Re^2: When DOESN'T "Use of uninitialized value" show up?

Autovivification only happens when required

Be careful with such statements:

$ perl -le 'my $a; defined $a->{b}{c}; print ref $a'
HASH
[download]

Autovivification is not required here, but still the hash access autovivifies $a to { b => {} }.

Perl 6 - second systems done right

Comment on Re^2: When DOESN'T "Use of uninitialized value" show up? Select or Download Code

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Re^3: When DOESN'T "Use of uninitialized value" show up? by GrandFather (Saint) on Dec 16, 2011 at 22:44 UTC
Actually that was exactly the case I had in mind when I made the statement. "b => {}" is required so that the presence of {c} can be checked. In fact I updated the OP's sample to include `exists $x->{notdef}{notdeftoo}` to demonstrate this case. True laziness is hard work	[reply] [d/l]
Re^4: When DOESN'T "Use of uninitialized value" show up? by moritz (Cardinal) on Dec 17, 2011 at 08:36 UTC
"b => {}" is required so that the presence of {c} can be checked. And that's the part where I disagree. If `$a->{b}` doesn't exist, we know that `$a->{b}{c}` cannot exist, and perl could know too. The fact that such reads autovivify are really only due to a quirk in the implementation, and not deducible from a general rule like "autovivification happens only when it needs to". Perl 6 - second systems done right	[reply] [d/l] [select]
Re^5: When DOESN'T "Use of uninitialized value" show up? by ikegami (Patriarch) on Dec 17, 2011 at 09:12 UTC
And that's the part where I disagree. If $a->{b} doesn't exist, we know that $a->{b}{c} cannot exist, But he isn't checking if `$a->{b}` exists, so it's possible that `$a->{b}{c}` can exist. Perl is being asked if `$a->{b}{c}` exists, and it's being done a few steps after autovivifying `$a->{b}`.	[reply] [d/l] [select]
Re^3: When DOESN'T "Use of uninitialized value" show up? by Skeeve (Parson) on Dec 16, 2011 at 22:49 UTC
But that's what Grandfather said: " However if a chain of references is required all the intermediate references will autovivify, but not the last item in the chain " didn't he? `s$$([},&%#}/&/]+}%&{});#$&&s&&$^X.($'^"%]=\&(\|?{%` `+`.+=%;.#_}\&"^"-+%*).}%:##%}={~=~:.")&e&&s""`$''`"e	[reply] [d/l] [select]