[racket] Making a Racket function "recallable"

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)
> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
> 
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