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<title>Expression Syntax (Guile Reference Manual)</title>
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<div class="subsubsection-level-extent" id="Expression-Syntax">
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Next: <a href="Comments.html" accesskey="n" rel="next">Comments</a>, Up: <a href="Scheme-Syntax.html" accesskey="u" rel="up">Scheme Syntax: Standard and Guile Extensions</a> &nbsp; [<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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<h4 class="subsubsection" id="Expression-Syntax-1"><span>6.16.1.1 Expression Syntax<a class="copiable-link" href="#Expression-Syntax-1"> &para;</a></span></h4>
<p>An expression to be evaluated takes one of the following forms.
</p>
<dl class="table">
<dt><code class="code"><var class="var">symbol</var></code></dt>
<dd><p>A symbol is evaluated by dereferencing. A binding of that symbol is
sought and the value there used. For example,
</p>
<div class="example">
<pre class="example-preformatted">(define x 123)
x &rArr; 123
</pre></div>
</dd>
<dt><code class="code">(<var class="var">proc</var> <var class="var">args</var>&hellip;)</code></dt>
<dd><p>A parenthesized expression is a function call. <var class="var">proc</var> and each
argument are evaluated, then the function (which <var class="var">proc</var> evaluated
to) is called with those arguments.
</p>
<p>The order in which <var class="var">proc</var> and the arguments are evaluated is
unspecified, so be careful when using expressions with side effects.
</p>
<div class="example">
<pre class="example-preformatted">(max 1 2 3) &rArr; 3
(define (get-some-proc) min)
((get-some-proc) 1 2 3) &rArr; 1
</pre></div>
<p>The same sort of parenthesized form is used for a macro invocation,
but in that case the arguments are not evaluated. See the
descriptions of macros for more on this (see <a class="pxref" href="Macros.html">Macros</a>, and
see <a class="pxref" href="Syntax-Rules.html">Syntax-rules Macros</a>).
</p>
</dd>
<dt><code class="code"><var class="var">constant</var></code></dt>
<dd><p>Number, string, character and boolean constants evaluate &ldquo;to
themselves&rdquo;, so can appear as literals.
</p>
<div class="example">
<pre class="example-preformatted">123 &rArr; 123
99.9 &rArr; 99.9
&quot;hello&quot; &rArr; &quot;hello&quot;
#\z &rArr; #\z
#t &rArr; #t
</pre></div>
<p>Note that an application must not attempt to modify literal strings,
since they may be in read-only memory.
</p>
</dd>
<dt><a class="index-entry-id" id="index-_0027"></a>
<a id="index-quote"></a><span><code class="code">(quote <var class="var">data</var>)</code><a class="copiable-link" href="#index-quote"> &para;</a></span></dt>
<dt><code class="code">'<var class="var">data</var></code></dt>
<dd><p>Quoting is used to obtain a literal symbol (instead of a variable
reference), a literal list (instead of a function call), or a literal
vector. <code class="code">'</code> is simply a shorthand for a <code class="code">quote</code> form.
For example,
</p>
<div class="example">
<pre class="example-preformatted">'x &rArr; x
'(1 2 3) &rArr; (1 2 3)
'#(1 (2 3) 4) &rArr; #(1 (2 3) 4)
(quote x) &rArr; x
(quote (1 2 3)) &rArr; (1 2 3)
(quote #(1 (2 3) 4)) &rArr; #(1 (2 3) 4)
</pre></div>
<p>Note that an application must not attempt to modify literal lists or
vectors obtained from a <code class="code">quote</code> form, since they may be in
read-only memory.
</p>
</dd>
<dt><a class="index-entry-id" id="index-_0060"></a>
<a id="index-quasiquote"></a><span><code class="code">(quasiquote <var class="var">data</var>)</code><a class="copiable-link" href="#index-quasiquote"> &para;</a></span></dt>
<dt><code class="code">`<var class="var">data</var></code></dt>
<dd><p>Backquote quasi-quotation is like <code class="code">quote</code>, but selected
sub-expressions are evaluated. This is a convenient way to construct
a list or vector structure most of which is constant, but at certain
points should have expressions substituted.
</p>
<p>The same effect can always be had with suitable <code class="code">list</code>,
<code class="code">cons</code> or <code class="code">vector</code> calls, but quasi-quoting is often easier.
</p>
<dl class="table">
<dt><a class="index-entry-id" id="index-_002c"></a>
<a id="index-unquote"></a><span><code class="code">(unquote <var class="var">expr</var>)</code><a class="copiable-link" href="#index-unquote"> &para;</a></span></dt>
<dt><code class="code">,<var class="var">expr</var></code></dt>
<dd><p>Within the quasiquote <var class="var">data</var>, <code class="code">unquote</code> or <code class="code">,</code> indicates
an expression to be evaluated and inserted. The comma syntax <code class="code">,</code>
is simply a shorthand for an <code class="code">unquote</code> form. For example,
</p>
<div class="example">
<pre class="example-preformatted">`(1 2 (* 9 9) 3 4) &rArr; (1 2 (* 9 9) 3 4)
`(1 2 ,(* 9 9) 3 4) &rArr; (1 2 81 3 4)
`(1 (unquote (+ 1 1)) 3) &rArr; (1 2 3)
`#(1 ,(/ 12 2)) &rArr; #(1 6)
</pre></div>
</dd>
<dt><a class="index-entry-id" id="index-_002c_0040"></a>
<a id="index-unquote_002dsplicing"></a><span><code class="code">(unquote-splicing <var class="var">expr</var>)</code><a class="copiable-link" href="#index-unquote_002dsplicing"> &para;</a></span></dt>
<dt><code class="code">,@<var class="var">expr</var></code></dt>
<dd><p>Within the quasiquote <var class="var">data</var>, <code class="code">unquote-splicing</code> or
<code class="code">,@</code> indicates an expression to be evaluated and the elements of
the returned list inserted. <var class="var">expr</var> must evaluate to a list. The
&ldquo;comma-at&rdquo; syntax <code class="code">,@</code> is simply a shorthand for an
<code class="code">unquote-splicing</code> form.
</p>
<div class="example">
<pre class="example-preformatted">(define x '(2 3))
`(1 ,x 4) &rArr; (1 (2 3) 4)
`(1 ,@x 4) &rArr; (1 2 3 4)
`(1 (unquote-splicing (map 1+ x))) &rArr; (1 3 4)
`#(9 ,@x 9) &rArr; #(9 2 3 9)
</pre></div>
<p>Notice <code class="code">,@</code> differs from plain <code class="code">,</code> in the way one level of
nesting is stripped. For <code class="code">,@</code> the elements of a returned list
are inserted, whereas with <code class="code">,</code> it would be the list itself
inserted.
</p></dd>
</dl>
</dd>
</dl>
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