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<title>Delayed evaluation - Revised(5) Scheme</title>
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<h4 class="subsection">4.2.5 Delayed evaluation</h4>
<p><a name="index-g_t_0040w_007bdelayed-evaluation_007d-132"></a>
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— library syntax: <b>delay</b><var> <expression><a name="index-delay-133"></a></var><br>
<blockquote>
<p>The ‘<samp><span class="samp">delay</span></samp>’ construct is used together with the procedure <code>force</code> to
<a name="index-g_t_0040w_007bforce_007d-134"></a>implement <dfn>lazy evaluation</dfn> or <dfn>call by need</dfn>.
<a name="index-g_t_0040w_007bcall-by-need_007d-135"></a><a name="index-g_t_0040w_007blazy-evaluation_007d-136"></a><tt>(delay </tt><span class="roman"><expression></span><tt>)</tt> returns an object called a
<dfn>promise</dfn> which at some point in the future may be asked (by
<a name="index-g_t_0040w_007bpromise_007d-137"></a>the ‘<samp><span class="samp">force</span></samp>’ procedure)
to evaluate
<span class="roman"><expression></span>, and deliver the resulting value.
The effect of <span class="roman"><expression></span> returning multiple values
is unspecified.
<p>See the description of ‘<samp><span class="samp">force</span></samp>’ (section see <a href="Control-features.html#Control-features">Control features</a>) for a
more complete description of ‘<samp><span class="samp">delay</span></samp>’.
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