Hint on local DIE handlers in Eval

[Scott Penrose]

Thought I might share some wisdom on a common code error I have seen in 
my own code as well as others...

DIE handlers are great to change so that you can gracefully tell the 
user. However you must remember that module writers tend to expect the 
die handler to actually do a die, and not an "print STDERR ...; exit 
1;".

It is common to find in module code things like...
	eval q{
		use Fred;
	};
	unless ($@) {
		print "Cool Fred exists on this machine";
	}

but of course with the DIE handler set this may fail to pick up the 
error, calling your DIE handler and exiting, when it should have 
continued.

You probably find yourself often writing code like this

	eval {
          local $SIG{__DIE__} = sub { die $_[0] };
		Do Stuff Here...
	};
	print "Errors checking here" if ($@);

Which correctly picks up errors in the eval.

So us smart developers start doing our own local DIE handler along the 
lines of...

	eval q{
          local $SIG{__DIE__} = sub { die $_[0] };
		use Fred;
	};
	unless ($@) {
		print "Cool Fred exists on this machine";
	}

Of course the problem with our little bit of code above is that when 
perl compiles our string it has not yet set the local die handler. So 
any errors in compile (eg: Fred does not exist as a module) causes the 
root DIE handler to get called, again missing the error and not 
handling it correctly.

So the correct approach is of course...
   {
     local $SIG{__DIE__} = sub { die $_[0] };
	eval q{
		use Fred;
	};
	unless ($@) {
		print "Cool Fred exists on this machine";
	}
   }

(ie: move the local $SIG{__DIE__} outside the string eval, but inside a 
block).

This may seem very obvious to some, but you would be surprised how many 
times I have seen it done and then cause REALLY obscure errors when 
upgrading a CPAN module (Cache::Cache uses the eval q{ use ...} 
approach to check for modules, and does NOT set a local DIE handler)

:-)

Scott
-- 
Scott Penrose
Anthropomorphic Personification Expert
http://search.cpan.org/search?author=SCOTT
scott at cpan.org

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