[plt-scheme] Hash-tables and call/cc

Thanks Eli, Danny and Jos,

> On Mon, 7 May 2007, Jens Axel S?gaard wrote:
> 
>> (define (hash-table->generator ht)
>>   (let ([continue 'start])
>>     (lambda ()
>>       (case continue
>>         [(done)    #f]
>>         [(start)   (let/ec return
>>                      (hash-table-for-each
>>                       ht
>>                       (lambda (x v)
>>                         (call/cc
>>                          (lambda (k)
>>                            (set! continue k)
>>                            (return x)))))
>>                      (set! continue 'done)
>>                      #f)]
>>         [else      (continue)]))))
> 
> 
> Hi Jens,
> 
> "return" looks a little funny: I think we need to reset it every time we 
> re-enter the continuation.

Ahh! Thanks for the explanation.

> We also have to make sure to use return on every exit point, including 
> when we're "done".  So something like:
> 
> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
> (module hash-table-continuations mzscheme
>   (define (hash-table->generator ht)
>     (let ([continue 'start]
>           [return #f])
>       (lambda ()
>         (let/cc ret
>           (set! return ret)
>           (case continue
>             [(done)
>              (return #f)]
>             [(start)
>              (hash-table-for-each
>               ht
>               (lambda (x v)
>                 (call/cc
>                  (lambda (k)
>                    (set! continue k)
>                    (return x)))))
>              (set! continue 'done)
>              (return #f)]
>             [else (continue)]))))))v
> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
> 
> appears to work better.

And you did with minimal changes.


> As a self-plug, if you use my generator.plt from PLaneT, the code looks 
> like:
> 
> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
>   (require (planet "generator.ss" ("dyoo" "generator.plt" 2)))
> 
>   (define-generator (hash-table->generator-2 ht)
>     (hash-table-for-each ht (lambda (x v) (yield x))))
> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

Both this and Eli's version are really nice.

This solved my immediate problem. I'll post on the downside later.

Thanks,
-- 
Jens Axel Søgaard