Perl Language eval and die


Example

This is the built-in way to deal with "exceptions" without relying on third party libraries like Try::Tiny.

my $ret;

eval {
  $ret = some_function_that_might_die();
  1;
} or do {
  my $eval_error = $@ || "Zombie error!";
  handle_error($eval_error);
};

# use $ret

We "abuse" the fact that die has a false return value, and the return value of the overall code block is the value of the last expression in the code block:

  • if $ret is assigned to successfully, then the 1; expression is the last thing that happens in the eval code block. The eval code block thus has a true value, so the or do block does not run.
  • if some_function_that_might_die() does die, then the last thing that happens in the eval code block is the die. The eval code block thus has a false value and the or do block does run.
  • The first thing you must do in the or do block is read $@. This global variable will hold whatever argument was passed to die. The || "Zombie Error" guard is popular, but unnecessary in the general case.

This is important to understand because some not all code does fail by calling die, but the same structure can be used regardless. Consider a database function that returns:

  • the number of rows affected on success
  • '0 but true' if the query is successful but no rows were affected
  • 0 if the query was not successful.

In that case you can still use the same idiom, but you have to skip the final 1;, and this function has to be the last thing in the eval. Something like this:

eval {
  my $value = My::Database::retrieve($my_thing); # dies on fail
  $value->set_status("Completed");
  $value->set_completed_timestamp(time());
  $value->update(); # returns false value on fail
} or do { # handles both the die and the 0 return value
  my $eval_error = $@ || "Zombie error!";
  handle_error($eval_error);
};