[racket] Trouble with nack-guard-evt

Sometimes, `sync' picks the first evt that it is given before even
trying the second evt enough to call the evt-generating function. That
is, `sync' looks far enough to see all arguments are `evt?', but
calling the evt-generator part of a guard evt is part of the attempt to
choose the specific guard evt, and that action doesn't happen if some
other evt succeeds first.

I'll work on the docs to clarify.

Yes, `proc' should be `generator'. But, `generator' also should be
changed, because "generator" now means something specific and
different.

At Thu, 28 Mar 2013 16:08:09 +0100, Tobias Hammer wrote:
> Hi,
> 
> i am trying to understand how nack-guard-evt works. Therefore i created  
> the appended test program. Sometimes it stops with the error message and  
> sometimes it prints the three values. It is clear to me what the error  
> means and why it is raised.
> What i am not understanding is, why the nack generator is not run every  
> time sync is called. I would have expected from the docs that it is run on  
> every sync.
> Can someone clarify why my assumptions are wrong and how nack-guard-evt is  
> expected to behave?
> 
> Related question: The docs for the function talk abound 'proc', should it  
> be 'generator'?
> 
> Tobias
> 
> 
> 
> ;; -----------------------------
> 
> #lang racket
> 
> (define nack #f)
> 
> (sync (handle-evt
>         always-evt
>         (lambda (_) 'ALWAYS))
>        (nack-guard-evt
>         (lambda (e)
>           (set! nack e)
>           never-evt)))
> 
> nack
> (printf "~a\n" (sync/timeout 0. nack))
> 
> > 'ALWAYS
> #f
> . . sync/timeout: contract violation
>    expected: evt?
>    given: #f
>    argument position: 2nd
>    other arguments...:
>     0.0
> 
> OR
> 
> > 'ALWAYS
> #<evt>
> #<void>
> 
> 
> 
> -- 
> ---------------------------------------------------------
> Tobias Hammer
> DLR / Robotics and Mechatronics Center (RMC)
> Muenchner Str. 20, D-82234 Wessling
> Tel.: 08153/28-1487
> Mail: tobias.hammer at dlr.de
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