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    <h2>Labels and named lets if your Lisp optimizes tail calls</h2> <p>Just as a matter of playing "stupid Lisp tricks", I'd point out that in Scheme, the analog to </p> <pre><code>sum a b = let loop s i = if i &gt; b then sum else loop (s + i) (i + 1) in loop 0 a </code></pre> <p>is letrec or a named let:</p> <pre><code>(define (sum a b) (letrec ((loop (lambda (s i) (if (&gt; i b) s (loop (+ s i) (+ i 1)))))) (loop 0 a))) (define (sum a b) (let loop ((s 0) (i a)) (if (&gt; i b) s (loop (+ s i) (+ i 1))))) </code></pre> <p>Letrec, because Scheme is a Lisp-1, gives you the functionality of <code>labels</code>, which <a href="https://stackoverflow.com/a/19535069/1281433">Baggers described</a>. The named let can be done in Common Lisp with a macro around <code>labels</code>:</p> <pre><code>(defmacro named-let (name bindings &amp;body body) `(labels ((,name ,(mapcar 'first bindings) ,@body)) (,name ,@(mapcar 'second bindings)))) (pprint (macroexpand '(named-let loop ((s 0) (i a)) (if (&gt; i b) s (loop (+ s i) (+ i 1)))))) ;; (LABELS ((LOOP (S I) ;; (IF (&gt; I B) ;; S ;; (LOOP (+ S I) ;; (+ I 1))))) ;; (LOOP 0 A)) </code></pre> <h2>Do and Do* should be efficient everywhere</h2> <p>However, tail calls aren't necessarily optimized away in Common Lisp, so this kind of recursion for iteration isn't all that common. The iterative analog is <code>do</code>:</p> <pre><code>(defun sum (a b) (do ((s 0 (+ s i)) (i a (+ i 1))) ((&gt; i b) s))) </code></pre> <h2>Loop works too</h2> <p>You can use <code>loop</code> too, but it's a bit more verbose (but also probably more readable if you're familiar with <code>do</code>:</p> <pre><code>(defun sum (a b) (loop for s = 0 then (+ s i) for i = a then (+ i 1) when (&gt; i b) return s)) </code></pre>
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    1. POhelper nested functions in CL
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    1. USJoshua Taylor
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    1. COthank you for your list of alternatives, I appreciate it an +1'd your answer.
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      1. USArtem Pelenitsyn
 

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