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2010-12-30T00:21:52.133
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2010-12-31T05:25:14.317
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2010-12-31T05:25:14.317
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I don't know CL well enough, but will Scheme macros do? Here's a while loop in Scheme: <pre><code>(define-syntax while (syntax-rules () ((while pred body ...) (let loop () (if pred (begin body ... (loop))))))) </code></pre> In this case, the example demonstrates that you can easily write your own control structures using macros. <a href="http://mumble.net/~campbell/scheme/foof-loop.txt" rel="nofollow"><code>foof-loop</code></a> is a collection of even more useful looping constructs (probably nothing new given CL's ones, but still good for a demonstration). <hr> Another use case: picking values out of associative lists. Say users pass in an alist as options to your function. You can easily pick values out by using this macro: <pre><code>(define-syntax let-assq (syntax-rules () ((let-assq alist (key) body ...) (let ((key (assq-ref alist 'key))) body ...)) ((let-assq alist (key rest ...) body ...) (let ((key (assq-ref alist 'key))) (let-assq alist (rest ...) body ...))))) ;; Guile built-in (define (assq-ref alist key) (cond ((assq key alist) => cdr) (else #f))) </code></pre> Example usage: <pre><code>(define (binary-search tree needle (lt? <)) (let loop ((node tree)) (and node (let-assq node (value left right) (cond ((lt? needle value) (loop left)) ((lt? value needle) (loop right)) (else value)))))) </code></pre> Notice how the <code>let-assq</code> macro allows picking out the <code>value</code>, <code>left</code>, and <code>right</code> keys from the "node" without having to write a much longer <code>let</code> form.
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1. POIs there a simple example for explaining Lisp macros to a "generic" programmer?
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1. USChris Jester-Young
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1. USChris Jester-Young
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