[racket] Making a Racket function "recallable"

Thanks Erik.  Very nice!  Although I miss the recursion.

please help me understand how this works, especially (let loop ([vs '()]).
What exactly is going on there?

Thanks again,
-joe

On Mon, Feb 13, 2012 at 9:51 PM, Erik Silkensen <eriksilkensen at gmail.com>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))))))
>
> - Erik
>
> On Feb 13, 2012, at 10:41 PM, Joe Gilray wrote:
>
> Erik,
>
> Intriguing idea and it works "flat", but if I write the following I end up
> with an infinite loop:
>
> (define (fib-less-than-n n)
>   (let ([f1 (mk-fib)])
>     (let ([fib-val (f1)])
>       (if (>= fib-val n) '() (cons fib-val (fib-less-than-n n))))))
>
> How would I use your idea in this case?
>
> thanks!
> -joe
>
> On Mon, Feb 13, 2012 at 9:18 PM, Erik Silkensen <eriksilkensen at gmail.com>wrote:
>
>> Another option is you could turn change your fib function to be 'mk-fib'
>> that returns a new instance of the fib function each time it's called:
>>
>> (define (mk-fib)
>>   (let ([n0 -1] [n1 1])
>>    (lambda ()
>>      (let ([next (+ n0 n1)])
>>        (set! n0 n1)
>>        (set! n1 next))
>>      n1)))
>>
>> - Erik
>>
>> On Feb 13, 2012, at 10:10 PM, Danny Yoo wrote:
>>
>> > On Mon, Feb 13, 2012 at 11:52 PM, Joe Gilray <jgilray at gmail.com> wrote:
>> >> Warning: extreme newbie question ahead.
>> >>
>> >> I wrote the following fibonacci function:
>> >>
>> >> ; function that returns the next fibonacci number each time it is
>> called
>> >> ; invoke as (fib)
>> >> (define fib
>> >>   (let ([n0 -1] [n1 1])
>> >>     (lambda ()
>> >>       (let ([next (+ n0 n1)])
>> >>         (set! n0 n1)
>> >>         (set! n1 next))
>> >>       n1)))
>> >
>> >
>> > One thing you can do is turn fib into a "sequence", and then from a
>> > sequence into a stream that knows how to remember its previous values.
>> >
>> > Here's what it might look like:
>> >
>> > ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
>> > #lang racket
>> > (require racket/sequence)
>> >
>> > (define fib
>> >  (let ([n0 -1] [n1 1])
>> >    (lambda ()
>> >      (let ([next (+ n0 n1)])
>> >        (set! n0 n1)
>> >        (set! n1 next))
>> >      n1)))
>> >
>> > (define fib-stream
>> >  ;; Here's a sequence of the function:
>> >  (let ([fib-sequence (in-producer fib 'donttellmecauseithurts)])
>> >
>> >    ;; Let's wrap it and turn it into a stream that remembers...
>> >    (sequence->stream fib-sequence)))
>> >
>> >
>> > ;; Ok, we've got a stream.  Let's look at its first few elements.
>> > (define (peek-fibs n)
>> >  (for ([elt fib-stream]
>> >        [i (in-range n)])
>> >    (displayln elt)))
>> >
>> > (peek-fibs 10)
>> > (printf "-----\n")
>> > (peek-fibs 20)
>> >
>> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
>> >
>> > ____________________
>> >  Racket Users list:
>> >  http://lists.racket-lang.org/users
>>
>>
>
>
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