Perl 6 and Parrot Essentials, 2nd Edition

Lvalue Subroutines

Lvalue subroutines pretend to be assignable values, just like any ordinary variable. They do this by returning a proxy variable which handles the lvalue behavior for the subroutine (fetch, store, etc.). You declare an lvalue subroutine with the is rw property:

sub storage is rw { . . . }

storage( ) = 5;

An lvalue sub can return an ordinary variable which acts as a proxy, return the return value from another lvalue sub, or it can return a tied proxy variable defined within the sub:

my sub assignable is rw {
    my $proxy is Proxy(
        FETCH => { . . . },
        STORE => { . . . },
         . . . 
    );
    return $proxy;
}

This example defines an lvalue sub named assignable. It creates a proxy variable tied to a Proxy class that defines FETCH and STORE tie methods on the fly.