There are, it seems, only a few nodes which speak of lvalue subroutines, such as this, which is surprising since they can be quite useful.

The question is, though, how do you know what context the lvalue subroutine is being called in? Here's a simple example: 
my $foo;
my $foo_new;

sub fooz : lvalue
{
     $foo;
}

$foo = "Bar";      # Initialization
print fooz,"\n";   # Reading
fooz = "Foo";      # Assignment
print fooz,"\n";   # Reading

[download]
Within the subroutine, I'm looking for a mechanism similar to wantarray which can return some information about what side of the assignment the sub is on. Here's what I'd like to be able to do: 
my $foo;
my $foo_new;

sub fooz : lvalue
{
     defined ($foo_new)?
        $foo_new
        : (islvalue?
              $foo_new = $foo
              : $foo);
}

$foo = "Bar";      # Initialization
print fooz,"\n";   # Reading
fooz = "Foo";      # Assignment
print fooz,"\n";   # Reading

[download]
Here the idea is that any writes go into $foo_new, while the reads come from $foo, unless of course there is a $foo_new, in which case they will read from $foo_new.

This all hinges on their being some sort of hint about what side the sub is appearing on, left or right.

Admittely, this is oversimplified, and in this example there will be issues if one decides to assign 'undef' to fooz(). The code is merely for demonstration purposes.

If there is no mechanism presently, is this kind of thing worth implementing as a patch?

In reply to Side Context in an 'lvalue' Subroutine by tadman

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