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    <title>Local binding constructs</title>
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    <section class="sect1" title="Local binding constructs" epub:type="subchapter" id="Local-binding-constructs">
      <div class="titlepage">
        <div>
          <div>
            <h2 class="title" style="clear: both">Local binding constructs</h2>
          </div>
        </div>
      </div>
      <p>The binding constructs <code class="literal">let</code>, <code class="literal">let*</code>, <code class="literal">letrec</code>,
and <code class="literal">letrec*</code> give Scheme a block structure, like Algol 60.
The syntax of these four constructs
is identical, but they differ in the regions they establish
for their variable bindings. In a <code class="literal">let</code> expression, the initial
values are computed before any of the variables become
bound; in a <code class="literal">let*</code> expression, the bindings and evaluations
are performed sequentially; while in <code class="literal">letrec</code> and <code class="literal">letrec*</code>
expressions, all the bindings are in effect while their initial
values are being computed, thus allowing mutually recursive definitions.


</p>
      <p class="synopsis" kind="Syntax"><span class="kind">Syntax</span><span class="ignore">: </span><a id="idm139667878162832" class="indexterm"/> <code class="function">let</code> <em class="replaceable"><code><span class="bold"><strong>((</strong></span></code></em><em class="replaceable"><code><a class="link" href="Variables-and-Patterns.xhtml#meta-pattern"><em class="replaceable"><code>pattern</code></em></a></code></em> <em class="replaceable"><code><em class="replaceable"><code>init</code></em></code></em><em class="replaceable"><code><span class="bold"><strong>)</strong></span></code></em> <em class="replaceable"><code>...</code></em><em class="replaceable"><code><span class="bold"><strong>)</strong></span></code></em> <em class="replaceable"><code><a class="link" href="Bodies.xhtml#meta-body"><em class="replaceable"><code>body</code></em></a></code></em></p>
      <div class="blockquote">
        <blockquote class="blockquote">
          <p>Declare new local variables as found in the <em class="replaceable"><code>pattern</code></em>s.
Each <em class="replaceable"><code>pattern</code></em> is matched against the corresponding <em class="replaceable"><code>init</code></em>.
The <em class="replaceable"><code>init</code></em>s are evaluated in the current environment (in left-to-right
onder), the <em class="replaceable"><code>variable</code></em>s in the <em class="replaceable"><code>patterns</code></em>s are bound to fresh
locations holding the matched results,
the <em class="replaceable"><code>body</code></em> is evaluated in the extended environment, and the values of
the last expression of body are returned.
Each binding of a variable has <em class="replaceable"><code>body</code></em> as its region.
</p>
          <pre class="screen">(let ((x 2) (y 3))
  (* x y)) ⇒ 6
</pre>
          <pre class="screen">(let ((x 2) (y 3))
  (let ((x 7)
        (z (+ x y)))
    (* z x)))   ⇒ 35
</pre>
          <p>An example with a non-trivial pattern:
</p>
          <pre class="screen">(let (([a::double b::integer] (vector 4 5)))
  (cons b a))  ⇒ (5 . 4.0)
</pre>
        </blockquote>
      </div>
      <p class="synopsis" kind="Syntax"><span class="kind">Syntax</span><span class="ignore">: </span><a id="idm139667878149392" class="indexterm"/> <code class="function">let*</code> <em class="replaceable"><code><span class="bold"><strong>((</strong></span></code></em><em class="replaceable"><code><a class="link" href="Variables-and-Patterns.xhtml#meta-pattern"><em class="replaceable"><code>pattern</code></em></a></code></em> <em class="replaceable"><code>init</code></em><em class="replaceable"><code><span class="bold"><strong>)</strong></span></code></em> <em class="replaceable"><code>...</code></em><em class="replaceable"><code><span class="bold"><strong>)</strong></span></code></em> <em class="replaceable"><code><a class="link" href="Bodies.xhtml#meta-body"><em class="replaceable"><code>body</code></em></a></code></em></p>
      <div class="blockquote">
        <blockquote class="blockquote">
          <p>The <code class="literal">let*</code> binding construct is similar to <code class="literal">let</code>,
but the bindings are performed sequentially from left to
right, and the region of a <em class="replaceable"><code>variable</code></em>s in a <em class="replaceable"><code>pattern</code></em>
is that part of the <code class="literal">let*</code> expression to the right of
the <em class="replaceable"><code>pattern</code></em>. Thus the second pattern is matched in an environment
in which the bindings from the first pattern are visible, and so on.
</p>
          <pre class="screen">(let ((x 2) (y 3))
  (let* ((x 7)
         (z (+ x y)))
    (* z x)))  ⇒ 70
</pre>
        </blockquote>
      </div>
      <p class="synopsis" kind="Syntax"><span class="kind">Syntax</span><span class="ignore">: </span><a id="idm139667878138240" class="indexterm"/> <code class="function">letrec</code> <em class="replaceable"><code><span class="bold"><strong>((</strong></span></code></em><em class="replaceable"><code>variable</code></em> [<em class="replaceable"><code><span class="bold"><strong>::</strong></span></code></em> <em class="replaceable"><code><a class="link" href="Types.xhtml#meta-type"><em class="replaceable"><code>type</code></em></a></code></em>] <em class="replaceable"><code>init</code></em><em class="replaceable"><code><span class="bold"><strong>)</strong></span></code></em> <em class="replaceable"><code>...</code></em><em class="replaceable"><code><span class="bold"><strong>)</strong></span></code></em> <em class="replaceable"><code><a class="link" href="Bodies.xhtml#meta-body"><em class="replaceable"><code>body</code></em></a></code></em></p>
      <p class="synopsis" kind="Syntax"><span class="kind">Syntax</span><span class="ignore">: </span><a id="idm139667878129968" class="indexterm"/> <code class="function">letrec*</code> <em class="replaceable"><code><span class="bold"><strong>((</strong></span></code></em><em class="replaceable"><code>variable</code></em> [<em class="replaceable"><code><span class="bold"><strong>::</strong></span></code></em> <em class="replaceable"><code><a class="link" href="Types.xhtml#meta-type"><em class="replaceable"><code>type</code></em></a></code></em>] <em class="replaceable"><code>init</code></em><em class="replaceable"><code><span class="bold"><strong>)</strong></span></code></em> <em class="replaceable"><code>...</code></em><em class="replaceable"><code><span class="bold"><strong>)</strong></span></code></em> <em class="replaceable"><code><a class="link" href="Bodies.xhtml#meta-body"><em class="replaceable"><code>body</code></em></a></code></em></p>
      <div class="blockquote">
        <blockquote class="blockquote">
          <p>The <em class="replaceable"><code>variable</code></em>s are bound to fresh locations,
each <em class="replaceable"><code>variable</code></em> is assigned in left-to-right order
to the result of the corresponding <em class="replaceable"><code>init</code></em>,
the <em class="replaceable"><code>body</code></em> is evaluated in the resulting environment,
and the values of the last expression in body are returned.
Despite the left-to-right evaluation and assignment order, each binding of a
<em class="replaceable"><code>variable</code></em> has the entire <code class="literal">letrec</code> or <code class="literal">letrec*</code>
expression as its region,
making it possible to define mutually recursive procedures.
</p>
          <p>In Kawa <code class="literal">letrec</code> is defined as the same as <code class="literal">letrec*</code>.
In standard Scheme the order of evaluation of the <em class="replaceable"><code>init</code></em>s
is undefined, as is the order of assignments.
If the order matters, you should use <code class="literal">letrec*</code>.
</p>
          <p>If it is not possible to evaluate each <em class="replaceable"><code>init</code></em> without assigning
or referring to the value of the corresponding <em class="replaceable"><code>variable</code></em>
or the variables that follow it, it is an error.
</p>
          <pre class="screen">(letrec ((even?
          (lambda (n)
            (if (zero? n)
                #t
                (odd? (- n 1)))))
         (odd?
          (lambda (n)
            (if (zero? n)
                #f
                (even? (- n 1))))))
  (even? 88))
     ⇒ #t
</pre>
        </blockquote>
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