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PODynamic and Lexical variables in Common Lisp
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  1. PODynamic and Lexical variables in Common Lisp
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    copied!<p>I am reading the book 'Practical Common Lisp' by Peter Seibel.</p> <p>In Chapter 6, "Variables" sections "Lexical Variables and Closures" and "Dynamic, a.k.a. Special, Variables". <a href="http://www.gigamonkeys.com/book/variables.html" rel="noreferrer">http://www.gigamonkeys.com/book/variables.html</a></p> <p>My problem is that the examples in both sections show how (let ...) can shadow global variables and doesn't really tell the difference between the Dynamic and Lexical vars.</p> <p>I understand how closures work but I don't really get whats so special about let in this example:</p> <pre><code>(defvar *x* 10) (defun foo () (format t "Before assignment~18tX: ~d~%" *x*) (setf *x* (+ 1 *x*)) (format t "After assignment~18tX: ~d~%" *x*)) (defun bar () (foo) (let ((*x* 20)) (foo)) (foo)) CL-USER&gt; (foo) Before assignment X: 10 After assignment X: 11 NIL CL-USER&gt; (bar) Before assignment X: 11 After assignment X: 12 Before assignment X: 20 After assignment X: 21 Before assignment X: 12 After assignment X: 13 NIL </code></pre> <p>I feel like there is nothing special is going on here. The outer <em>foo</em> in <em>bar</em> increments the global <em>x</em>, and <em>foo</em> surrounded by <em>let</em> in <em>bar</em> increments the shadowed <em>x</em>. What's the big deal? I don't see how is this supposed to explain the difference between lexical and dynamic vars. Yet the book continues like this:</p> <blockquote> <p>So how does this work? How does LET know that when it binds <em>x</em> it's supposed to create a dynamic binding rather than a normal lexical binding? It knows because the name has been declared special.12 The name of every variable defined with DEFVAR and DEFPARAMETER is automatically declared globally special.</p> </blockquote> <p>What whould happen if <em>let</em> would bind <em>x</em> using <strong>"normal lexical binding"</strong>? All in all, what are the differences between dynamic and lexical binding and how is this example special regarding dynamic binding?</p>
 

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