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"
<matth...@ccs.neu.edu>
To: "Rüdiger Asche" <r...@ruediger-asche.de>
Cc: "users" <users@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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