Using grep on undefined array

[Matt Lawrence]

On 14/08/13 08:33, William Blunn wrote:
> On 14/08/2013 00:09, Andrew Beverley wrote:
>> Could someone please explain to me why the following outputs an empty
>> string rather than "*"?
>
> In this case it may be wise to show a “use strict;”, which I assume is 
> in effect here, otherwise people can just say that @$a on an undefined 
> $a in the absence of "use strict;" would fine anyway.
>
>> get();
>> sub get($)
>> {   my $fields = shift;
>>      my @fields = grep $_ ne 'domain', @$fields;
>>      my $select_fields = $fields ? join(',', map { 'users.' . $_ } 
>> @fields) : '*';
>>      print "$select_fields\n";
>> }
>>
>> I would have expected $fields to remain undefined, but it seems to be
>> turning into an empty array during the grep.
>
> We can write a simpler case to illustrate the point.
>
> This demonstrates the effect of "strict" on @$a when $a is undefined:
>
>     $ perl -E 'use strict; my $a; say @$a;'
>     Can't use an undefined value as an ARRAY reference at -e line 1.
>
> but things seem to work differently when @$a an argument to "grep":
>
>     $ perl -E 'use strict; my $a; say grep { 1 } @$a; say $a'
>
>     ARRAY(0x7fd3b4003ed0)
>
> "strict" doesn't appear to apply in this case and @$a appears to be 
> being autovivified leaving a reference in $a.
>
> That being the case, I'd also be interested to know why.
>
My understanding is that autovivification only happens in lvalue 
contexts, which applies in the case of arguments to grep and similar 
situations because of aliasing (or something *waves hands*)

    $ perl -Mstrict -wE 'my $foo; say for keys %$foo;'

Matt