planetp planetp - 1 month ago 6
Perl Question

Why does map() mask the 'undefined value' error?

I'm wondering why a call to

map
in the second snippet makes the 'undefined value' error gone?

use strict;
use warnings;
my $x;
my @a = @{ $x }; # error: Can't use an undefined value as an ARRAY reference


Compare to:

use strict;
use warnings;
my $x;
my @a = map $_, @{ $x }; # no error, @a is empty

Answer

This is due to the way that map() does aliasing (it's essentially using a for() loop). What's happening is that the aref is being used in l-value context, and therefore is being auto-vivified into existence.

In your former example, you're attempting to use the aref directly in r-value context, which is why it generates the error (because no auto-vivification happens).

You can simplify your test to use for(), and you'll get the same result as with map():

use warnings;
use strict;

my $x;

for (@{ $x }){
    print "$_\n";
}

...no output.

To visually see that $x was auto-vivified as an array reference, you can use the ref() function:

my $x;
my @a = map $_, @{ $x };

print ref $x;

Output:

ARRAY