I actually don't fully understand the code you've provided, but I'll certainly try to remedy that when I get the chance.

First up, I wanted to remove the "experimental" requirement - assuming it's not a crucial piece of the demo. (I'm not at all familiar with experimental stuff, and I don't have much interest in it.)
I've also added some dumps of some more subtractions.
So, I've arrived at this modified version of choroba's script :

use warnings;
use strict;
#use experimental qw( signatures );

{   package My::A;
    use overload '-' => \−

#   sub minus($x, $y, $swap) { ############ Replaced by next line ##
    sub minus { my($x, $y, $swap) = (shift, shift, shift);
        return ref($x)->new(
            ($y->isa('My::B') ? $x->[0] - $y->{value}
                              : ($x->[0] - $y->[0])
            ) * (1, -1)[$swap]
        )
    }

#   sub new($class, $value) { ############ Replaced by next line ##
    sub new { my($class, $value) = (shift, shift);
        bless [$value], $class
    }

    # We can change My::B outside of B, in fact.
    eval q{ package My::B;
            my $m = My::B->can('minus') or die 'My::B not loaded';
            use overload "-" => sub  { my($x, $y, $s) = (shift, shift,
+ shift);
                return $y->isa('My::A') ? $y->minus($x, 1) : $m->($x, 
+$y, $s)
            }
    };
}

{   package My::B; # Can't be changed!
    use overload '-' => \−

#   sub minus($x, $y, $swap) { ############ Replaced by next line ##
    sub minus { my($x, $y, $swap) = (shift, shift, shift);
        my $subtr = $x->{value} - $y->{value};
        return ref($x)->new($swap ? -$subtr : $subtr)
    }

#   sub new($class, $value) { ############ Replaced by next line ##
    sub new { my($class, $value) = (shift, shift);
        bless {value => $value}, $class
    }
}

my $a0 = 'My::A'->new(16);
my $a1 = 'My::A'->new(6);
my $b0 = 'My::B'->new(16);
my $b1 = 'My::B'->new(6);

use Data::Dumper;
print Dumper A => $a0 - $a1;
print Dumper B => $a0 - $b1;
print Dumper C => $b0 - $b1;
print Dumper D => $b0 - $a1;
print Dumper AA => $a1 - $a0;
print Dumper BB => $b1 - $a0; 
print Dumper CC => $b1 - $b0;
print Dumper DD => $a1 - $b0;
[download]

At this point, I have 2 questions:
1. Where is can documented ?
2. How is it that I can use isa even though no features have been enabled ?

Regarding the first question:

>perldoc -f can
No documentation for perl function 'can' found
[download]

WRT the second question, the perlop documentation says this regarding isa:

    This feature is available from Perl 5.31.6 onwards when enabled by
+ "use
    feature 'isa'". This feature is enabled automatically by a "use v5
+.36"
    (or higher) declaration in the current scope.
[download]

I have done neither of those things. Has perl-5.42.0 automatically called that feature for me ? ... or does it automatically call "use v5.36" ?

Cheers,
Rob

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.