165 lines
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165 lines
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<title>
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Revised^5 Report on the Algorithmic Language Scheme
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</title>
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<link rel="stylesheet" type="text/css" href="r5rs-Z-C.css" title=default>
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<body>
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<p><div class=navigation>[Go to <span><a href="r5rs.html">first</a>, <a href="r5rs-Z-H-4.html">previous</a></span><span>, <a href="r5rs-Z-H-6.html">next</a></span> page<span>; </span><span><a href="r5rs-Z-H-2.html#%_toc_start">contents</a></span><span><span>; </span><a href="r5rs-Z-H-15.html#%_index_start">index</a></span>]</div><p>
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<a name="%_chap_2"></a>
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<h1 class=chapter>
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<div class=chapterheading><a href="r5rs-Z-H-2.html#%_toc_%_chap_2">Chapter 2</a></div><p>
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<a href="r5rs-Z-H-2.html#%_toc_%_chap_2">Lexical conventions</a></h1><p>
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This section gives an informal account of some of the lexical
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conventions used in writing Scheme programs. For a formal syntax of
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Scheme, see section <a href="r5rs-Z-H-10.html#%_sec_7.1">7.1</a>.<p>
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Upper and lower case forms of a letter are never distinguished
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except within character and string constants. For example, <tt>Foo</tt> is
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the same identifier as <tt>FOO</tt>, and <tt>#x1AB</tt> is the same number as
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<tt>#X1ab</tt>.<p>
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<a name="%_sec_2.1"></a>
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<h2><a href="r5rs-Z-H-2.html#%_toc_%_sec_2.1">2.1 Identifiers</a></h2><p>
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<p>
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Most identifiers<a name="%_idx_14"></a> allowed by other programming
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languages are also acceptable to Scheme. The precise rules for forming
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identifiers vary among implementations of Scheme, but in all
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implementations a sequence of letters, digits, and ``extended alphabetic
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characters'' that begins with a character that cannot begin a number is
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an identifier. In addition, <tt>+</tt>, <tt>-</tt>, and <tt>...</tt> are identifiers.
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Here are some examples of identifiers:<p>
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<tt><p>lambda q<br>
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list->vector soup<br>
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+ V17a<br>
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<=? a34kTMNs<br>
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the-word-recursion-has-many-meanings<p></tt><p>
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Extended alphabetic characters may be used within identifiers as if
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they were letters. The following are extended alphabetic characters:<p>
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<tt><p>! $ % <code class=verbatim>&</code> * + - . / : < = > ? @ <code class=verbatim>^</code> <code class=verbatim>_</code> <code class=verbatim>~</code> <p></tt><p>
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See section <a href="r5rs-Z-H-10.html#%_sec_7.1.1">7.1.1</a> for a formal syntax of identifiers.<p>
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Identifiers have two uses within Scheme programs:
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<p><ul>
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<li>Any identifier may be used as a variable<a name="%_idx_16"></a>
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or as a syntactic keyword<a name="%_idx_18"></a>
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(see sections <a href="r5rs-Z-H-6.html#%_sec_3.1">3.1</a> and <a href="r5rs-Z-H-7.html#%_sec_4.3">4.3</a>).<p>
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<li>When an identifier appears as a literal or within a literal
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(see section <a href="r5rs-Z-H-7.html#%_sec_4.1.2">4.1.2</a>), it is being used to denote a <em>symbol</em>
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(see section <a href="r5rs-Z-H-9.html#%_sec_6.3.3">6.3.3</a>).<p>
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</ul><p><p>
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<p>
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<a name="%_sec_2.2"></a>
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<h2><a href="r5rs-Z-H-2.html#%_toc_%_sec_2.2">2.2 Whitespace and comments</a></h2><p>
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<a name="%_idx_20"></a><em>Whitespace</em> characters are spaces and newlines.
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(Implementations typically provide additional whitespace characters such
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as tab or page break.) Whitespace is used for improved readability and
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as necessary to separate tokens from each other, a token being an
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indivisible lexical unit such as an identifier or number, but is
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otherwise insignificant. Whitespace may occur between any two tokens,
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but not within a token. Whitespace may also occur inside a string,
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where it is significant.<p>
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A semicolon (<tt>;</tt>) indicates the start of a
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comment.<a name="%_idx_22"></a><a name="%_idx_24"></a> The comment continues to the
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end of the line on which the semicolon appears. Comments are invisible
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to Scheme, but the end of the line is visible as whitespace. This
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prevents a comment from appearing in the middle of an identifier or
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number.<p>
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<tt><p>;;; The FACT procedure computes the factorial<br>
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;;; of a non-negative integer.<br>
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(define fact<br>
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(lambda (n)<br>
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(if (= n 0)<br>
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1 ;Base case: return 1<br>
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(* n (fact (- n 1))))))<p></tt><p>
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<a name="%_sec_2.3"></a>
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<h2><a href="r5rs-Z-H-2.html#%_toc_%_sec_2.3">2.3 Other notations</a></h2><p>
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<p>
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For a description of the notations used for numbers, see
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section <a href="r5rs-Z-H-9.html#%_sec_6.2">6.2</a>.<p>
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<p><ul><p>
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<li><b><tt>. + -</tt></b>
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These are used in numbers, and may also occur anywhere in an identifier
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except as the first character. A delimited plus or minus sign by itself
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is also an identifier.
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A delimited period (not occurring within a number or identifier) is used
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in the notation for pairs (section <a href="r5rs-Z-H-9.html#%_sec_6.3.2">6.3.2</a>), and to indicate a
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rest-parameter in a formal parameter list (section <a href="r5rs-Z-H-7.html#%_sec_4.1.4">4.1.4</a>).
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A delimited sequence of three successive periods is also an identifier.<p>
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<li><b><tt>( )</tt></b>
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Parentheses are used for grouping and to notate lists
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(section <a href="r5rs-Z-H-9.html#%_sec_6.3.2">6.3.2</a>).<p>
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<li><b><tt>'</tt></b>
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The single quote character is used to indicate literal data (section <a href="r5rs-Z-H-7.html#%_sec_4.1.2">4.1.2</a>).<p>
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<li><b><tt>`</tt></b>
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The backquote character is used to indicate almost-constant
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data (section <a href="r5rs-Z-H-7.html#%_sec_4.2.6">4.2.6</a>).<p>
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<li><b><tt>, ,@</tt></b>
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The character comma and the sequence comma at-sign are used in conjunction
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with backquote (section <a href="r5rs-Z-H-7.html#%_sec_4.2.6">4.2.6</a>).<p>
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<li><b><tt>"</tt></b>
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The double quote character is used to delimit strings (section <a href="r5rs-Z-H-9.html#%_sec_6.3.5">6.3.5</a>).<p>
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<li><b><tt>\</tt></b>
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Backslash is used in the syntax for character constants
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(section <a href="r5rs-Z-H-9.html#%_sec_6.3.4">6.3.4</a>) and as an escape character within string
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constants (section <a href="r5rs-Z-H-9.html#%_sec_6.3.5">6.3.5</a>).<p>
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<li><b><code class=verbatim>[ ] { } |</code></b>
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Left and right square brackets and curly braces and vertical bar
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are reserved for possible future extensions to the language.<p>
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<li><b><tt>#</tt></b> Sharp sign is used for a variety of purposes depending on
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the character that immediately follows it:<p>
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<li><b><tt>#t</tt> <tt>#f</tt></b>
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These are the boolean constants (section <a href="r5rs-Z-H-9.html#%_sec_6.3.1">6.3.1</a>).<p>
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<li><b><tt>#</tt><tt>\</tt></b>
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This introduces a character constant (section <a href="r5rs-Z-H-9.html#%_sec_6.3.4">6.3.4</a>).<p>
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<li><b><tt>#</tt><tt>(</tt></b>
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This introduces a vector constant (section <a href="r5rs-Z-H-9.html#%_sec_6.3.6">6.3.6</a>). Vector constants
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are terminated by <tt>)</tt> .<p>
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<li><b><tt>#e #i #b #o #d #x</tt></b>
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These are used in the notation for numbers (section <a href="r5rs-Z-H-9.html#%_sec_6.2.4">6.2.4</a>).<p>
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</ul><p>
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<p>
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<p><div class=navigation>[Go to <span><a href="r5rs.html">first</a>, <a href="r5rs-Z-H-4.html">previous</a></span><span>, <a href="r5rs-Z-H-6.html">next</a></span> page<span>; </span><span><a href="r5rs-Z-H-2.html#%_toc_start">contents</a></span><span><span>; </span><a href="r5rs-Z-H-15.html#%_index_start">index</a></span>]</div><p>
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