Will code after eval(die "some error message") continue to be executed?

Question

I know that in java language ,if an exception is catched successfully ,the code after the try-catch-clause will still run.In perl ,it uses eval to catch exception.So ,I write two simple programs to test it.

testEval1.pl:

 $exp = '$i = 3; die "error message"; $k = $i + $j';

push ( @program, '$i = 3; die "error message"; $k = $i + $j');
 $rtn =eval($exp);
    if ( ! defined ( $rtn))
    {
       print "Exception: " , $@,"\n";
    }
    else
    {
       print $rtn,"\n";
    }

output of testEval1.pl:

code continue to run after die!
Exception: error message at (eval 1) line 1.

testEval2.pl

$baseDir = "/home/wuchang/newStore1";
my $eval_rtn = eval(opendir(BASEDIR,$baseDir) or die "dir doesn't exist!\n");
print "code continue to run after die!\n";
if(!defined($eval_rtn)){
print $@;
}
 else
    {
       print $rtn,"\n";
    }

output of testEval2.pl:

dir doesn't exist!

you can see that in the two code examples , the code block of eval both has die expressions.But in testEval1.pl,the code after eval can be excuted,while in testEval2.pl,it's not! So ,my question is ,what's the difference ? What can I do to make the program continue to run even if a "dir doesn't exist" exception happeded ?

thank you.

Answer 1

You're evaling result of

opendir(BASEDIR,$baseDir) or die "dir doesn't exist!\n"

code. If it would succeed that would be equivalent of eval(1).

What you want is eval BLOCK:

my $eval_rtn = eval{ opendir(BASEDIR,$baseDir) or die "dir doesn't exist!\n" };

Check perldoc -f eval for difference between eval EXPR and eval BLOCK

Answer 2

To answer your question title:

Will code after eval(die “some error message”) continue to be executed?

The answer is "No". But please read on, because this is not a problem, but a misunderstanding about the Perl syntax involved.

The line:

my $eval_rtn = eval( opendir(BASEDIR,$baseDir) or die "dir doesn't exist!\n" );

Does not get as far as running the eval. The syntax you have used with (..) brackets takes a scalar value, and before the eval does anything at all, it is waiting for the opendir...or die expression to return a string (which will then be evaluated). To make it equivalent to your other example, you could make the param a string:

my $eval_rtn = eval( q{opendir(BASEDIR,$baseDir) or die "dir doesn't exist!\n"} );

You could also use the block form instead:

my $eval_rtn = eval { opendir(BASEDIR,$baseDir) or die "dir doesn't exist!\n"; };

I would recommend using the block form where possible, it is usually easier to debug, and in your case better matches the exception handling semantics that you want to achieve.