;</li>
<li>when it is an operator about to be applied as a procedure.</li>
</ul></li>
<li>This is similar to <a href="../3/5.html#3.5.1">streams</a>, but uniform and automatic throughout the language.</li>
<li>For efficiency, we’ll make our interpreter <em>memoize</em> thunks.
<ul>
<li>This is called <em>call-by-need</em>, as opposed to non-memoized <em>call-by-name</em>.</li>
<li>It raises subtle and confusing issues in the presence of assignments.</li>
</ul></li>
</ul>
<div id="q1" class="highlight">
<a