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<h1><a href="https://srfi.schemers.org/"><img class="srfi-logo" src="https://srfi.schemers.org/srfi-logo.svg" alt="SRFI surfboard logo"/></a>258: Uninterned Symbols</h1>
<p>by Wolfgang Corcoran-Mathe</p>
<h2 id="status">Status</h2>
<p>This SRFI is currently in <em>final</em> status. Here is <a href="https://srfi.schemers.org/srfi-process.html">an explanation</a> of each status that a SRFI can hold. To provide input on this SRFI, please send email to <code><a href="mailto:srfi+minus+258+at+srfi+dotschemers+dot+org">srfi-258@<span class="antispam">nospam</span>srfi.schemers.org</a></code>. To subscribe to the list, follow <a href="https://srfi.schemers.org/srfi-list-subscribe.html">these instructions</a>. You can access previous messages via the mailing list <a href="https://srfi-email.schemers.org/srfi-258/">archive</a>.</p>
<ul>
<li>Received: 2025-01-22</li>
<li>Draft #1 published: 2025-01-23</li>
<li>Draft #2 published: 2025-01-27</li>
<li>Draft #3 published: 2025-02-24</li>
<li>Finalized: 2025-03-26</li>
</ul>
<h2 id="abstract">Abstract</h2>
<p>An uninterned symbol is not the same as any other symbol, even one
with the same name. These symbols are useful in macro programming and
in other situations where guaranteed-unique names are needed.
A survey of uninterned and uniquely named symbols
in Scheme is also provided.</p>
<h2 id="table-of-contents">Table of Contents</h2>
<ul>
<li><a href="#rationale">Rationale</a></li>
<li><a href="#caveats">Caveats</a></li>
<li><a href="#specification">Specification</a></li>
<li>
<a href="#procedures">Procedures</a>
<ul>
<li>
<a href="#string-to-uninterned-symbol">
<code>string->uninterned-symbol</code>
</a>
</li>
<li>
<a href="#symbol-interned"><code>symbol-interned?</code></a>
</li>
<li>
<a href="#generate-uninterned-symbol">
<code>generate-uninterned-symbol</code>
</a>
</li>
</ul>
</li>
<li>
<a href="#prior-art">Prior art</a>
<ul>
<li><a href="#chezscheme">ChezScheme</a></li>
<li><a href="#common-lisp">Common Lisp</a></li>
</ul>
</li>
<li><a href="#implementation">Implementation</a></li>
<li><a href="#acknowledgments">Acknowledgments</a></li>
<li><a href="#references">References</a></li>
<li><a href="#copyright">Copyright</a></li>
</ul>
<h2 id="rationale">Rationale</h2>
<p>A characteristic property of symbols in Scheme is that “two symbols
are identical (in the sense of <code>eqv?</code>) if and only if their
names are spelled the same way” (<cite>R7RS</cite> section 6.5). Such
symbols are traditionally known as <dfn>interned</dfn> symbols. It is
sometimes useful, however, to create <dfn>uninterned</dfn> symbols
that are different from all other symbols, even those with the same
textual name.</p>
<p>Many Scheme implementations provide uninterned symbols, and they
are also part of the ANSI Common Lisp standard. Their use is almost
mandatory with unhygienic macro systems (like that of Common Lisp),
where they are used as hidden names that cannot be accidentally
captured. This issue is largely solved by Scheme’s hygienic macro
systems, but uninterned symbols still have their uses in Scheme macro
programming. They can be used as unique keys shared between
communicating macros or procedures, for example,
since there is no possibility of
collision between uninterned and user-created symbols.</p>
<p>The traditional way to create a unique symbol is through a
<code>gensym</code> procedure. The name of this procedure is
ubiquitous: it dates back at least as far as LISP 1.5, and a variant of
<code>gensym</code> is provided by almost every Scheme implementation.
It should come as no surprise that the behavior of these
<code>gensym</code> procedures varies
widely; in particular, many <code>gensym</code>s return interned
symbols (see the <a href="#prior-art">survey</a>
below for details). To increase
compatibility with existing implementations, this document specifies
<code>generate-uninterned-symbol</code>, a <code>gensym</code>-like
procedure, and
another constructor, <code>string->uninterned-symbol</code>, which
is consistent across the Scheme implementations that provide it.</p>
<h2 id="caveats">Caveats</h2>
<p>Adding uninterned symbols to a Scheme implementation changes the
semantics of symbols materially. They break the defining property of
symbols, namely, that two symbols are the same if they have the same
name. In the presence of uninterned symbols, symbol
equality (and thus general object equality in the senses of
<code>eqv?</code> and <code>equal?</code>) becomes harder to specify.
(An attempt is made below.)</p>
<p>Uninterned symbols have a long history in Lisp and Scheme, and
their behavior is mostly consistent across the Scheme implementations
that provide them. Nevertheless, the author of this SRFI is not sure
whether their benefits outweigh their drawbacks.</p>
<h2 id="specification">Specification</h2>
<p>The words “must”, “may”, etc., though not capitalized in this
SRFI, are to be interpreted as described in
<a href="https://datatracker.ietf.org/doc/html/rfc2119">RFC
2119</a>.</p>
<p>In this document, a symbol’s <dfn>textual name</dfn> refers to
the string returned when <code>symbol->string</code> is invoked
on the symbol.</p>
<p>An uninterned symbol is a symbol (in the sense of
<code>symbol?</code>) which is not identical (in the sense of
<code>symbol=?</code>) with any other symbol, even a symbol with
the same textual name. An uninterned symbol is “identical to
itself”. That is, if <var>s</var> and <var>t</var> are uninterned
symbols, <code>(symbol=?</code> <var>s</var>
<var>t</var><code>)</code> returns true if and only if <var>s</var>
and <var>t</var> were created by the same invocation of
<code>string->uninterned-symbol</code> or
<code>generate-uninterned-symbol</code>.</p>
<p>The exact external representation of an uninterned symbol is
unspecified, but the <code>read</code> procedure (<cite>R7RS</cite>
section 6.13.2) must signal an error (satisfying
<code>read-error?</code>) if it encounters such an external
representation. Thus uninterned symbols do not support the
write/read invariance described in <cite>R7RS</cite> section
6.5.</p>
<h2 id="procedures">Procedures</h2>
<p>The procedures in this section are exported by the
<code>(srfi :258 uninterned-symbols)</code> library.</p>
<p id="string-to-uninterned-symbol">
<code>(string->uninterned-symbol</code> <var>string</var><code>)</code>
→ <span class="type-meta">uninterned-symbol</span></p>
<p>Returns an uninterned symbol with a textual name given by <var>string</var>.</p>
<p id="symbol-interned"><code>(symbol-interned?</code> <var>symbol</var><code>)</code>
→ <span class="type-meta">boolean</span></p>
<p>Returns <code>#t</code> if <var>symbol</var> is an interned (ordinary)
symbol, and <code>#f</code> if it is uninterned.</p>
<p id="generate-uninterned-symbol"><code>(generate-uninterned-symbol</code>
[<var>prefix</var>]<code>)</code> →
<span class="type-meta">uninterned-symbol</span></p>
<p>Returns an uninterned symbol with a textual name that is likely to be
unique. If the optional <var>prefix</var> argument is provided and is a
string / symbol, then <var>prefix</var> / the name of <var>prefix</var> is
prepended to the resulting symbol’s name.</p>
<h4>Rationale:</h4>
<p><code>generate-uninterned-symbol</code> duplicates the behavior of
<code>gensym</code> in Scheme implementations that provide uninterned
symbols (and in which <code>gensym</code> returns an uninterned
symbol).
<h4>Implementation (informational discussion):</h4>
<p>Following <a href="#common-lisp">Common Lisp</a>, many
<code>gensym</code> implementations use a non-negative integer
counter, a representation of the value of which is appended to
the name of the returned symbol. Each call to <code>gensym</code>
increments this counter. Other implementations use Universally
Unique Identifiers (UUIDs) (ITU-T Rec. X.667, ISO/IEC
9834-8:2014) as names.</p>
<h2 id="prior-art">Prior art</h2>
<table>
<caption>Summary of existing support for uninterned symbols,
<code>gensym</code>, and related forms in Scheme.</caption>
<thead>
<tr>
<th>Implementation</th>
<th>Uninterned symbols</th>
<th><code>gensym</code></th>
<th><code>gensym</code> prefix argument</th>
<th><code>gensym</code> interned</th>
<th><code>string->uninterned-symbol</code></th>
</tr>
</thead>
<tbody>
<tr>
<td>Bigloo</td>
<td class="no">No</td>
<td class="yes">Yes</td>
<td>String or symbol</td>
<td>Yes</td>
<td class="no">No</td>
</tr>
<tr>
<td>ChezScheme</td>
<td class="yes">Yes</td>
<td class="yes">Yes</td>
<td>String</td>
<td>Yes</td>
<td class="yes">Yes</td>
</tr>
<tr>
<td>CHICKEN</td>
<td class="yes">Yes</td>
<td class="yes">Yes</td>
<td>String or symbol</td>
<td>No</td>
<td class="yes">Yes</td>
</tr>
<tr>
<td>Cyclone</td>
<td class="no">No</td>
<td class="yes">Yes</td>
<td>String or symbol</td>
<td>Yes</td>
<td class="no">No</td>
</tr>
<tr>
<td>Gambit</td>
<td class="yes">Yes</td>
<td class="yes">Yes</td>
<td>Symbol</td>
<td>No</td>
<td class="yes">Yes</td>
</tr>
<tr>
<td>Gauche</td>
<td class="yes">Yes</td>
<td class="yes">Yes</td>
<td>String</td>
<td>No</td>
<td class="yes">Yes</td>
</tr>
<tr>
<td>Guile</td>
<td class="yes">Yes</td>
<td class="yes">Yes</td>
<td>String</td>
<td>Yes</td>
<td class="no">No*</td>
<tr>
<td>Ikarus</td>
<td class="no">No</td>
<td class="yes">Yes</td>
<td>String</td>
<td>Yes</td>
<td class="no">No</td>
</tr>
<tr>
<td>Kawa</td>
<td class="no">No</td>
<td class="no">No†</td>
<td>n/a</td>
<td>n/a</td>
<td class="no">No</td>
</tr>
<tr>
<td>Larceny</td>
<td class="yes">Yes</td>
<td class="yes">Yes</td>
<td>String</td>
<td>No</td>
<td class="no">No</td>
</tr>
<tr>
<td>Loko</td>
<td class="yes">Yes</td>
<td class="yes">Yes</td>
<td>None</td>
<td>No</td>
<td class="no">No</td>
</tr>
<tr>
<td>MIT/GNU Scheme</td>
<td class="yes">Yes</td>
<td class="no">No‡</td>
<td>n/a</td>
<td>n/a</td>
<td class="yes">Yes</td>
<tr>
<td>Racket</td>
<td class="yes">Yes</td>
<td class="yes">Yes</td>
<td>String or symbol</td>
<td>No</td>
<td class="yes">Yes</td>
</tr>
<tr>
<td>STklos</td>
<td class="yes">Yes</td>
<td class="yes">Yes</td>
<td>String or symbol</td>
<td>No</td>
<td class="yes">Yes</td>
</tr>
<tr>
<td>TinySCHEME</td>
<td class="no">No</td>
<td class="yes">Yes</td>
<td>None</td>
<td>Yes</td>
<td class="no">No</td>
</tr>
</tbody>
</table>
<p>*: Guile’s <code>make-symbol</code> procedure is identical to
<code>string->uninterned-symbol</code>.</p>
<p>†: Kawa includes a
<a href="https://www.gnu.org/software/kawa/Macros.html#idm45230724012608">
zero-argument <code>gentemp</code> procedure</a> that returns an interned
symbol.</p>
<p>‡: MIT/GNU Scheme provides
<a href="https://web.mit.edu/scheme/scheme_v9.2/doc/mit-scheme-ref/Symbols.html">generate-uninterned-symbol</a>,
an extended version of
<a href="#generate-uninterned-symbol">the procedure described
above.</a></p>
<h3 id="chezscheme">ChezScheme</h3>
<p>Chez provides interned “gensyms” which are distinguished by the
<code>gensym?</code> predicate. Chez’s gensyms have “pretty” and
“unique” names. The former are created immediately and the latter are
generated lazily using an internal prefix and counter, which are
accessible through parameter objects.
<code>gensym->unique-string</code> returns the unique name of a
gensym.</p>
<p>Chez’s <code>gensym</code> procedure takes optional
<var>pretty-name</var> and <var>unique-name</var> string arguments.
The latter argument allows the unique name of one gensym to be given
to another; thus, distinct gensyms that are equal in the sense of
<code>symbol=?</code> can be created in Chez.</p>
<p>The lexical syntax <code>#:</code><var class="syn">name</var> is
used to write gensyms; <var class="syn">name</var> is the pretty
name of the resulting gensym.</p>
<p>See section 7.9 of the <cite>ChezScheme User’s Guide</cite> for
further details.</p>
<h3 id="common-lisp">Common Lisp</h3>
<p>Uninterned symbols are used extensively in Common Lisp macro
programming. The Common Lisp
HyperSpec specifies
<a href="http://clhs.lisp.se/Body/f_mk_sym.htm#make-symbol"><code>MAKE-SYMBOL</code></a>
(analogous to <code>string->uninterned-symbol</code>) and
<a href="http://clhs.lisp.se/Body/f_gensym.htm#gensym"><code>GENSYM</code></a>
procedures that construct uninterned symbols. The names of the
symbols returned by <code>GENSYM</code> usually include the value
of <code>*GENSYM-COUNTER*</code>, a non-negative integer variable,
which is incremented by each call to <code>GENSYM</code>. (This
behavior can be circumvented by providing an explicit suffix to
<code>GENSYM</code>. See the CLHS for details.)</p>
<p>Since Common Lisp does not have Scheme’s distinction between
symbols and identifiers, the implications of internedness are
much broader in Common Lisp. For example, objects named by
uninterned symbols are, in effect, private to the package in
which they were defined. A full discussion of uninterned symbols
in Common Lisp is beyond the scope of this SRFI.</p>
<h2 id="implementation">Implementation</h2>
<p>A portable implementation of uninterned symbols is
impossible. In Scheme implementations that provide them (see the
table above), uninterned symbols
and <code>string->uninterned-symbol</code> are generally
compatible with this SRFI. A <a href="srfi-258.sls">sample
implementation for ChezScheme</a> and
a <a href="tests/test-portable.scm">portable test suite</a> are
included in this SRFI’s repository.</p>
<p>Many Scheme implementations maintain a table of symbols, indexed
by name. When a symbol is created, its name is looked up in this
table; if a symbol with that name exists, it is returned;
otherwise, a fresh symbol is added to the table. In implementations
that use this or a similar strategy, uninterned symbols may be
provided by simply skipping the table-lookup and insertion
steps.</p>
<h2 id="acknowledgments">Acknowledgments</h2>
<p>Thanks to Daphne Preston-Kendal for pushing for this SRFI and to
Marc Nieper-Wißkirchen for challenging almost everything in it.</p>
<p>Thanks to those who provided reviews and commentary via the SRFI
mailing list or the <span class="channel">#scheme</span> IRC
channel.</p>
<h2 id="references">References</h2>
<p>Alex Shinn, John Cowan, & Arthur A. Gleckler, eds.,
<cite>Revised<sup>7</sup> Report on the Algorithmic Language Scheme</cite>
(R7RS Small) (2013). Available <a href="https://small.r7rs.org/">on the
Web</a>.</p>
<p>Kent M. Pitman, et al., eds., <cite>Common Lisp HyperSpec</cite>™
(CLHS) (1994). Available <a href="http://clhs.lisp.se/">on the
Web</a>.</p>
<p>Cisco Systems, Inc., <cite>Chez Scheme User’s Guide</cite> (2015).
Available <a href="https://cisco.github.io/ChezScheme/csug9.5/">on the
Web</a>.</p>
<p>S. Bradner, <cite>Key words for use in RFCs to Indicate Requirement
Levels</cite>. (RFC 2119) (1997).
https://datatracker.ietf.org/doc/html/rfc2119</p>
<h2 id="copyright">Copyright</h2>
<p>© 2025 Wolfgang Corcoran-Mathe</p>
<p>
Permission is hereby granted, free of charge, to any person
obtaining a copy of this software and associated documentation files
(the "Software"), to deal in the Software without restriction,
including without limitation the rights to use, copy, modify, merge,
publish, distribute, sublicense, and/or sell copies of the Software,
and to permit persons to whom the Software is furnished to do so,
subject to the following conditions:</p>
<p>
The above copyright notice and this permission notice (including the
next paragraph) shall be included in all copies or substantial
portions of the Software.</p>
<p>
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.</p>
<hr>
<address>Editor: <a href="mailto:srfi-editors+at+srfi+dot+schemers+dot+org">Arthur A. Gleckler</a></address>
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