Inconsistent autovivification

[p5pRT]

Migrated from rt.perl.org#126432 (status was 'rejected')

Searchable as RT126432$

[p5pRT]

From hjp@hjp.at

Consider the attached test script.

It tries to use a non-existing node in a (deep) hash as an arrayref. First in the condition of a for loop​:

for my $node (@​{ $cache->{$facttablename}{$parent_selection_key}{$relevant_roles}{path}

Then in list context in an expression​:

...
  = [
  @​{ $cache->{$facttablename}{$parent_selection_key}{$relevant_roles}{path} },
  ...
  ];

The first one works (autovivifying ...->{path} to []), the second dies with the message

Can't use an undefined value as an ARRAY reference at ./foo line 36

I don't see a meaningful difference between those two use cases. Either both should autovivify or both should die.

[p5pRT]

From hjp@hjp.at

foo

[p5pRT]

From @iabyn

On Fri, Oct 23, 2015 at 01​:54​:33AM -0700, Peter J. Holzer wrote​:

# New Ticket Created by Peter J. Holzer
# Please include the string​: [perl #126432]
# in the subject line of all future correspondence about this issue.
# <URL​: https://rt-archive.perl.org/perl5/Ticket/Display.html?id=126432 >

Consider the attached test script.

It tries to use a non-existing node in a (deep) hash as an arrayref. First in the condition of a for loop​:

for my $node (@​{ $cache->{$facttablename}{$parent_selection_key}{$relevant_roles}{path}

Then in list context in an expression​:

...
= [
@​{ $cache->{$facttablename}{$parent_selection_key}{$relevant_roles}{path} },
...
];

The first one works (autovivifying ...->{path} to []), the second dies with the message

Can't use an undefined value as an ARRAY reference at ./foo line 36

I don't see a meaningful difference between those two use cases. Either
both should autovivify or both should die.

Using a reduced version of your example​:

  use strict;
  my $r = {};
  for my $node (@​{ $r->{foo} }) {}
  () = @​{ $r->{bar} };

In the for loop, the array dereference is in lvalue context, since $node
becomes a writeable alias to each element in @​{$r->foo} in turn.

In the empty list assignment, the array derefernce is in rvalue context,
ans so doesn't auto-vivify.

It's basically the same as

  @​{ $r->{foo} } = @​a; # lvalue
  @​a = @​{ $r->{bar} }; # rvalue

--
Never do today what you can put off till tomorrow.

[p5pRT]

The RT System itself - Status changed from 'new' to 'open'

[p5pRT]

@iabyn - Status changed from 'open' to 'rejected'