use Perl

• It's a well-known problem (Score:1)

  by Ed Avis (3134) <ed@membled.com> on 2008.08.29 4:36 (#64649) Homepage

  Yes, this is a well-known problem with eval. You cannot, as the manual page suggests, do eval { ... } and then test $@ to see whether there was an error. The correct incantation is something like
  
  my $r = eval { ... };
  my $error = $@;
  $error //= 'unknown error' if not $r;
  if ($error) { ... }
  
  By well-known I mean known to a tiny minority of cognoscenti, while most perl programmers and the documentation are entirely ignorant of it. Which is the default situation for any trap or gotcha in the perl language. But I digress...
  
  The Error module on CPAN provides syntactic sugar 'try', 'catch', and 'except'. It handles the case of eval {} in DESTROY half-correctly: an exception is thrown, but the original message is lost. See <http://rt.cpan.org/Ticket/Display.html?id=38836>.
  
  The good news is that if you use Error everywhere and have try {} in your destructors instead of eval {}, it appears to work correctly.

  --
  -- Ed Avis ed@membled.com

  Reply to This

  • Re: (Score:1)

    by Ed Avis (3134)

    Duuh, I meant
    
    my $r = eval { ...; 1 };
    my $error = $@;
    $error //= 'unknown error' if not $r;
    if ($error) { ... }
    
    Note the trailing '1' in the eval block.
    
    (It doesn't help that the comment engine here is doing something weird so you lose all your text if you go Back and Forward in Firefox. And those submit buttons are not buttons at all but look like links. What's going on?)

    --
    -- Ed Avis ed@membled.com
  • Re: (Score:1)

    by rjbs (4671)

    Yeah, I generally do that, and definitely suggest that anyone else do it, too. What I was wondering about, though, is prevention. People are going to *forget* to do that, and if my destructors localize $@, they won't clobber it. Where else is there clobbering potential?

    I'm not even sure the destructor in quesiton did call eval. I guess maybe somewhere down *its* calls. Blech!

    --
    rjbs
    • Re: (Score:1)

      by Ed Avis (3134)

      For prevention, you just have to run perlcritic over your code. If you're calling other people's code that isn't perlcritic-clean, then 'local $@' in every DESTROY method might save you. I suppose there should be a perlcritic policy for that too.
      
      I don't like the way that the perl core has all sorts of traps for the unwary and can seemingly never be fixed for fear of breaking backwards compatibility, but perlcritic provides a useful sticking plaster.

      --
      -- Ed Avis ed@membled.com