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Next: <a href="Internal-Definitions.html" accesskey="n" rel="next">Internal definitions</a>, Previous: <a href="Top-Level.html" accesskey="p" rel="prev">Top Level Variable Definitions</a>, Up: <a href="Binding-Constructs.html" accesskey="u" rel="up">Definitions and Variable Bindings</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="subsection" id="Local-Variable-Bindings"><span>6.10.2 Local Variable Bindings<a class="copiable-link" href="#Local-Variable-Bindings"> &para;</a></span></h4>
<a class="index-entry-id" id="index-local-bindings"></a>
<a class="index-entry-id" id="index-local-variables"></a>
<p>As opposed to definitions at the top level, which creates bindings that
are visible to all code in a module, it is also possible to define
variables which are only visible in a well-defined part of the program.
Normally, this part of a program will be a procedure or a subexpression
of a procedure.
</p>
<p>With the constructs for local binding (<code class="code">let</code>, <code class="code">let*</code>,
<code class="code">letrec</code>, and <code class="code">letrec*</code>), the Scheme language has a block
structure like most other programming languages since the days of
<small class="sc">ALGOL 60</small>. Readers familiar to languages like C or Java should
already be used to this concept, but the family of <code class="code">let</code>
expressions has a few properties which are well worth knowing.
</p>
<p>The most basic local binding construct is <code class="code">let</code>.
</p>
<dl class="first-deffn">
<dt class="deffn" id="index-let"><span class="category-def">syntax: </span><span><strong class="def-name">let</strong> <var class="def-var-arguments">bindings body</var><a class="copiable-link" href="#index-let"> &para;</a></span></dt>
<dd><p><var class="var">bindings</var> has the form
</p>
<div class="example lisp">
<pre class="lisp-preformatted">((<var class="var">variable1</var> <var class="var">init1</var>) ...)
</pre></div>
<p>that is zero or more two-element lists of a variable and an arbitrary
expression each. All <var class="var">variable</var> names must be distinct.
</p>
<a class="index-entry-id" id="index-body_002c-of-a-let-expression"></a>
<p><var class="var">body</var> is a sequence of expressions and definitions, ending in an
expression.
</p>
<p>A <code class="code">let</code> expression is evaluated as follows.
</p>
<ul class="itemize mark-bullet">
<li>All <var class="var">init</var> expressions are evaluated.
</li><li>New storage is allocated for the <var class="var">variables</var>.
</li><li>The values of the <var class="var">init</var> expressions are stored into the variables.
</li><li>The expressions and definitions in <var class="var">body</var> are evaluated in order
(see <a class="pxref" href="Internal-Definitions.html">Internal definitions</a>), and the values of the last expression
are returned as the result of the <code class="code">let</code> expression.
</li></ul>
<p>The <var class="var">init</var> expressions are not allowed to refer to any of the
<var class="var">variables</var>.
</p></dd></dl>
<p>The other binding constructs are variations on the same theme: making new
values, binding them to variables, and executing a body in that new,
extended lexical context.
</p>
<dl class="first-deffn">
<dt class="deffn" id="index-let_002a"><span class="category-def">syntax: </span><span><strong class="def-name">let*</strong> <var class="def-var-arguments">bindings body</var><a class="copiable-link" href="#index-let_002a"> &para;</a></span></dt>
<dd><p>Similar to <code class="code">let</code>, but the variable bindings are performed
sequentially, that means that all <var class="var">init</var> expression are allowed to
use the variables defined on their left in the binding list.
</p>
<p>A <code class="code">let*</code> expression can always be expressed with nested <code class="code">let</code>
expressions.
</p>
<div class="example lisp">
<pre class="lisp-preformatted">(let* ((a 1) (b a))
b)
&equiv;
(let ((a 1))
(let ((b a))
b))
</pre></div>
</dd></dl>
<dl class="first-deffn">
<dt class="deffn" id="index-letrec"><span class="category-def">syntax: </span><span><strong class="def-name">letrec</strong> <var class="def-var-arguments">bindings body</var><a class="copiable-link" href="#index-letrec"> &para;</a></span></dt>
<dd><p>Similar to <code class="code">let</code>, but it is possible to refer to the <var class="var">variable</var>
from lambda expression created in any of the <var class="var">inits</var>. That is,
procedures created in the <var class="var">init</var> expression can recursively refer to
the defined variables.
</p>
<div class="example lisp">
<pre class="lisp-preformatted">(letrec ((even? (lambda (n)
(if (zero? n)
#t
(odd? (- n 1)))))
(odd? (lambda (n)
(if (zero? n)
#f
(even? (- n 1))))))
(even? 88))
&rArr;
#t
</pre></div>
<p>Note that while the <var class="var">init</var> expressions may refer to the new
variables, they may not access their values. For example, making the
<code class="code">even?</code> function above creates a closure (see <a class="pxref" href="About-Closure.html">The Concept of Closure</a>)
referencing the <code class="code">odd?</code> variable. But <code class="code">odd?</code> can&rsquo;t be called
until after execution has entered the body.
</p></dd></dl>
<dl class="first-deffn">
<dt class="deffn" id="index-letrec_002a"><span class="category-def">syntax: </span><span><strong class="def-name">letrec*</strong> <var class="def-var-arguments">bindings body</var><a class="copiable-link" href="#index-letrec_002a"> &para;</a></span></dt>
<dd><p>Similar to <code class="code">letrec</code>, except the <var class="var">init</var> expressions are bound to
their variables in order.
</p>
<p><code class="code">letrec*</code> thus relaxes the letrec restriction, in that later
<var class="var">init</var> expressions may refer to the values of previously bound
variables.
</p>
<div class="example lisp">
<pre class="lisp-preformatted">(letrec ((a 42)
(b (+ a 10))) ;; Illegal access
(* a b))
;; The behavior of the expression above is unspecified
(letrec* ((a 42)
(b (+ a 10)))
(* a b))
&rArr; 2184
</pre></div>
</dd></dl>
<p>There is also an alternative form of the <code class="code">let</code> form, which is used
for expressing iteration. Because of the use as a looping construct,
this form (the <em class="dfn">named let</em>) is documented in the section about
iteration (see <a class="pxref" href="while-do.html">Iteration</a>)
</p>
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Next: <a href="Internal-Definitions.html">Internal definitions</a>, Previous: <a href="Top-Level.html">Top Level Variable Definitions</a>, Up: <a href="Binding-Constructs.html">Definitions and Variable Bindings</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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