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<tr><td align="center" bgcolor="#8381de"><div class="skribetitle"><strong><big><big><big>2. R5Rs<br/>Scheme Revised(5) Report on the Algorithmic Language Scheme -- Lexical conventions</big></big></big></strong></div><center>
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>Lexical conventions</strong></font></th></tr>
<tr bgcolor="#ffffff"><td id="tc7580" align="center" colspan="1"><table cellspacing="1" cellpadding="1" width="100%" class="toc">
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<tr><td valign="top" align="left">2.1</td><td colspan="4" width="100%"><a href="r5rs-5.html#Identifiers">Identifiers</a></td></tr>
<tr><td valign="top" align="left">2.2</td><td colspan="4" width="100%"><a href="r5rs-5.html#Whitespace-and-comments">Whitespace and comments</a></td></tr>
<tr><td valign="top" align="left">2.3</td><td colspan="4" width="100%"><a href="r5rs-5.html#Other-notations">Other notations</a></td></tr>
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<tr><td valign="top" align="left"></td><td colspan="4" width="100%"><a href="r5rs-1.html#Summary">Summary</a></td></tr>
<tr><td valign="top" align="left"></td><td colspan="4" width="100%"><a href="r5rs-2.html#Introduction">Introduction</a></td></tr>
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<tr><td valign="top" align="left">1</td><td colspan="4" width="100%"><a href="r5rs-4.html#Overview-of-Scheme">Overview of Scheme</a></td></tr>
<tr><td valign="top" align="left">2</td><td colspan="4" width="100%"><a href="r5rs-5.html#Lexical-conventions">Lexical conventions</a></td></tr>
<tr><td valign="top" align="left">3</td><td colspan="4" width="100%"><a href="r5rs-6.html#Basic-concepts">Basic concepts</a></td></tr>
<tr><td valign="top" align="left">4</td><td colspan="4" width="100%"><a href="r5rs-7.html#Expressions">Expressions</a></td></tr>
<tr><td valign="top" align="left">5</td><td colspan="4" width="100%"><a href="r5rs-8.html#Program-structure">Program structure</a></td></tr>
<tr><td valign="top" align="left">6</td><td colspan="4" width="100%"><a href="r5rs-9.html#Standard-procedures">Standard procedures</a></td></tr>
<tr><td valign="top" align="left">7</td><td colspan="4" width="100%"><a href="r5rs-10.html#Formal-syntax-and-semantics">Formal syntax and semantics</a></td></tr>
<tr><td valign="top" align="left">8</td><td colspan="4" width="100%"><a href="r5rs-11.html#Concepts">Concepts</a></td></tr>
<tr><td valign="top" align="left">9</td><td colspan="4" width="100%"><a href="r5rs-12.html#Variables-and-Procedures">Variables and Procedures</a></td></tr>
<tr><td valign="top" align="left"></td><td colspan="4" width="100%"><a href="r5rs-13.html#Notes">Notes</a></td></tr>
<tr><td valign="top" align="left"></td><td colspan="4" width="100%"><a href="r5rs-14.html#Additional-material">Additional material</a></td></tr>
<tr><td valign="top" align="left"></td><td colspan="4" width="100%"><a href="r5rs-15.html#Example">Example</a></td></tr>
<tr><td valign="top" align="left"></td><td colspan="4" width="100%"><a href="r5rs-16.html#Bibliography">Bibliography</a></td></tr>
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<td align="left" valign="top" class="skribe-body"><div class="skribe-body">
<a name="Lexical-conventions" class="mark"></a>
This section gives an informal account of some of the lexical
conventions used in writing Scheme programs. For a formal syntax of
Scheme, see section <a href="r5rs-10.html#Formal-syntax" class="inbound">Formal syntax</a>.<br/><br/>Upper and lower case forms of a letter are never distinguished
except within character and string constants. For example, <samp id='samp1299'
>Foo</samp> is
the same identifier as <samp id='samp1300'
>FOO</samp>, and <tt id='tt1301'
>#x1AB</tt> is the same number as
<tt id='tt1302'
>#X1ab</tt>.<br/><br/><!-- Identifiers -->
<a name="Identifiers"></a>
<div class="section-atitle"><table width="100%"><tr><td bgcolor="#dedeff"><h3><font color="black">2.1 Identifiers</font>
</h3></td></tr></table>
</div><div class="section">
<a name="Identifiers" class="mark"></a>
Most identifiers allowed by other programming
<a name="g1305" class="mark"></a>languages are also acceptable to Scheme. The precise rules for forming
identifiers vary among implementations of Scheme, but in all
implementations a sequence of letters, digits, and ``extended alphabetic
characters'' that begins with a character that cannot begin a number is
an identifier. In addition, <code id='code1307'
>+</code>, <code id='code1308'
>-</code>, and <code id='code1309'
>...</code> are identifiers.
<a name="g1311" class="mark"></a><a name="g1314" class="mark"></a><a name="g1317" class="mark"></a>Here are some examples of identifiers:<br/><br/><center id='center1322'
><table cellspacing="0" class="color" cellpadding="0" width="95%"><tbody>
<tr><td bgcolor="#ffffcc"><pre class="prog" id='prog1320'
>lambda q
list->vector soup
+ V17a
<=? a34kTMNs
the-word-recursion-has-many-meanings
</pre>
</td></tr>
</tbody></table></center>
Extended alphabetic characters may be used within identifiers as if
they were letters. The following are extended alphabetic characters:<br/><br/><center id='center1326'
><table cellspacing="0" class="color" cellpadding="0" width="95%"><tbody>
<tr><td bgcolor="#ffffcc"><pre class="prog" id='prog1324'
>! $ % & * + - . / : < = > ? @ ^ _ ~
</pre>
</td></tr>
</tbody></table></center>
See section <a href="r5rs-10.html#Lexical-structure" class="inbound">Lexical structure</a> for a formal syntax of identifiers.<br/><br/>Identifiers have two uses within Scheme programs:<br/><br/><ul class="itemize" id='itemize1334'
><li>Any identifier may be used as a variable
or as a syntactic keyword
(see sections <a href="r5rs-6.html#Variables;-syntactic-keywords;-and-regions" class="inbound">Variables; syntactic keywords; and regions</a> and <a href="r5rs-7.html#Macros" class="inbound">Macros</a>).<br/><br/></li>
<li>When an identifier appears as a literal or within a literal
(see section <a href="r5rs-7.html#Literal-expressions" class="inbound">Literal expressions</a>), it is being used to denote a <em id='emph1331'
>symbol</em>
(see section <a href="r5rs-9.html#Symbols" class="inbound">Symbols</a>).<br/><br/></li>
</ul>
<a name="g1336" class="mark"></a><a name="g1339" class="mark"></a>
</div><br>
<!-- Whitespace and comments -->
<a name="Whitespace-and-comments"></a>
<div class="section-atitle"><table width="100%"><tr><td bgcolor="#dedeff"><h3><font color="black">2.2 Whitespace and comments</font>
</h3></td></tr></table>
</div><div class="section">
<a name="Whitespace-and-comments" class="mark"></a>
"Whitespace" characters are spaces and newlines.
<a name="g1342" class="mark"></a>(Implementations typically provide additional whitespace characters such
as tab or page break.) Whitespace is used for improved readability and
as necessary to separate tokens from each other, a token being an
indivisible lexical unit such as an identifier or number, but is
otherwise insignificant. Whitespace may occur between any two tokens,
but not within a token. Whitespace may also occur inside a string,
where it is significant.<br/><br/>A semicolon (<tt id='tt1345'
>;</tt>) indicates the start of a
comment. The comment continues to the
<a name="g1347" class="mark"></a><a name="g1350" class="mark"></a>end of the line on which the semicolon appears. Comments are invisible
to Scheme, but the end of the line is visible as whitespace. This
prevents a comment from appearing in the middle of an identifier or
number.<br/><br/><center id='center1358'
><table cellspacing="0" class="color" cellpadding="0" width="95%"><tbody>
<tr><td bgcolor="#ffffcc"><pre class="prog" id='prog1356'
><font color="#ffa600"><em id='it7594'
>;;; The FACT procedure computes the factorial</em></font>
<font color="#ffa600"><em id='it7596'
>;;; of a non-negative integer.</em></font>
(<font color="#6959cf"><strong id='bold7598'
>define</strong></font> <font color="#6959cf"><strong id='bold7600'
>fact</strong></font>
(<strong id='bold7602'
>lambda</strong> (n)
(<strong id='bold7603'
>if</strong> (= n 0)
1 ;Base case: return 1
(* n (fact (- n 1))))))
</pre>
</td></tr>
</tbody></table></center>
</div><br>
<!-- Other notations -->
<a name="Other-notations"></a>
<div class="section-atitle"><table width="100%"><tr><td bgcolor="#dedeff"><h3><font color="black">2.3 Other notations</font>
</h3></td></tr></table>
</div><div class="section">
<a name="Other-notations" class="mark"></a>
For a description of the notations used for numbers, see
section <a href="r5rs-9.html#Numbers" class="inbound">Numbers</a>.<br/><br/><dl class="description" id='description1428'
> <dt><tt id='tt1361'
><tt id='tt1360'
>.: + -</tt></tt></dt> <dt><tt id='tt1362'
>
</tt></dt><dd>These are used in numbers, and may also occur anywhere in an identifier
except as the first character. A delimited plus or minus sign by itself
is also an identifier.
A delimited period (not occurring within a number or identifier) is used
in the notation for pairs (section <a href="r5rs-9.html#Pairs-and-lists" class="inbound">Pairs and lists</a>), and to indicate a
rest-parameter in a formal parameter list (section <a href="r5rs-7.html#Procedures" class="inbound">Procedures</a>).
A delimited sequence of three successive periods is also an identifier.<br/><br/></dd>
<dt><tt id='tt1366'
><tt id='tt1365'
>( )</tt></tt></dt> <dt><tt id='tt1367'
>
</tt></dt><dd>Parentheses are used for grouping and to notate lists
(section <a href="r5rs-9.html#Pairs-and-lists" class="inbound">Pairs and lists</a>).<br/><br/></dd>
<dt><tt id='tt1371'
><tt id='tt1370'
>'</tt></tt></dt> <dt><tt id='tt1372'
>
</tt></dt><dd>The single quote character is used to indicate literal data (section <a href="r5rs-7.html#Literal-expressions" class="inbound">Literal expressions</a>).<br/><br/></dd>
<dt><tt id='tt1376'
><tt id='tt1375'
>`</tt></tt></dt> <dt><tt id='tt1377'
>
</tt></dt><dd>The backquote character is used to indicate almost-constant
data (section <a href="r5rs-7.html#Quasiquotation" class="inbound">Quasiquotation</a>).<br/><br/></dd>
<dt><tt id='tt1381'
><tt id='tt1380'
>, ,@</tt></tt></dt> <dt><tt id='tt1382'
>
</tt></dt><dd>The character comma and the sequence comma at-sign are used in conjunction
with backquote (section <a href="r5rs-7.html#Quasiquotation" class="inbound">Quasiquotation</a>).<br/><br/></dd>
<dt><tt id='tt1386'
><tt id='tt1385'
>"</tt></tt></dt> <dt><tt id='tt1387'
>
</tt></dt><dd>The double quote character is used to delimit strings (section <a href="r5rs-9.html#Strings" class="inbound">Strings</a>).<br/><br/></dd>
<dt><tt id='tt1390'
>\</tt></dt> <dt><tt id='tt1391'
>
</tt></dt><dd>Backslash is used in the syntax for character constants
(section <a href="r5rs-9.html#Characters" class="inbound">Characters</a>) and as an escape character within string
constants (section <a href="r5rs-9.html#Strings" class="inbound">Strings</a>).<br/><br/>
</dd>
<dt><tt id='tt1396'
><strong id='bold1395'
><tt id='tt1394'
>[ ] { } |</tt></strong></tt></dt> <dt><tt id='tt1397'
>
</tt></dt><dd>Left and right square brackets and curly braces and vertical bar
are reserved for possible future extensions to the language.<br/><br/></dd>
<dt><tt id='tt1400'
>#</tt></dt> <dt><tt id='tt1401'
>
</tt></dt><dd> Sharp sign is used for a variety of purposes depending on
the character that immediately follows it:<br/><br/></dd>
<dt><tt id='tt1405'
><tt id='tt1404'
>#t</tt></tt></dt> <dt><tt id='tt1406'
> </tt></dt> <dt><tt id='tt1408'
><tt id='tt1407'
>#f</tt></tt></dt> <dt><tt id='tt1409'
>
</tt></dt><dd>These are the boolean constants (section <a href="r5rs-9.html#Booleans" class="inbound">Booleans</a>).<br/><br/></dd>
<dt><tt id='tt1412'
>#\</tt></dt> <dt><tt id='tt1413'
>
</tt></dt><dd>This introduces a character constant (section <a href="r5rs-9.html#Characters" class="inbound">Characters</a>).<br/><br/></dd>
<dt><tt id='tt1416'
>#</tt></dt> <dt><tt id='tt1418'
><tt id='tt1417'
>(</tt></tt></dt> <dt><tt id='tt1419'
>
</tt></dt><dd>This introduces a vector constant (section <a href="r5rs-9.html#Vectors" class="inbound">Vectors</a>). Vector constants
are terminated by <tt id='tt1420'
>)</tt> .<br/><br/></dd>
<dt><tt id='tt1424'
><tt id='tt1423'
>#e #i #b #o #d #x #l</tt></tt></dt> <dt><tt id='tt1425'
>
</tt></dt><dd>These are used in the notation for numbers (section <a href="r5rs-9.html#Syntax-of-numerical-constants" class="inbound">Syntax of numerical constants</a>).<br/><br/></dd>
</dl>
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