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! > > > > ____________________ > Racket Users list: > http://lists.racket-lang.org/users ____________________ Racket Users list: http://lists.racket-lang.org/users
