>INTERN.html -- ACL2 Version 3.1</title></head>
<body text=#000000 bgcolor="#FFFFFF">
<h2>INTERN</h2>create a new symbol in a given package
<pre>Major Section: <a href="PROGRAMMING.html">PROGRAMMING</a>
</pre><p>
<code>(intern symbol-name symbol-package-name)</code> returns a symbol with
the given <code><a href="SYMBOL-NAME.html">symbol-name</a></code> and the given <code><a href="SYMBOL-PACKAGE-NAME.html">symbol-package-name</a></code>. We
restrict Common Lisp's <code>intern</code> so that the second argument is
either the symbol *main-lisp-package-name*, the value of that
constant, or is one of "ACL2", "ACL2-INPUT-CHANNEL",
"ACL2-OUTPUT-CHANNEL", or "KEYWORD". To avoid that restriction,
see <a href="INTERN$.html">intern$</a>.
<p>
In ACL2 <code>intern</code> is actually implemented as a macro that expands to
a call of a similar function whose second argument is a symbol.
Invoke <code>:pe intern</code> to see the definition, or
see <a href="INTERN-IN-PACKAGE-OF-SYMBOL.html">intern-in-package-of-symbol</a>.<p>
To see why is <code>intern</code> so restricted consider
<code>(intern "X" "P")</code>. In particular, is it a symbol and if so,
what is its <code><a href="SYMBOL-PACKAGE-NAME.html">symbol-package-name</a></code>? One is tempted to say ``yes, it
is a symbol in the package <code>"P"</code>.'' But if package <code>"P"</code> has
not yet been defined, that would be premature because the imports to
the package are unknown. For example, if <code>"P"</code> were introduced
with
<pre>
(defpkg "P" '(LISP::X))
</pre>
then in Common Lisp <code>(symbol-package-name (intern "X" "P"))</code> returns
<code>"LISP"</code>.<p>
The obvious restriction on <code>intern</code> is that its second argument be
the name of a package known to ACL2. We cannot express such a
restriction (except, for example, by limiting it to those packages
known at some fixed time, as we do). Instead, we provide
<code><a href="INTERN-IN-PACKAGE-OF-SYMBOL.html">intern-in-package-of-symbol</a></code> which requires a ``witness symbol''
for the package instead of the package. The witness symbol is any
symbol (expressible in ACL2) and uniquely specifies a package
necessarily known to ACL2.
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