[racket] again: timeouts and exceptions

From: Matthias Felleisen (matthias at ccs.neu.edu)
Date: Thu Jul 5 14:06:46 EDT 2012

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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