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    <p>The simple "standard" trick is</p> <pre><code>(apply append (map (lambda(x)(if (eq? x 'c) (list x) '())) '(a b c d))) ;Value 12: (c) </code></pre> <p>which returns <code>(c)</code>. The <code>apply append ... map</code> combo is known as <code>mapcan</code> in Common Lisp ("mapcan" for "map and concatenate"):</p> <pre><code>[1]&gt; (mapcan #'(lambda(x)(if (eq x 'c) (list x))) '(a b c d)) (C) </code></pre> <p>MIT Scheme has this function too.</p> <p><code>apply append</code> flattens one level of its argument list, <code>(apply append '((a) () (c) ()))</code> <strong>==</strong> <code>(append '(a) '() '(c) '())</code> <strong>--></strong> <code>(a c)</code>. Since empty lists disappear, it is useful for eliminating elements with <code>map</code>. This achieves the same effect as <code>filter</code> if you have one available (it is not in <a href="http://www.schemers.org/Documents/Standards/R5RS/HTML/r5rs-Z-H-15.html#%_chap_Temp_11" rel="nofollow">R5RS</a>, but it is in <a href="http://srfi.schemers.org/srfi-1/srfi-1.html#FilteringPartitioning" rel="nofollow">SRFI1</a>).</p> <p>It can be used for other effects too, like doubling:</p> <pre><code>[2]&gt; (mapcan #'(lambda(x)(if (evenp x) (list x x))) '(1 2 3 4)) (2 2 4 4) </code></pre> <hr> <p>As an aside, <code>mapcan</code> is what's known as list monad's "bind" (with arguments flipped), and <code>(apply append ...)</code> as its "join" operation. Indeed as it must be for any monad, for lists too <code>bind m f == join (map f m)</code>.</p> <p>This is also the basis for e.g. Haskell's <em>list comprehensions</em>: </p> <pre><code>Prelude&gt; [y | x&lt;-[1,2,3,4], even x, y&lt;-[x,x]] [2,2,4,4] </code></pre>
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    1. COThe Racket equivalent of `mapcan` is `append-map`.
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      1. USJeremiah Willcock
 

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