Fellow monks,

I just spent a hour trying to find a bug in some code that I was eval-ing. It turns out that there was no bug in the eval code; the bug was in how I checked the result of the eval. I am posting it to the monastery since I've seen this type of bug in LOTS of Perl code before, and I never realized it was a bug until now.

First, some code: 

use strict;
my $num = 1;
print "Surprise!\n" if (!defined(eval { $num++; return;}));
print "$num\r\n";

[download]
As most people know, eval returns undef if it fails, and the value of the last operation if it succeeds. In the code above, the last operation is an argument-less return, so eval returns undef even though it succeeded. Hence, we can't check the return value to determine if an eval succeeded; we need to check $@: 
use strict;
my $num = 1;
eval { $num++; return;};
print "Surprise!\n" if ($@);
print "$num\r\n";

[download]
This gives the desired result.

-Ton
-----
Be bloody, bold, and resolute; laugh to scorn
The power of man...

In reply to Checking the success of eval by ton

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