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    <p><em>R: "Look, in the sky! The Lambda Signal! A citizen is in trouble Lambda Man!"</em></p> <p><em>LM: "I see it! And I've got just the box art they need."</em></p> <hr> <p>In Lisps, lists are singly-linked data structures, comprised of elements called <em>cons cells</em>. Each of these cells is a structure that consists of</p> <ol> <li>a pointer to a value </li> <li>a pointer to the next cell </li> </ol> <p>These are called the <em>car</em> and <em>cdr</em> respectively for historical reasons. Here's the traditional box art representing a 3-element list:</p> <pre><code>Structure: (car . cdr -)---&gt;(car . cdr -)---&gt;(car . cdr) | | | | v v v v Values: 1 2 3 nil </code></pre> <p>The <code>car</code> and <code>cdr</code> functions allow you to work with lists from this low level of abstraction, and return the values of the respective cells. Thus, <code>car</code> returns the 'value' of the cell, and <code>cdr</code> dereferences to the remainder of the list. <code>nthcdr</code> is a generalisation on top of <code>cdr</code> for convenience.</p> <p>The value returned by <code>cdr</code> is a reference to a raw data structure, which is mutable at this level. If you change the value of a cons-cell's cdr, you are changing the underlying structure of the list.</p> <p>Given:</p> <pre><code> let A = '(1 2) ~= (1 . -)--&gt;(2 . nil) let B = '(3 4) ~= (3 . -)--&gt;(4 . nil) </code></pre> <p>Setting the cdr of <code>(cdr A)</code> to B will destructively concatenate A and B such that A is now the structure below:</p> <pre><code> A B (1 . -)--&gt;(2 . -)--&gt;(3 . -)--&gt;(4 . nil) </code></pre> <p>As we've shown, a <code>nil</code> value in a cell's cdr represents the end of the list - there's nothing more that can be traversed. If we set the cdr of A to <code>nil</code>, we lobotomise the list, such that A is now</p> <pre><code>A (1 . nil) &lt;- [Not pointing to anything - the rest of the list shall go wanting] </code></pre> <p>This is pretty much what you've done - you've mutated the underlying data structures using low-level functions. :) By setting one of the cell's cdrs to <code>nil</code>, you've trimmed the end off your list.</p>
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    1. POwhy the change is in place for the list with elisp?
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    1. USChris Barrett
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    1. COOne rule of thumb is that functions containing `car` or `cdr` in their name are often lower-level and destructive in elisp.
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