Advanced Perl Programming

The Block Form: Exception Handling

In this form, eval is followed by a block of code, not a scalar containing a string. It is used for handling run-time errors, or exceptions. Errors can be internal built-in ones (out-of-memory, divide-by-zero) or user-defined ones produced by die.

The following example shows how you can use the block form eval to trap a run-time divide-by-zero error:

eval {

$a = 10; $b = 0;
     $c = $a / $b;     # Causes a run-time error, 
                       # which is trapped by eval
};
print $@;   # Prints  "Illegal division by 0 at try.pl line 3"

When the script is compiled, Perl syntax-checks the block of code and generates code. If it encounters a run-time error, Perl skips the rest of the eval block and sets $@ to the corresponding error text.

To signal your own errors, you use die. Perl knows whether a piece of code is currently executing inside an eval, and so, when die is called, Perl simply gives the error string — die’s argument — to the global $@, and jumps to the statement following the eval block. In the following example, open_file invokes die if it has trouble opening a file. To use this function, wrap it inside an eval.

sub open_file {
    open (F, $_[0]) || die "Could not open file: $!";
}

$f = 'test.dat';
while (1) {    
    eval {
                       open_file($f);# if open_file dies, the program doesn't quit
    };
    last unless $@;     # no error: break out of the loop.
    print "$f is not present. Please enter new file name $f";
    chomp($f = <STDIN>);
}

Java/C++ programmers would of course recognize the parallel to ...