[racket] Making a Racket function "recallable"

From: Stephen Bloch (bloch at adelphi.edu)
Date: Tue Feb 14 06:45:38 EST 2012

On Feb 14, 2012, at 12:51 AM, Erik Silkensen wrote:

> Instead of calling fib-less-than-n recursively, you could add a loop inside, for example something like,
> (define (fib-less-than-n n)
>   (define fib (mk-fib))
>   (let loop ([vs '()])
>     (let ([fib-val (fib)])
>       (if (>= fib-val n)
>           (reverse vs)
>           (loop (cons fib-val vs))))))

Let's generalize and abstract this to "take-while":

(define (take-while generator test?)
      (let loop [[vs '()]]
         (let [[value (generator)]]
            (if (test? value)
                (loop (cons value vs))

(define (fib-less-than-n n)
   (take-while (mk-fib) (lambda (value) (< value n))))

[I haven't typed this code in and tested it, so there may be typos.]

Stephen Bloch
sbloch at adelphi.edu

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