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Re: local $@ has an unwanted side effect

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Mark Mielke
March 21, 2008 10:51
Re: local $@ has an unwanted side effect
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Zefram wrote:
> Abigail wrote:
>> Inspecting $@ to check whether an eval die()d is wrong. It can
>> trigger both false positives, and false negatives:
> By that logic, there is no way at all to determine what exception
> was thrown.  It's not a practical approach.

That's not correct.

I find Abigail's comment a little surprising given the way that eval{} 
and exception handling is documented. I think Abigail may mean "if you 
require local $@, then inspecting $@ is wrong, otherwise if you do not 
require local $@, I have found in my personal experience, that there are 
other techniques for handling $@ than checking if ($@) that seem to work 

In my own experience, I find if ($@) to not always be the best approach. 
For example, I'll often do:

    my $variable_expected_to_be_truthful = eval {...}
        or die "....\n";

I may or may not make use of $@. I find the above to be cleaner to read 
than the attempt at mapping eval{}/if($@){} to try/catch, and I find it 
to be more exact. After all, what happens if:

   my $variable;
   eval {
       $variable = ...;
   if ($@) {
       die "...\n";


What if $variable is undef by the time it reaches the bottom? I believe 
I agree with Abigail, that if you really want to know whether $variable 
contains a usable value, checking $@ is a backwards way to do it, and in 
cases such as described in this thread, it's a broken way to do it.

> The possibility of DESTROY functions clobbering $@ is a design bug.
> I suggest that $@ should be automatically preserved across (localised
> to) DESTROY functions, and in the absence of such a change to the core
> I have argued for all DESTROY functions to explicitly localise $@ and
> the other global status variables

What might this break?

To me, it's a major change in behaviour. Perhaps a good one - but it 
would have been better suggested before Perl-5.000 was released.


Mark Mielke <>

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