Yes! This was one of the biggest things I missed when I moved to Ruby. I try and...

kbenson · on Nov 10, 2015

I love it as well, and would hate to do without it, but it does come with it's own warts. Such as this:

    use Data::Dumper;
    my %h;
    if ( $h{foo}{bar}{baz} ) { say "Never happens"; }
    say Dumper \%h;

And you get this:

    $VAR1 = {
              'foo' => {
                         'bar' => {}
                       }
            };

colonelguy · on Nov 10, 2015

You'd want to use the exists operator in that case. It checks if the hash key is present without auto vivifying it.

  my %hash = ();
  if (exists $hash{foo}) {print "This doesn't run!";}

kbenson · on Nov 10, 2015

I'm well aware of exists, and how it functions, and that it in no way solves the problem presented. Exists tests for the existence of a hash key, so you can tell if it exists but is possibly undefined, but it does not stop autovivification in any way.

Your example doesn't cause autovivification even without exists. Autovivification is the automatic creation of the underlying hashes and arrays in a multiple level data structure when they are used while accessing a nested data-structure.

For example, given an empty hash %hash, $hash{foo} does not cause autovivification, but $hash{foo}{bar} will automatically create an empty hash and assign a reference to it to $hash{foo}.

raiph · on Nov 11, 2015

Fwiw Perl 6 only autovivifies anything, including intermediary data structure levels, when writing to a data structure. So nothing happens in this case:

  my %h;
  dd %h
  if %h<foo><bar><baz> { say "Never happens" }
  dd %h

prints:

  Hash $var = {}
  Hash $var = {}

kbenson · on Nov 11, 2015

Nice, I wasn't aware of that feature, but the features I'm still not aware of in Perl 6 could fill a small book, since I still haven't gotten around to using it for much. :/