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277 lines
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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<title>Symbol Primitives (Guile Reference Manual)</title>
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Symbols.html" rel="up" title="Symbols">
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<div class="subsubsection-level-extent" id="Symbol-Primitives">
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<div class="nav-panel">
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<p>
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Next: <a href="Symbol-Read-Syntax.html" accesskey="n" rel="next">Extended Read Syntax for Symbols</a>, Previous: <a href="Symbol-Variables.html" accesskey="p" rel="prev">Symbols as Denoting Variables</a>, Up: <a href="Symbols.html" accesskey="u" rel="up">Symbols</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<h4 class="subsubsection" id="Operations-Related-to-Symbols"><span>6.6.6.4 Operations Related to Symbols<a class="copiable-link" href="#Operations-Related-to-Symbols"> ¶</a></span></h4>
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<p>Given any Scheme value, you can determine whether it is a symbol using
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the <code class="code">symbol?</code> primitive:
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</p>
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<a class="index-entry-id" id="index-symbol_003f-3"></a>
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<dl class="first-deffn">
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<dt class="deffn" id="index-symbol_003f"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">symbol?</strong> <var class="def-var-arguments">obj</var><a class="copiable-link" href="#index-symbol_003f"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fsymbol_005fp"><span class="category-def">C Function: </span><span><strong class="def-name">scm_symbol_p</strong> <var class="def-var-arguments">(obj)</var><a class="copiable-link" href="#index-scm_005fsymbol_005fp"> ¶</a></span></dt>
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<dd><p>Return <code class="code">#t</code> if <var class="var">obj</var> is a symbol, otherwise return
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<code class="code">#f</code>.
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</p></dd></dl>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-scm_005fis_005fsymbol"><span class="category-def">C Function: </span><span><code class="def-type">int</code> <strong class="def-name">scm_is_symbol</strong> <code class="def-code-arguments">(SCM val)</code><a class="copiable-link" href="#index-scm_005fis_005fsymbol"> ¶</a></span></dt>
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<dd><p>Equivalent to <code class="code">scm_is_true (scm_symbol_p (val))</code>.
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</p></dd></dl>
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<p>Once you know that you have a symbol, you can obtain its name as a
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string by calling <code class="code">symbol->string</code>. Note that Guile differs by
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default from R5RS on the details of <code class="code">symbol->string</code> as regards
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case-sensitivity:
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</p>
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<a class="index-entry-id" id="index-symbol_002d_003estring-2"></a>
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<dl class="first-deffn">
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<dt class="deffn" id="index-symbol_002d_003estring"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">symbol->string</strong> <var class="def-var-arguments">s</var><a class="copiable-link" href="#index-symbol_002d_003estring"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fsymbol_005fto_005fstring"><span class="category-def">C Function: </span><span><strong class="def-name">scm_symbol_to_string</strong> <var class="def-var-arguments">(s)</var><a class="copiable-link" href="#index-scm_005fsymbol_005fto_005fstring"> ¶</a></span></dt>
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<dd><p>Return the name of symbol <var class="var">s</var> as a string. By default, Guile reads
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symbols case-sensitively, so the string returned will have the same case
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variation as the sequence of characters that caused <var class="var">s</var> to be
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created.
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</p>
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<p>If Guile is set to read symbols case-insensitively (as specified by
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R5RS), and <var class="var">s</var> comes into being as part of a literal expression
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(see <a data-manual="r5rs" href="../r5rs_html/Literal-expressions.html#Literal-expressions">Literal expressions</a> in <cite class="cite">The Revised^5 Report on Scheme</cite>) or
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by a call to the <code class="code">read</code> or <code class="code">string-ci->symbol</code> procedures,
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Guile converts any alphabetic characters in the symbol’s name to
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lower case before creating the symbol object, so the string returned
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here will be in lower case.
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</p>
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<p>If <var class="var">s</var> was created by <code class="code">string->symbol</code>, the case of characters
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in the string returned will be the same as that in the string that was
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passed to <code class="code">string->symbol</code>, regardless of Guile’s case-sensitivity
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setting at the time <var class="var">s</var> was created.
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</p>
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<p>It is an error to apply mutation procedures like <code class="code">string-set!</code> to
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strings returned by this procedure.
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</p></dd></dl>
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<p>Most symbols are created by writing them literally in code. However it
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is also possible to create symbols programmatically using the following
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procedures:
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-symbol"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">symbol</strong> <var class="def-var-arguments">char…</var><a class="copiable-link" href="#index-symbol"> ¶</a></span></dt>
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<dd><a class="index-entry-id" id="index-symbol-1"></a>
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<p>Return a newly allocated symbol made from the given character arguments.
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</p>
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<div class="example">
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<pre class="example-preformatted">(symbol #\x #\y #\z) ⇒ xyz
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</pre></div>
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</dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-list_002d_003esymbol"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">list->symbol</strong> <var class="def-var-arguments">lst</var><a class="copiable-link" href="#index-list_002d_003esymbol"> ¶</a></span></dt>
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<dd><a class="index-entry-id" id="index-list_002d_003esymbol-1"></a>
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<p>Return a newly allocated symbol made from a list of characters.
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</p>
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<div class="example">
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<pre class="example-preformatted">(list->symbol '(#\a #\b #\c)) ⇒ abc
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</pre></div>
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</dd></dl>
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<a class="index-entry-id" id="index-symbol_002dappend-1"></a>
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<dl class="first-deffn">
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<dt class="deffn" id="index-symbol_002dappend"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">symbol-append</strong> <var class="def-var-arguments">arg …</var><a class="copiable-link" href="#index-symbol_002dappend"> ¶</a></span></dt>
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<dd><p>Return a newly allocated symbol whose characters form the
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concatenation of the given symbols, <var class="var">arg</var> <small class="enddots">...</small>.
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</p>
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<div class="example">
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<pre class="example-preformatted">(let ((h 'hello))
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(symbol-append h 'world))
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⇒ helloworld
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</pre></div>
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</dd></dl>
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<a class="index-entry-id" id="index-string_002d_003esymbol-3"></a>
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<dl class="first-deffn">
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<dt class="deffn" id="index-string_002d_003esymbol"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">string->symbol</strong> <var class="def-var-arguments">string</var><a class="copiable-link" href="#index-string_002d_003esymbol"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fstring_005fto_005fsymbol"><span class="category-def">C Function: </span><span><strong class="def-name">scm_string_to_symbol</strong> <var class="def-var-arguments">(string)</var><a class="copiable-link" href="#index-scm_005fstring_005fto_005fsymbol"> ¶</a></span></dt>
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<dd><p>Return the symbol whose name is <var class="var">string</var>. This procedure can create
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symbols with names containing special characters or letters in the
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non-standard case, but it is usually a bad idea to create such symbols
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because in some implementations of Scheme they cannot be read as
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themselves.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-string_002dci_002d_003esymbol"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">string-ci->symbol</strong> <var class="def-var-arguments">str</var><a class="copiable-link" href="#index-string_002dci_002d_003esymbol"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fstring_005fci_005fto_005fsymbol"><span class="category-def">C Function: </span><span><strong class="def-name">scm_string_ci_to_symbol</strong> <var class="def-var-arguments">(str)</var><a class="copiable-link" href="#index-scm_005fstring_005fci_005fto_005fsymbol"> ¶</a></span></dt>
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<dd><p>Return the symbol whose name is <var class="var">str</var>. If Guile is currently
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reading symbols case-insensitively, <var class="var">str</var> is converted to lowercase
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before the returned symbol is looked up or created.
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</p></dd></dl>
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<p>The following examples illustrate Guile’s detailed behavior as regards
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the case-sensitivity of symbols:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(read-enable 'case-insensitive) ; R5RS compliant behavior
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(symbol->string 'flying-fish) ⇒ "flying-fish"
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(symbol->string 'Martin) ⇒ "martin"
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(symbol->string
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(string->symbol "Malvina")) ⇒ "Malvina"
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(eq? 'mISSISSIppi 'mississippi) ⇒ #t
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(string->symbol "mISSISSIppi") ⇒ mISSISSIppi
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(eq? 'bitBlt (string->symbol "bitBlt")) ⇒ #f
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(eq? 'LolliPop
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(string->symbol (symbol->string 'LolliPop))) ⇒ #t
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(string=? "K. Harper, M.D."
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(symbol->string
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(string->symbol "K. Harper, M.D."))) ⇒ #t
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(read-disable 'case-insensitive) ; Guile default behavior
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(symbol->string 'flying-fish) ⇒ "flying-fish"
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(symbol->string 'Martin) ⇒ "Martin"
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(symbol->string
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(string->symbol "Malvina")) ⇒ "Malvina"
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(eq? 'mISSISSIppi 'mississippi) ⇒ #f
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(string->symbol "mISSISSIppi") ⇒ mISSISSIppi
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(eq? 'bitBlt (string->symbol "bitBlt")) ⇒ #t
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(eq? 'LolliPop
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(string->symbol (symbol->string 'LolliPop))) ⇒ #t
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(string=? "K. Harper, M.D."
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(symbol->string
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(string->symbol "K. Harper, M.D."))) ⇒ #t
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</pre></div>
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<p>From C, there are lower level functions that construct a Scheme symbol
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from a C string in the current locale encoding.
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</p>
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<p>When you want to do more from C, you should convert between symbols
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and strings using <code class="code">scm_symbol_to_string</code> and
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<code class="code">scm_string_to_symbol</code> and work with the strings.
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</p>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-scm_005ffrom_005flatin1_005fsymbol"><span class="category-def">C Function: </span><span><code class="def-type">SCM</code> <strong class="def-name">scm_from_latin1_symbol</strong> <code class="def-code-arguments">(const char *name)</code><a class="copiable-link" href="#index-scm_005ffrom_005flatin1_005fsymbol"> ¶</a></span></dt>
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<dt class="deftypefnx def-cmd-deftypefn" id="index-scm_005ffrom_005futf8_005fsymbol"><span class="category-def">C Function: </span><span><code class="def-type">SCM</code> <strong class="def-name">scm_from_utf8_symbol</strong> <code class="def-code-arguments">(const char *name)</code><a class="copiable-link" href="#index-scm_005ffrom_005futf8_005fsymbol"> ¶</a></span></dt>
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<dd><p>Construct and return a Scheme symbol whose name is specified by the
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null-terminated C string <var class="var">name</var>. These are appropriate when
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the C string is hard-coded in the source code.
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</p></dd></dl>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-scm_005ffrom_005flocale_005fsymbol"><span class="category-def">C Function: </span><span><code class="def-type">SCM</code> <strong class="def-name">scm_from_locale_symbol</strong> <code class="def-code-arguments">(const char *name)</code><a class="copiable-link" href="#index-scm_005ffrom_005flocale_005fsymbol"> ¶</a></span></dt>
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<dt class="deftypefnx def-cmd-deftypefn" id="index-scm_005ffrom_005flocale_005fsymboln"><span class="category-def">C Function: </span><span><code class="def-type">SCM</code> <strong class="def-name">scm_from_locale_symboln</strong> <code class="def-code-arguments">(const char *name, size_t len)</code><a class="copiable-link" href="#index-scm_005ffrom_005flocale_005fsymboln"> ¶</a></span></dt>
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<dd><p>Construct and return a Scheme symbol whose name is specified by
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<var class="var">name</var>. For <code class="code">scm_from_locale_symbol</code>, <var class="var">name</var> must be null
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terminated; for <code class="code">scm_from_locale_symboln</code> the length of <var class="var">name</var> is
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specified explicitly by <var class="var">len</var>.
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</p>
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<p>Note that these functions should <em class="emph">not</em> be used when <var class="var">name</var> is a
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C string constant, because there is no guarantee that the current locale
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will match that of the execution character set, used for string and
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character constants. Most modern C compilers use UTF-8 by default, so
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in such cases we recommend <code class="code">scm_from_utf8_symbol</code>.
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</p></dd></dl>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-scm_005ftake_005flocale_005fsymbol"><span class="category-def">C Function: </span><span><code class="def-type">SCM</code> <strong class="def-name">scm_take_locale_symbol</strong> <code class="def-code-arguments">(char *str)</code><a class="copiable-link" href="#index-scm_005ftake_005flocale_005fsymbol"> ¶</a></span></dt>
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<dt class="deftypefnx def-cmd-deftypefn" id="index-scm_005ftake_005flocale_005fsymboln"><span class="category-def">C Function: </span><span><code class="def-type">SCM</code> <strong class="def-name">scm_take_locale_symboln</strong> <code class="def-code-arguments">(char *str, size_t len)</code><a class="copiable-link" href="#index-scm_005ftake_005flocale_005fsymboln"> ¶</a></span></dt>
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<dd><p>Like <code class="code">scm_from_locale_symbol</code> and <code class="code">scm_from_locale_symboln</code>,
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respectively, but also frees <var class="var">str</var> with <code class="code">free</code> eventually.
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Thus, you can use this function when you would free <var class="var">str</var> anyway
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immediately after creating the Scheme string. In certain cases, Guile
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can then use <var class="var">str</var> directly as its internal representation.
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</p></dd></dl>
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<p>The size of a symbol can also be obtained from C:
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</p>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-scm_005fc_005fsymbol_005flength"><span class="category-def">C Function: </span><span><code class="def-type">size_t</code> <strong class="def-name">scm_c_symbol_length</strong> <code class="def-code-arguments">(SCM sym)</code><a class="copiable-link" href="#index-scm_005fc_005fsymbol_005flength"> ¶</a></span></dt>
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<dd><p>Return the number of characters in <var class="var">sym</var>.
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</p></dd></dl>
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<p>Finally, some applications, especially those that generate new Scheme
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code dynamically, need to generate symbols for use in the generated
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code. The <code class="code">gensym</code> primitive meets this need:
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-gensym"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">gensym</strong> <var class="def-var-arguments">[prefix]</var><a class="copiable-link" href="#index-gensym"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fgensym"><span class="category-def">C Function: </span><span><strong class="def-name">scm_gensym</strong> <var class="def-var-arguments">(prefix)</var><a class="copiable-link" href="#index-scm_005fgensym"> ¶</a></span></dt>
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<dd><p>Create a new symbol with a name constructed from a prefix and a counter
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value. The string <var class="var">prefix</var> can be specified as an optional
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argument. Default prefix is ‘<samp class="samp"> g<!-- /@w --></samp>’. The counter is increased by 1
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at each call. There is no provision for resetting the counter.
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</p></dd></dl>
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<p>The symbols generated by <code class="code">gensym</code> are <em class="emph">likely</em> to be unique,
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since their names begin with a space and it is only otherwise possible
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to generate such symbols if a programmer goes out of their way to do
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so. Uniqueness can be guaranteed by instead using uninterned symbols
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(see <a class="pxref" href="Symbol-Uninterned.html">Uninterned Symbols</a>), though they can’t be usefully written out
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and read back in.
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</p>
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</div>
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<hr>
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<div class="nav-panel">
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<p>
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Next: <a href="Symbol-Read-Syntax.html">Extended Read Syntax for Symbols</a>, Previous: <a href="Symbol-Variables.html">Symbols as Denoting Variables</a>, Up: <a href="Symbols.html">Symbols</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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