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copied!<p>Try this, implemented using lazy code by means of <a href="http://docs.racket-lang.org/reference/Delayed_Evaluation.html" rel="nofollow">delayed evaluation</a>:</p> <pre><code>(define (each-fib fn) (letrec ((next (lambda (a b) (fn a) (delay (next b (+ a b)))))) (next 0 1))) (define (take-n-fibs n fn) (let loop ((i n) (promise (each-fib fn))) (when (positive? i) (loop (sub1 i) (force promise))))) </code></pre> <p>As has been mentioned, <code>each-fib</code> can be further simplified by using a <a href="http://www.r6rs.org/final/html/r6rs/r6rs-Z-H-14.html#node_sec_11.16" rel="nofollow">named <code>let</code></a>:</p> <pre><code>(define (each-fib fn) (let next ((a 0) (b 1)) (fn a) (delay (next b (+ a b))))) </code></pre> <p>Either way, it was necessary to modify <code>each-fib</code> a little for using the <code>delay</code> primitive, which creates a promise:</p> <blockquote> <p>A <em>promise</em> encapsulates an expression to be evaluated on demand via <code>force</code>. After a promise has been <code>force</code>d, every later force of the promise produces the same result.</p> </blockquote> <p>I can't think of a way to stop the original (unmodified) procedure from iterating indefinitely. But with the above change in place, <code>take-n-fibs</code> can keep forcing the lazy evaluation of as many values as needed, and no more.</p> <p>Also, <code>take-n-fibs</code> now receives a function for printing or processing each value in turn, use it like this:</p> <pre><code>(take-n-fibs 10 (lambda (n) (printf "~a " n))) > 0 1 1 2 3 5 8 13 21 34 55 </code></pre>

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