undefined value as an ARRAY reference sometimes ok

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

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Re: undefined value as an ARRAY reference sometimes ok by ikegami (Patriarch) on Dec 14, 2011 at 06:47 UTC
Autovifivication means that `EXPR1->[ EXPR2 ]` [download] is short for `( EXPR1 //= [] )->[ EXPR2 ]` [download] This makes sense: `$list //= []` [download] This doesn't make sense: `get_no_list() //= []` [download] You can't assign to a non-lvalue function. If you want the first element of a list, you can use a list slice. `( some_list() )[0] # Returns the first element or an empty list.` [download]	[reply] [d/l] [select]
Re: undefined value as an ARRAY reference sometimes ok by chilledham (Friar) on Dec 13, 2011 at 23:13 UTC
See: What does Autovivify mean? Calling `$list->[0]` causes `$list` to autovivify. You can see this happening here: `#!perl use strict; my $list = undef; print ref $list, "\n"; # should print only a blank line my $item = $list->[0]; print ref $list, "\n"; # should print ARRAY` [download]	[reply] [d/l] [select]
Re^2: undefined value as an ARRAY reference sometimes ok by Anonymous Monk on Dec 13, 2011 at 23:52 UTC
Maybe I missed it, but the refs you gave didn't explain why this didn't autovivify: `my $item = get_no_list()->[0]; #no way` [download]	[reply] [d/l]
Re^3: undefined value as an ARRAY reference sometimes ok by chilledham (Friar) on Dec 13, 2011 at 23:57 UTC
Sorry for the lack of clarity in my first response. It's the attempt to dereference `$list` that causes the autovivification. In your last example, you are attempting to dereference your function call, not `$list`.	[reply] [d/l] [select]
Re: undefined value as an ARRAY reference sometimes ok by Anonymous Monk on Dec 14, 2011 at 00:31 UTC
It is not an inconsistency only lvalues get autovivified, and undef is not an lvalue `$ perl -le " sub dang : lvalue { undef } dang()=1; " Can't return undef from lvalue subroutine at -e line 1. $ perl -le " sub dang : lvalue { undef } dang()->[0] " Can't use an undefined value as an ARRAY reference at -e line 1. $ perl -le " undef->[0] " Can't use an undefined value as an ARRAY reference at -e line 1. $ perl -le " my $variable = undef; $variable->[0] "` [download] Even though undef can resemble an lvalue (hey, its on the left of an assignment), `( undef, undef, my $mode ) = stat 'filename';` [download] you can't assing a value to undef, so undef is not an lvalue `$ perl -le " undef = 1; " Modification of a read-only value attempted at -e line 1.` [download] See References quick reference, autovivification, perlglossary#autovivification, autovivification, lvalue	[reply] [d/l] [select]