r5rs-z-h-7.html

scheme 标准（r5rs）。Scheme是MIT发布的基于Lambda运算的语言

<variable>.  If this restriction is violated, then it is an error.  The
restriction is necessary because Scheme passes arguments by value rather than by
name.  In the most common uses of <tt>letrec</tt>, all the &lt;init&gt;s are
<tt>lambda</tt> expressions and the restriction is satisfied automatically.<p>


<p><p>

<a name="%_sec_4.2.3"></a>
<h3><a href="r5rs-Z-H-2.html#%_toc_%_sec_4.2.3">4.2.3&nbsp;&nbsp;Sequencing</a></h3><p>

<p><p><p>

<p><div align=left><u>library syntax:</u>&nbsp;&nbsp;<tt>(<a name="%_idx_136"></a>begin<i> &lt;expression<sub>1</sub>&gt; &lt;expression<sub>2</sub>&gt; <tt>...</tt></i>)</tt>&nbsp;</div>
<p>

The &lt;expression&gt;s are evaluated sequentially from left to right,
and the value(s) of the last &lt;expression&gt; is(are) returned.  This
expression type is used to sequence side effects such as input and
output.<p>

<tt><p>(define&nbsp;x&nbsp;0)<br>
<br>
(begin&nbsp;(set!&nbsp;x&nbsp;5)<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(+&nbsp;x&nbsp;1))&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;6<br>
<br>
(begin&nbsp;(display&nbsp;&quot;4&nbsp;plus&nbsp;1&nbsp;equals&nbsp;&quot;)<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(display&nbsp;(+&nbsp;4&nbsp;1)))&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;<i>unspecified</i><br>
&nbsp;<em>&nbsp;and&nbsp;prints</em>&nbsp;&nbsp;4&nbsp;plus&nbsp;1&nbsp;equals&nbsp;5<p></tt><p>

<p><p>

<a name="%_sec_4.2.4"></a>
<h3><a href="r5rs-Z-H-2.html#%_toc_%_sec_4.2.4">4.2.4&nbsp;&nbsp;Iteration</a></h3><p>


<div align=left><u>library syntax:</u>&nbsp;&nbsp;<tt>(do ((&lt;variable<sub>1</sub>&gt; &lt;init<sub>1</sub>&gt; &lt;step<sub>1</sub>&gt;)</tt>&nbsp;</div>

<a name="%_idx_138"></a><tt>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<tt>...</tt>)<br>
&nbsp;&nbsp;&nbsp;(&lt;test&gt;&nbsp;&lt;expression&gt;&nbsp;<tt>...</tt>)<br>
&nbsp;&lt;command&gt;&nbsp;<tt>...</tt>)</tt><p>

<tt>Do</tt> is an iteration construct.  It specifies a set of variables to
be bound, how they are to be initialized at the start, and how they are
to be updated on each iteration.  When a termination condition is met,
the loop exits after evaluating the &lt;expression&gt;s.<p>

<tt>Do</tt> expressions are evaluated as follows:
The &lt;init&gt; expressions are evaluated (in some unspecified order),
the &lt;variable&gt;s are bound to fresh locations, the results of the
&lt;init&gt; expressions are stored in the bindings of the
&lt;variable&gt;s, and then the iteration phase begins.<p>

Each iteration begins by evaluating &lt;test&gt;; if the result is
false (see section&nbsp;<a href="r5rs-Z-H-9.html#%_sec_6.3.1">6.3.1</a>), then the &lt;command&gt;
expressions are evaluated in order for effect, the &lt;step&gt;
expressions are evaluated in some unspecified order, the
&lt;variable&gt;s are bound to fresh locations, the results of the
&lt;step&gt;s are stored in the bindings of the
&lt;variable&gt;s, and the next iteration begins.<p>

If &lt;test&gt; evaluates to a true value, then the
&lt;expression&gt;s are evaluated from left to right and the value(s) of
the last &lt;expression&gt; is(are) returned.  If no &lt;expression&gt;s
are present, then the value of the <tt>do</tt> expression is unspecified.<p>

The region<a name="%_idx_140"></a> of the binding of a &lt;variable&gt;
consists of the entire <tt>do</tt> expression except for the &lt;init&gt;s.
It is an error for a &lt;variable&gt; to appear more than once in the
list of <tt>do</tt> variables.<p>

A &lt;step&gt; may be omitted, in which case the effect is the
same as if <tt>(&lt;variable&gt; &lt;init&gt; &lt;variable&gt;)</tt> had
been written instead of <tt>(&lt;variable&gt; &lt;init&gt;)</tt>.<p>

<tt><p>(do&nbsp;((vec&nbsp;(make-vector&nbsp;5))<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(i&nbsp;0&nbsp;(+&nbsp;i&nbsp;1)))<br>
&nbsp;&nbsp;&nbsp;&nbsp;((=&nbsp;i&nbsp;5)&nbsp;vec)<br>
&nbsp;&nbsp;(vector-set!&nbsp;vec&nbsp;i&nbsp;i))&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;#(0&nbsp;1&nbsp;2&nbsp;3&nbsp;4)<br>
<br>
(let&nbsp;((x&nbsp;'(1&nbsp;3&nbsp;5&nbsp;7&nbsp;9)))<br>
&nbsp;&nbsp;(do&nbsp;((x&nbsp;x&nbsp;(cdr&nbsp;x))<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(sum&nbsp;0&nbsp;(+&nbsp;sum&nbsp;(car&nbsp;x))))<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;((null?&nbsp;x)&nbsp;sum)))&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;25<p></tt><p>

<p>

<p><div align=left><u>library syntax:</u>&nbsp;&nbsp;<tt>(let<i> &lt;variable&gt; &lt;bindings&gt; &lt;body&gt;</i>)</tt>&nbsp;</div>
<p>


``Named <tt>let</tt>'' is a variant on the syntax of <tt>let</tt> which provides
a more general looping construct than <tt>do</tt> and may also be used to express
recursions.
It has the same syntax and semantics as ordinary <tt>let</tt>
except that &lt;variable&gt; is bound within &lt;body&gt; to a procedure
whose formal arguments are the bound variables and whose body is
&lt;body&gt;.  Thus the execution of &lt;body&gt; may be repeated by
invoking the procedure named by &lt;variable&gt;.<p>

<tt><p>(let&nbsp;loop&nbsp;((numbers&nbsp;'(3&nbsp;-2&nbsp;1&nbsp;6&nbsp;-5))<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(nonneg&nbsp;'())<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(neg&nbsp;'()))<br>
&nbsp;&nbsp;(cond&nbsp;((null?&nbsp;numbers)&nbsp;(list&nbsp;nonneg&nbsp;neg))<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;((&gt;=&nbsp;(car&nbsp;numbers)&nbsp;0)<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(loop&nbsp;(cdr&nbsp;numbers)<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(cons&nbsp;(car&nbsp;numbers)&nbsp;nonneg)<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;neg))<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;((&lt;&nbsp;(car&nbsp;numbers)&nbsp;0)<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(loop&nbsp;(cdr&nbsp;numbers)<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;nonneg<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(cons&nbsp;(car&nbsp;numbers)&nbsp;neg)))))&nbsp;&nbsp;&nbsp;<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;((6&nbsp;1&nbsp;3)&nbsp;(-5&nbsp;-2))<p></tt><p>

<p><p>

<a name="%_sec_4.2.5"></a>
<h3><a href="r5rs-Z-H-2.html#%_toc_%_sec_4.2.5">4.2.5&nbsp;&nbsp;Delayed evaluation</a></h3><p>

<p><p><p>

<p><div align=left><u>library syntax:</u>&nbsp;&nbsp;<tt>(<a name="%_idx_142"></a>delay<i> &lt;expression&gt;</i>)</tt>&nbsp;</div>
<p>

<p>

The <tt>delay</tt> construct is used together with the procedure <tt>force</tt> to
implement <a name="%_idx_144"></a><em>lazy evaluation</em> or <a name="%_idx_146"></a><em>call by need</em>.
<tt>(delay&nbsp;&lt;expression&gt;)</tt> returns an object called a
<a name="%_idx_148"></a><em>promise</em> which at some point in the future may be asked (by
the <tt>force</tt> procedure)  to evaluate
&lt;expression&gt;, and deliver the resulting value.
The effect of &lt;expression&gt; returning multiple values
is unspecified.<p>

See the description of <tt>force</tt> (section&nbsp;<a href="r5rs-Z-H-9.html#%_sec_6.4">6.4</a>) for a
more complete description of <tt>delay</tt>.<p>

<p><p>

<a name="%_sec_4.2.6"></a>
<h3><a href="r5rs-Z-H-2.html#%_toc_%_sec_4.2.6">4.2.6&nbsp;&nbsp;Quasiquotation</a></h3><p>

<p><p><p>

<p><div align=left><u>syntax:</u>&nbsp;&nbsp;<tt>(<a name="%_idx_150"></a>quasiquote<i> &lt;qq template&gt;</i>)</tt>&nbsp;</div>
 <div align=left><u>syntax:</u>&nbsp;&nbsp;<tt><tt>`</tt>&lt;qq template&gt;</tt>&nbsp;</div>
<p>

``Backquote'' or ``quasiquote''<a name="%_idx_152"></a> expressions are useful
for constructing a list or vector structure when most but not all of the
desired structure is known in advance.  If no
commas<a name="%_idx_154"></a> appear within the &lt;qq template&gt;, the result of
evaluating
<tt>`</tt>&lt;qq template&gt; is equivalent to the result of evaluating
<tt>'</tt>&lt;qq template&gt;.  If a comma<a name="%_idx_156"></a> appears within the
&lt;qq template&gt;, however, the expression following the comma is
evaluated (``unquoted'') and its result is inserted into the structure
instead of the comma and the expression.  If a comma appears followed
immediately by an at-sign (<tt>@</tt>),<a name="%_idx_158"></a> then the following
expression must evaluate to a list; the opening and closing parentheses
of the list are then ``stripped away'' and the elements of the list are
inserted in place of the comma at-sign expression sequence.  A comma
at-sign should only appear within a list or vector &lt;qq template&gt;.<p>


<tt><p>`(list&nbsp;,(+&nbsp;1&nbsp;2)&nbsp;4)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;(list&nbsp;3&nbsp;4)<br>
(let&nbsp;((name&nbsp;'a))&nbsp;`(list&nbsp;,name&nbsp;',name))&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;(list&nbsp;a&nbsp;(quote&nbsp;a))<br>
`(a&nbsp;,(+&nbsp;1&nbsp;2)&nbsp;,@(map&nbsp;abs&nbsp;'(4&nbsp;-5&nbsp;6))&nbsp;b)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;(a&nbsp;3&nbsp;4&nbsp;5&nbsp;6&nbsp;b)<br>
`((<tt>&nbsp;foo</tt>&nbsp;,(-&nbsp;10&nbsp;3))&nbsp;,@(cdr&nbsp;'(c))&nbsp;.&nbsp;,(car&nbsp;'(cons)))&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;((foo&nbsp;7)&nbsp;.&nbsp;cons)<br>
`#(10&nbsp;5&nbsp;,(sqrt&nbsp;4)&nbsp;,@(map&nbsp;sqrt&nbsp;'(16&nbsp;9))&nbsp;8)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;#(10&nbsp;5&nbsp;2&nbsp;4&nbsp;3&nbsp;8)<p></tt><p>

Quasiquote forms may be nested.  Substitutions are made only for
unquoted components appearing at the same nesting level
as the outermost backquote.  The nesting level increases by one inside
each successive quasiquotation, and decreases by one inside each
unquotation.<p>

<tt><p>`(a&nbsp;`(b&nbsp;,(+&nbsp;1&nbsp;2)&nbsp;,(foo&nbsp;,(+&nbsp;1&nbsp;3)&nbsp;d)&nbsp;e)&nbsp;f)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;(a&nbsp;`(b&nbsp;,(+&nbsp;1&nbsp;2)&nbsp;,(foo&nbsp;4&nbsp;d)&nbsp;e)&nbsp;f)<br>
(let&nbsp;((name1&nbsp;'x)<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(name2&nbsp;'y))<br>
&nbsp;&nbsp;`(a&nbsp;`(b&nbsp;,,name1&nbsp;,',name2&nbsp;d)&nbsp;e))&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;(a&nbsp;`(b&nbsp;,x&nbsp;,'y&nbsp;d)&nbsp;e)<p></tt><p>

The two notations
<tt>`</tt>&lt;qq template&gt; and <tt>(quasiquote &lt;qq template&gt;)</tt>
are identical in all respects.
<tt>,&lt;expression&gt;</tt> is identical to <tt>(unquote &lt;expression&gt;)</tt>,
and
<tt>,@&lt;expression&gt;</tt> is identical to <tt>(unquote-splicing &lt;expression&gt;)</tt>.
The external syntax generated by <tt>write</tt> for two-element lists whose
car is one of these symbols may vary between implementations.
<a name="%_idx_160"></a><p>

<tt><p>(quasiquote&nbsp;(list&nbsp;(unquote&nbsp;(+&nbsp;1&nbsp;2))&nbsp;4))&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;(list&nbsp;3&nbsp;4)<br>
'(quasiquote&nbsp;(list&nbsp;(unquote&nbsp;(+&nbsp;1&nbsp;2))&nbsp;4))&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;`(list&nbsp;,(+&nbsp;1&nbsp;2)&nbsp;4)<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<em>i.e.,</em>&nbsp;(quasiquote&nbsp;(list&nbsp;(unquote&nbsp;(+&nbsp;1&nbsp;2))&nbsp;4))<p></tt><p>

Unpredictable behavior can result if any of the symbols
<tt>quasiquote</tt>, <tt>unquote</tt>, or <tt>unquote-splicing</tt> appear in
positions within a &lt;qq template&gt; otherwise than as described above.<p>

<p><p>

<a name="%_sec_4.3"></a>
<h2><a href="r5rs-Z-H-2.html#%_toc_%_sec_4.3">4.3&nbsp;&nbsp;Macros</a></h2><p>

<p>

Scheme programs can define and use new derived expression types,
called <em>macros</em>.<a name="%_idx_162"></a>
Program-defined expression types have the syntax
<tt><p>(&lt;keyword&gt;&nbsp;&lt;datum&gt;&nbsp;...)<p></tt>where &lt;keyword&gt; is an identifier that uniquely determines the
expression type.  This identifier is called the <em>syntactic
keyword</em><a name="%_idx_164"></a>, or simply <em>keyword</em><a name="%_idx_166"></a>, of the macro<a name="%_idx_168"></a>.  The
number of the &lt;datum&gt;s, and their syntax, depends on the
expression type.<p>

Each instance of a macro is called a <em>use</em><a name="%_idx_170"></a>
of the macro.
The set of rules that specifies
how a use of a macro is transcribed into a more primitive expression
is called the <em>transformer</em><a name="%_idx_172"></a>
of the macro.<p>

The macro definition facility consists of two parts:<p>

<p><ul>
<li>A set of expressions used to establish that certain identifiers
are macro keywords, associate them with macro transformers, and control
the scope within which a macro is defined, and<p>

<li>a pattern language for specifying macro transformers.
</ul><p><p>

The syntactic keyword of a macro may shadow variable bindings, and local
variable bindings may shadow keyword bindings.  <a name="%_idx_174"></a>  All macros
defined using the pattern language  are ``hygienic'' and ``referentially
transparent'' and thus preserve Scheme's lexical scoping&nbsp;[<a href="r5rs-Z-H-14.html#%_sec_7.3">14</a>,&nbsp;<a href="r5rs-Z-H-14.html#%_sec_7.3">15</a>,&nbsp;<a href="r5rs-Z-H-14.html#%_sec_7.3">2</a>,&nbsp;<a href="r5rs-Z-H-14.html#%_sec_7.3">7</a>,&nbsp;<a href="r5rs-Z-H-14.html#%_sec_7.3">9</a>]:
<a name="%_idx_176"></a>
<a name="%_idx_178"></a><p>

<p><ul><p>

<li>If a macro transformer inserts a binding for an identifier
(variable or keyword), the identifier will in effect be renamed
throughout its scope to avoid conflicts with other identifiers.
Note that a <tt>define</tt> at top level may or may not introduce a binding;
see section&nbsp;<a href="r5rs-Z-H-8.html#%_sec_5.2">5.2</a>.<p>

<li>If a macro transformer inserts a free reference to an
identifier, the reference refers to the binding that was visible
where the transformer was specified, regardless of any local
bindings that may surround the use of the macro.<p>

</ul><p><p>