From: Hugh Sasse <hgs@...>
Date: 2005-11-03T04:44:14+09:00
Subject: Re: Trapping errors.

On Thu, 3 Nov 2005, Robert Klemme wrote:

> Hugh Sasse <hgs@dmu.ac.uk> wrote:
> > On Thu, 3 Nov 2005, Robert Klemme wrote:
> > 
> > > Hugh Sasse wrote:
> > > > On Thu, 3 Nov 2005, Robert Klemme wrote:
> > > > 
> > > > > Hugh Sasse wrote:
> > > > > > I see my changes to fileutils are now in the Ruby CVS.
> > > > > > However, even with
> > > > > >   rescue Exception, SystemCallError => e
> > > > > > or
> > > > > >   rescue Exception, Errno::EACCES, Errno::EBUSY => e
> > > > > > 
> > > > > > I still cannot trap this error.  From the call stack this part of
> > > > > > the code is being used, so why won't the error cause ruby to go
> > > > > > back up the callstack until it finds this rescue clause?
> > > > > 
> > > > > Maybe there's another rescue clause that is closer to the place
> > > > > where the exception is thrown...
> > > > 
> > > > But unless I have completely misunderstood the point of rescue, even
> > > > if that calls raise, the exception will still be caught by my
> > > > enclosing rescue.
> > > 
> > > But who guarantees that the other rescue clause actually throws
> > > again?  If it doesn't you can't catch it.
> > > 
> > > ....
> > > rescue Exception => e
> > >   puts "Catch me if you can >:-}"
> > > end
> > > 
> > > ;-)
> > > 
> > 
> > Then it's already caught, and I should never see the error.
> 
> Why not?  If it's printed in the other rescue clause you would see it.  Or am
> I missing something here?

I wouldn't see it as an error which kills my program.  It would have
been caught, and dealt with, or caught and re-raised, in which case
the outer block would catch it.

1 We have established that a begin...rescue...end block
  will rescue any errors raised inside it, provided the rescue
  clause matches that error type. 

2 We have established that if a begin ... rescue ... raise ... end
  block re-raises an error, it can be caught by a surrounding
  begin...rescue...end block

3 My case is that I have a
   begin
      part1
   rescue Exception, SystemCallError, Errno::EACCES, Errno::EBUSY => e
      part2
   end

  block and it is not catching a Errno::EBUSY error from part1: The
  error crashes the program.  So:
    If that error has been raised in part1, and not caught in part1
      then I should be able to catch it. 
    If that error has been raised in part1, and caught in part1,
      part2 should never come into play, and the program should
      continue

4 How can a subclass of Exception not be caught by Exception?
5 Why doesn't SystemCallError catch it?
6 Why doesn't the Errno::%s form catch it?

i.e everything between "Exception" and " =>" should be unnecessary,
but even with those it doesn't catch the error.

> 
>    robert
> 
        Hugh