[racket] again: timeouts and exceptions

Well, '311' did create a common culture. Good. 


On Jul 6, 2012, at 3:15 AM, Rüdiger Asche wrote:

> by the way, you were absolutely right, of course. I must have been blind on both ears and deaf on both eyes not to think about CPS in this case. It works like a charm now!
> 
> Thanks again!
> 
>> 
>> ----- Original Message ----- From: "Matthias Felleisen"  <matthias at ccs.neu.edu>
>> To: "Rüdiger Asche" <rac at ruediger-asche.de>
>> Cc: "users" <users at racket-lang.org>
>> Sent: Thursday, July 05, 2012 8:06 PM
>> Subject: Re: [racket] again: timeouts and exceptions
>> 
>> 
>> 
>> 1. I don't know where this prose is but it reads rather confusing.
>> 
>> 2. If I were you, I'd use an escape continuation rather than an
>> exception handler here. What you seem to want is flow of control, and
>> let/ec is the right tool then.
>> 
>> 
>> 
>> 
>> 
>> On Jul 5, 2012, at 9:42 AM, Rüdiger Asche wrote:
>> 
>>> Hi there,
>>> 
>>> a few of you recommended to implement timeouting reads via  sync/timeout. That works neat, but it would be even nicer to  combine this w/ exception handling, like so:
>>> 
>>> 
>>> ...
>>> ((readbytetimeout)
>>>  (lambda (port)
>>>          (if (sync/timeout READTIMEOUT inport)
>>>      (read-byte inport)
>>>      (raise "read-byte" #t))))
>>> ...
>>> ((readsomething) ..... (readbytetimeout) ......
>>> 
>>> 
>>> (letrec ((repl (lambda ()
>>>      (dosomethingthatmustexecuteperiodically)
>>>              (let ((readresult
>>>     (call-with-exception-handler HandlerFn
>>> (readsomething))))
>>> (repl)))))
>>> (repl))
>>> 
>>> Iow, I want the REPL to always be operational but restart from the  beginning when the innermost read timeouts so that the periodic  computations get their turn.
>>> 
>>> I didn't get this to work though because according to the docs, "If the
>>> exception handler returns a value when invoked by raise, then raise
>>> propagates the value to the «previous» exception handler (still in the
>>> dynamic extent of the call to raise, and under the same barrier, if any)."
>>> 
>>> That is true; I get thrown back out of the entire thing because the  default uncaught exception handler is called. What ectually does it  mean "IF the
>>> exception handler..." How is it possible for a function NOT to  return a value?
>>> Any return value including #f and () will propagate the exception to the
>>> default handler; I simply want to stop it where it is.
>>> 
>>> What do I need to do (or paraphrased: What is the necessary  implementation of HandlerFn) in order to get this to work? Or do I  need yet another control flow or handling mechanism?
>>> 
>>> Thanks!
>>> 
>>> 
>>> 
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>>> http://lists.racket-lang.org/users
>> 
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> 
> 
>