[Bioperl-l] Grrrr. Another subtle overloading bug ...

[Lincoln Stein]

You can set $@ directly by assignment as a last resort.  It will remain there 
until the next eval{}

Lincoln

On Monday 07 January 2002 16:01, Chervitz, Steve wrote:
> Folks,
>
> Apologies for causing trouble. So much for my attempt at transparency!
> I've just checked in a fix to Bio::Root::Root::throw() that will ensure
> backward compatibility for eval{}-based exception handling.
>
> When I made changes in Root::throw(), I also added a variety of examples in
> examples/exceptions that demonstrated the new functionality and tested for
> backward compatibility (specifically, test2.pl and test4.pl in that dir).
> They were all working, or so I thought.
>
> I agree with Hilmar's comments re: exceptions. My aim was to be completely
> transparent with the current exception throwing/handling methodology, yet
> to enable users to get fancier if they want to, and have a standard way to
> get fancy, instead of creating multiple, incompatible Bioperl extensions. I
> realize this goal can be difficult to achieve. If the current system
> doesn't cut it, I'm OK with reverting back to the old ways.
>
> It would be nice if Error::throw() could be used within an eval{} block so
> that the traditional if($@) syntax would work with Error objects. This will
> take some digging into Error.pm to see what's going on here.
>
> In the meantime, eval{}-based exception handling should be working again
> after you cvs update. My examples/exceptions/test2.pl and test4.pl should
> suffice as  backward compatibility tests. Feel free to add to them as you
> see fit. Sorry again for my lapse in testing.
>
> Steve
>
> > -----Original Message-----
> > From: Ewan Birney [mailto:birney@ebi.ac.uk]
> > Sent: Sunday, January 06, 2002 10:39 AM
> > To: bioperl-l@bioperl.org
> > Subject: [Bioperl-l] Grrrr. Another subtle overloading bug ...
> >
> >
> >
> >
> > SteveC - you are going to get awards for introducing the most
> > bizarre bugs
> > across bioperl namespaces if we are not careful....
> >
> >
> > The new Exception class SteveC checked in (at my behest to get things
> > checked in early) now throws objects not scalars...
> >
> >
> > These objects have a stringify method which evaluates to the expected
> > string when printed - quite right
> >
> >
> > ... but there is a *VERY* nasty side-effect ....
> >
> >
> > which is that scenarios like
> >
> >
> >   eval {
> > 	# do complex stuff
> > 	# which might throw an exception
> >   };
> >
> >   if( $@ ) { # you would expect this to evaluate to "true" on
> > exception
> >     # do something else, like return failure gracefully
> >   } else {
> >     # do something sanely
> >   }
> >
> >
> > This construct WONT work. <sigh>
> >
> > The reason is that
> >
> >   if( $@ ) triggers the "stringify" method, which gives back
> > a nice piece
> > of text and then gets evaluated as a number (I think...),
> > which goes to 0
> > which evaluates as .... FALSE.
> >
> >
> >   ...Grrrrr...
> >
> >
> > I suspect world wide we have lots of
> >
> >   if( $@ ) {
> >
> >   }
> >
> >
> > (certainly there is alot in Ensembl...)
> >
> >
> > Now ... I have commited some tests to fix this but then I
> > realised that
> > this is probably a bad idea and we need to fix things so that
> >
> >
> >   if( $@ ) {
> >    #
> >   }
> >
> > Evaluates to TRUE. SteveC - do you know how to do this. I have spent a
> > fruitless half an hour in perl.com trying to find the overloading
> > documentation.
> >
> >
> >
> > This is a nasty bug and something we have to get right. It
> > also emphasises
> > how important it is to test the 0.9 series before we call it
> > 1.0 otherwise
> > we will just trigger alot of bugs world-wide...
> >
> >
> >
> > ewan
> >
> >
> > _______________________________________________
> > Bioperl-l mailing list
> > Bioperl-l@bioperl.org
> > http://bioperl.org/mailman/listinfo/bioperl-l
>
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-- 
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Lincoln D. Stein                           Cold Spring Harbor Laboratory
lstein@cshl.org			                  Cold Spring Harbor, NY
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