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    <p>If you represent a (binary) tree with some form of nodes, we have several possible ways to represent them in Common Lisp.</p> <p>We could define a node structure:</p> <pre><code>(defstruct node left right) </code></pre> <p>Then we have a function <code>MAKE-NODE</code>.</p> <p>We could define a node class:</p> <pre><code>(defclass node () (left right)) </code></pre> <p>Nodes would be made with <code>(make-instance 'node)</code>.</p> <p>We could make nodes with lists:</p> <pre><code>(defun make-node (left right) (list left right)) </code></pre> <p>Above uses two cons cells.</p> <p>We could make nodes for a binary tree a bit smaller with just one cons cell:</p> <pre><code>(defun make-node (left right) (cons left right)) </code></pre> <p>We could make nodes as a vector:</p> <pre><code>(defun make-node (left right) (vector left right)) </code></pre> <p>There are many possibilities. If the node should also have some other information, then the single cons cell approach is not enough. Something like a structure or class is fine, since the data item can have more information and the object knows its type: we can easily ask <code>(node-p something)</code> and get a useful answer.</p> <p>The style rules for choosing the right data structure for any slightly more advanced software you want to write:</p> <ul> <li>by default use CLOS</li> <li>if CLOS is too slow (for example slot access is too slow), then try to optimize CLOS</li> <li>if CLOS is still too slow, then use structures. Slot access should be faster</li> <li>try to avoid representing data structures as untyped lists or cons trees</li> </ul>
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    1. POCAR represent *right* subtree of a cons?
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    1. USRainer Joswig
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    1. COwhat does it mean, "try to optimize CLOS"? To optimize my CLOS-using user code, or to optimize how CLOS is implemented?
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      1. USWill Ness
    2. CO@Will Ness: some CLOS implementations have various ways to optimize slot access: primary classes, class sealing, circumventing slot-value-using-class, ... There are also other things which can be done to speed up CLOS-based code (like pre-fill caches for generic functions). For example I remember a talk by somebody whose company wrote a large CAD system using CLOS for the data structures. They needed some optimizations to CLOS - so that they did not have to use structures.
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      1. USRainer Joswig
    3. COThis is very good advice; I've fooled somewhat with Lisp data structure work, and CLOS never went wrong for me (albeit slightly slower). Using CAR/CDR was, as a rule, very fiddly and inflexible.
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      1. USPaul Nathan
 

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