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<h2 class="anchor anchorWithStickyNavbar_LWe7" id="2311-dynamic-control-of-the-lisp-reader">23.1.1 Dynamic Control of the Lisp Reader<a href="cd-b-reader-concepts.html#2311-dynamic-control-of-the-lisp-reader" class="hash-link" aria-label="Direct link to 23.1.1 Dynamic Control of the Lisp Reader" title="Direct link to 23.1.1 Dynamic Control of the Lisp Reader">​</a></h2>
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<p>Various aspects of the <em>Lisp reader</em> can be controlled dynamically. See Section 2.1.1 (Readtables) and Section 2.1.2 (Variables that affect the Lisp Reader).</p>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="2312-effect-of-readtable-case-on-the-lisp-reader">23.1.2 Effect of Readtable Case on the Lisp Reader<a href="cd-b-reader-concepts.html#2312-effect-of-readtable-case-on-the-lisp-reader" class="hash-link" aria-label="Direct link to 23.1.2 Effect of Readtable Case on the Lisp Reader" title="Direct link to 23.1.2 Effect of Readtable Case on the Lisp Reader">​</a></h2>
<!-- -->
<p>The <span><i>readtable case</i></span> of the <span><i>current readtable</i></span> affects the <em>Lisp reader</em> in the following ways: <!-- -->:upcase</p>
<p>When the <span><i>readtable case</i></span> is <!-- -->:upcase<!-- -->, unescaped constituent <span><i>characters</i></span> are converted to <span><i>uppercase</i></span>, as specified in Section 2.2 (Reader Algorithm).</p>
<p>:downcase</p>
<p>When the <span><i>readtable case</i></span> is <!-- -->:downcase<!-- -->, unescaped constituent <span><i>characters</i></span> are converted to <span><i>lowercase</i></span>.</p>
<p>:preserve</p>
<p>When the <span><i>readtable case</i></span> is <!-- -->:preserve<!-- -->, the case of all <span><i>characters</i></span> remains unchanged.</p>
<p>:invert</p>
<p>When the <span><i>readtable case</i></span> is <!-- -->:invert<!-- -->, then if all of the unescaped letters in the extended token are of the same <span><i>case</i></span>, those (unescaped) letters are converted to the opposite <span><i>case</i></span>.</p>
<h3 class="anchor anchorWithStickyNavbar_LWe7" id="23121-examples-of-effect-of-readtable-case-on-the-lisp-reader">23.1.2.1 Examples of Effect of Readtable Case on the Lisp Reader<a href="cd-b-reader-concepts.html#23121-examples-of-effect-of-readtable-case-on-the-lisp-reader" class="hash-link" aria-label="Direct link to 23.1.2.1 Examples of Effect of Readtable Case on the Lisp Reader" title="Direct link to 23.1.2.1 Examples of Effect of Readtable Case on the Lisp Reader">​</a></h3>
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<div class="language-lisp codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-lisp codeBlock_bY9V thin-scrollbar" style="color:#393A34;background-color:#f6f8fa"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token punctuation" style="color:#393A34">(</span><span class="token defun keyword" style="color:#00009f">defun</span><span class="token defun"> </span><span class="token defun function" style="color:#d73a49">test-readtable-case-reading</span><span class="token defun"> </span><span class="token defun punctuation" style="color:#393A34">(</span><span class="token defun arguments"></span><span class="token defun punctuation" style="color:#393A34">)</span><span class="token plain"> </span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token keyword" style="color:#00009f">let</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">\*readtable\* </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">copy-readtable</span><span class="token plain"> </span><span class="token boolean" style="color:#36acaa">nil</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">format</span><span class="token plain"> </span><span class="token boolean" style="color:#36acaa">t</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">&quot;</span><span class="token string argument" style="color:#e3116c">READTABLE-CASE</span><span class="token string" style="color:#e3116c"> Input Symbol-name~ </span><br></span><span class="token-line" style="color:#393A34"><span class="token string" style="color:#e3116c">~%–––––––––––––––––-~ </span><br></span><span class="token-line" style="color:#393A34"><span class="token string" style="color:#e3116c">~%&quot;</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">dolist</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">readtable-case</span><span class="token plain"> ’</span><span class="token punctuation" style="color:#393A34">(</span><span class="token lisp-property property" style="color:#36acaa">:upcase</span><span class="token plain"> </span><span class="token lisp-property property" style="color:#36acaa">:downcase</span><span class="token plain"> </span><span class="token lisp-property property" style="color:#36acaa">:preserve</span><span class="token plain"> </span><span class="token lisp-property property" style="color:#36acaa">:invert</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">setf</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">readtable-case</span><span class="token plain"> \*readtable\*</span><span class="token punctuation" style="color:#393A34">)<
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="2313-argument-conventions-of-some-reader-functions">23.1.3 Argument Conventions of Some Reader Functions<a href="cd-b-reader-concepts.html#2313-argument-conventions-of-some-reader-functions" class="hash-link" aria-label="Direct link to 23.1.3 Argument Conventions of Some Reader Functions" title="Direct link to 23.1.3 Argument Conventions of Some Reader Functions">​</a></h2>
<!-- -->
<!-- -->
<h3 class="anchor anchorWithStickyNavbar_LWe7" id="23131-the-eof">23.1.3.1 The EOF<a href="cd-b-reader-concepts.html#23131-the-eof" class="hash-link" aria-label="Direct link to 23.1.3.1 The EOF" title="Direct link to 23.1.3.1 The EOF">​</a></h3>
<!-- -->
<p><em>Eof-error-p</em> in input function calls controls what happens if input is from a file (or any other input source that has a definite end) and the end of the file is reached. If <em>eof-error-p</em> is <span><i>true</i></span> (the default), an error of <span><i>type</i></span> <span><b>end-of-file</b></span> is signaled at end of file. If it is <span><i>false</i></span>, then no error is signaled, and instead the function returns <em>eof-value</em>.</p>
<p>Functions such as <span><b>read</b></span> that read the representation of an <span><i>object</i></span> rather than a single character always signals an error, regardless of <em>eof-error-p</em>, if the file ends in the middle of an object representation. For example, if a file does not contain enough right parentheses to balance the left parentheses in it, <span><b>read</b></span> signals an error. If a file ends in a <span><i>symbol</i></span> or a <span><i>number</i></span> immediately followed by end-of-file, <span><b>read</b></span> reads the <span><i>symbol</i></span> or <span><i>number</i></span> successfully and when called again will act according to <em>eof-error-p</em>. Similarly, the <span><i>function</i></span> <span><b>read-line</b></span> successfully reads the last line of a file even if that line is terminated by end-of-file rather than the newline character. Ignorable text, such as lines containing only <span><i>whitespace</i></span><sub>2</sub> or comments, are not considered to begin an <span><i>object</i></span>; if <span><b>read</b></span> begins to read an <span><i>expression</i></span> but sees only such ignorable text, it does not consider the file to end in the middle of an <span><i>object</i></span>. Thus an <em>eof-error-p</em> argument controls what happens when the file ends between <span><i>objects</i></span>.</p>
<h3 class="anchor anchorWithStickyNavbar_LWe7" id="23132-the-recursive">23.1.3.2 The RECURSIVE<a href="cd-b-reader-concepts.html#23132-the-recursive" class="hash-link" aria-label="Direct link to 23.1.3.2 The RECURSIVE" title="Direct link to 23.1.3.2 The RECURSIVE">​</a></h3>
<!-- -->
<!-- -->
<p>If <em>recursive-p</em> is supplied and not <span><b>nil</b></span>, it specifies that this function call is not an outermost call to <span><b>read</b></span> but an embedded call, typically from a <span><i>reader macro function</i></span>. It is important to distinguish such recursive calls for three reasons.</p>
<p>1. An outermost call establishes the context within which the #<em>n</em>= and #<em>n</em># syntax is scoped. Consider, for example, the expression</p>
<p>(cons ’#3=(p q r) ’(x y . #3#))</p>
<p>If the <em>single-quote reader macro</em> were defined in this way:</p>
<p>(set-macro-character #\’ ;incorrect</p>
<p>#’(lambda (stream char)</p>
<p>(declare (ignore char))</p>
<p>(list ’quote (read stream))))</p>
<p>then each call to the <em>single-quote reader macro function</em> would establish independent contexts for the scope of <span><b>read</b></span> information, including the scope of identifications between markers like “#3=” and “#3#”. However, for this expression, the scope was clearly intended to be determined by the outer set of parentheses, so such a definition would be incorrect. The correct way to define the <em>single-quote reader macro</em> uses <em>recursive-p</em>:</p>
<p>(set-macro-character #\’ ;correct</p>
<p>#’(lambda (stream char)</p>
<p>(declare (ignore char))</p>
<p>(list ’quote (read stream t nil t))))</p>
<p>2. A recursive call does not alter whether the reading process is to preserve <span><i>whitespace</i></span><sub>2</sub> or not (as determined by whether the outermost call was to <span><b>read</b></span> or <span><b>read-preserving-whitespace</b></span>). Suppose again that <span><i>single-quote</i></span> were to be defined as shown above in the incorrect definition. Then a call to <span><b>read-preserving-whitespace</b></span> that read the expression ’foo*⟨Space⟩* would fail to preserve the space character following the symbol foo because the <em>single-quote reader macro function</em> calls <span><b>read</b></span>, not <span><b>read-preserving-whitespace</b></span>, to read the following expression (in this case foo). The correct definition, which passes the value <span><i>true</i></span> for <em>recursive-p</em> to <span><b>read</b></span>, allows the outermost call to determine whether <span><i>whitespace</i></span><sub>2</sub> is preserved.</p>
<p>3. When end-of-file is encountered and the <em>eof-error-p</em> argument is not <span><b>nil</b></span>, the kind of error that is signaled may depend on the value of <em>recursive-p</em>. If <em>recursive-p</em> is <span><i>true</i></span>, then the end-of-file is deemed to have occurred within the middle of a printed representation; if <em>recursive-p</em> is <span><i>false</i></span>, then the end-of-file may be deemed to have occurred between <span><i>objects</i></span> rather than within the middle of one.</p></div><footer class="theme-doc-footer docusaurus-mt-lg"><div class="theme-doc-footer-edit-meta-row row"><div class="col"><a href="https://github.com/lisp-docs/cl-language-reference/tree/main/docs/chap-23/cd-b-reader-concepts.md" target="_blank" rel="noopener noreferrer" class="theme-edit-this-page"><svg fill="currentColor" height="20" width="20" viewBox="0 0 40 40" class="iconEdit_Z9Sw" aria-hidden="true"><g><path d="m34.5 11.7l-3 3.1-6.3-6.3 3.1-3q0.5-0.5 1.2-0.5t1.1 0.5l3.9 3.9q0.5 0.4 0.5 1.1t-0.5 1.2z m-29.5 17.1l18.4-18.5 6.3 6.3-18.4 18.4h-6.3v-6.2z"></path></g></svg>Edit this page</a></div><div class="col lastUpdated_vwxv"><span class="theme-last-updated">Last updated<!-- --> by <b>daninus14</b></span></div></div></footer></article><nav class="pagination-nav docusaurus-mt-lg" aria-label="Docs pages"><a class="pagination-nav__link pagination-nav__link--prev" href="../chap-22/intro.html"><div class="pagination-nav__sublabel">Previous</div><div class="pagination-nav__label">intro</div></a><a class="pagination-nav__link pagination-nav__link--next" href="../category/232-reader-dictionary.html"><div class="pagination-nav__sublabel">Next</div><div class="pagination-nav__label">23.2 Reader Dictionary</div></a></nav></div></div><div class="col col--3"><div class="tableOfContents_bqdL thin-scrollbar theme-doc-toc-desktop"><ul class="table-of-contents table-of-contents__left-border"><li><a href="cd-b-reader-concepts.html#2311-dynamic-control-of-the-lisp-reader" class="table-of-contents__link toc-highlight">23.1.1 Dynamic Control of the Lisp Reader</a></li><li><a href="cd-b-reader-concepts.html#2312-effect-of-readtable-case-on-the-lisp-reader" class="table-of-contents__link toc-highlight">23.1.2 Effect of Readtable Case on the Lisp Reader</a><ul><li><a href="cd-b-reader-concepts.html#23121-examples-of-effect-of-readtable-case-on-the-lisp-reader" class="table-of-contents__link toc-highlight">23.1.2.1 Examples of Effect of Readtable Case on the Lisp Reader</a></li></ul></li><li><a href="cd-b-reader-concepts.html#2313-argument-conventions-of-some-reader-functions" class="table-of-contents__link toc-highlight">23.1.3 Argument Conventions of Some Reader Functions</a><ul><li><a href="cd-b-reader-concepts.html#23131-the-eof" class="table-of-contents__link toc-highlight">23.1.3.1 The EOF</a></li><li><a href="cd-b-reader-concepts.html#23132-the-recursive" class="table-of-contents__link toc-highlight">23.1.3.2 The RECURSIVE</a></li></ul></li></ul></div></div></div></div></main></div></div></div><footer class="footer footer--dark"><div class="container container-fluid"><div class="row footer__links"><div class="col footer__col"><div class="footer__title">Docs</div><ul class="footer__items clean-list"><li class="footer__item"><a href="../../docs/tutorial.html" target="_blank" rel="noopener noreferrer" class="footer__link-item">Common Lisp Tutorial</a></li><li class="footer__item"><a href="../index.html" target="_blank" rel="noopener noreferrer" class="footer__link-item">Common Lisp Technical Reference</a></li><li class="footer__item"><a href="../../docs/help.html" target="_blank" rel="noopener noreferrer" class="footer__link-item">Help</a></li></ul></div><div class="col footer__col"><div class="footer__title">Community</div><ul class="footer__items clean-list"><li class="footer__item"><a href="https://stackoverflow.com/questions/tagged/common-lisp" target="_blank" rel="noopener noreferrer" class="footer__link-item">Stack Overflow<svg width="13.5" height="13.5" aria-hidden="true" viewBox="0 0 24 24" class="iconExternalLink_n
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