[SOLVED] How do I use Perl keys() from a function return value?

How do I use Perl keys() from a function return value?

I know about the Perl keys() function and would like to use it on a hash I have, but don't see any reason to set a variable to it. Let me explain in code two ways:

Normal:

my %hash = ReturnsHash();
foreach (keys(%hash)) {
    ...code...
}

The way I'd like:

foreach (keys(ReturnsHash())) {
    ...code...
}

With the second way I get this error message(Type of arg 1 to keys must be hash or array (not subroutine entry)) Is the second way possible? If so how?

Solution

Contrary to its name, ReturnsHash doesn't return a hash. It's impossible for a sub to return a hash. Subs can only return a list of scalars. If you do

sub ReturnsHash {
   my %hash = ( a => 1, b => 2 );
   return %hash;
}

you're doing

return 'a', 1, 'b', 2;

Since you can't do

keys('a', 1, 'b', 2)

you can't do

keys(ReturnsHash())

Now, what you could do instead is return a reference to a hash

sub ReturnsHashRef {
   my %hash = ( a => 1, b => 2 );
   return \%hash;
}

aka

sub ReturnsHashRef {
   return { a => 1, b => 2 };
}

in which case you can do

keys(%{ ReturnsHashRef() })

Alternative

You can do

for my ( $k, $v ) ( a => 1, b => 2 ) {
   ...
}

so you can do

for my ( $k, $v ) ( ReturnsHash() ) {
   ...
}

This requires Perl 5.36+, although it produces a (safe-to-ignore) experimental warning until Perl 5.40.