There's nothing I can think of and the documentation suggests it's not something that is easy to do.

One could reverse the order so $A_obj is checked first: my $n2 = -$A_obj + $B_obj;. That's obviously not practical, though, and would also need neg overloading on $A_obj (and addition if not already done).

There's nothing I can think of and the documentation suggests it's not something that is easy to do.

Yes - the first point I read in that link to the documentation pretty much kills all hope:

1.If the first operand has declared a subroutine to overload the opera
+tor then use that implementation.
[download]

I suppose (untested) I could do:

my $n2 = (\$B_obj) - $A_obj;
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and that would at least call module A's oload_minus() subroutine .... which would then be structured to de-reference the first argument and return the intended result.
But that solution is no more practical than the alternative you provided.

Thanks for the reply. I had, of course, consulted the overloading docs but had stopped reading before reaching the bit to which you linked.
It's not the end of the world if I have to modify module B.

Cheers,
Rob

If you are working around it in the caller, this is clearer, if not as efficient:

my $n2 = -($A_obj - $B_obj);
[download]

#!/usr/bin/perl
use warnings;
use strict;
use experimental qw( signatures );

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

    sub minus($x, $y, $swap) {
        return ref($x)->new(
            ($y->isa('My::B') ? $x->[0] - $y->{value}
                              : ($x->[0] - $y->[0])
            ) * (1, -1)[$swap]
        )
    }

    sub new($class, $value) {
        bless [$value], $class
    }

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

    sub new($class, $value) {
        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;
[download]

You can do ugly things to B even from within A

Yes - and my overloading of the arithmetic operations are done in XSubs - which leads to a fair bit of code clutter, but is procedurally clean and simple (IMO, at least).
I actually don't fully understand the code you've provided, but I'll certainly try to remedy that when I get the chance. (I'm not really a proper perl programmer.)

Having decided that Math::MPC's basic arithmetic operators should be capable of handling Math::MPFR objects (since the MPC library already provides functions that perform such arithmetic), it's a fairly simple exercise to insert the few required lines of code into the relevant Math::MPC XSubs.
If not for this "conundrum", that would be the end of the exercise.
Math::MPFR, which has no business to engage in arithmetic with Math::MPC objects, could be left as is - with no need to even mention this new capability in the Math::MPFR docs.

The solution I use is simple enough, but it involves making changes to the Math::MPFR overloading XSubs.

It's only a matter of inserting one line of code into a new if(condition){} block in each of those XSubs. And it just does a callback to (eg) Math::MPC::overload_minus(mpc_object, mpfr_object, &PL_sv_yes) with the assistance of a 20-line macro.
I'm happy enough with that, and I've used that technique without issue in a number of cross-class overloading situations - though I have no idea how expensive that solution is.
There's even no need to check whether Math::MPC has been loaded. If a Math::MPC object has been passed to a Math::MPFR function then we know that perl has loaded Math::MPC.
It's just a bit annoying that I have to go that extra step.
It would be much better if I could simply tell perl's overloading process "hey, in this case, use the overloading subroutine of the second object". But ... I can't do that :-((

Cheers,
Rob

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