=head1 TITLE

Data: overloading via the SECOND operand if needed

=head1 VERSION

  Maintainer: Ilya Zakharevich <ilya@math.ohio-state.edu>
  Date: 15 Sep 2000
  Mailing List: perl6-language-data@perl.org
  Number: 234
  Version: 1
  Status: Developing

=head1 ABSTRACT

This RFC proposes a support of a situation when a more-knowledgable module may
steal overloading from a less-knowledgable module or visa versa;

=head1 DESCRIPTION

The problem: if both arguments to a binary operation are overloaded objects,
then with the current implementation the overloaded method registered for
the I<first> argument is loaded.

Sometimes it leads to problems: for example, C<Math::BigFloat> objects support
C<Math::BigInt> objects as the other arguments for binary operations, but
not visa versa, so

   print $bigfloat + $bigint;		# Works
   print $bigint   + $bigfloat;		# Does not

Solution: provide a pragma to inform the overloading mechanism that
C<Math::BigFloat> is more versatile than C<Math::BigInt>

   use overload ':override', 'Math::BigFloat', 'Math::BigInt';

This would make

    $bigint * $bigfloat

call 

    $bigfloat->MULTIPLY($bigint, 1)

instead of

    $bigint->MULTIPLY($bigfloat, 0)

(assuming that the MULTIPLY methods are registered for C<'*'> key).

This requires additional costly checks from the overloading engine.
However, it is not hard to implement this with the following features:

=over

=item

no cost if not used in the script;

=item

almost no cost if the first and the second argument are in the same package;

=item

almost no cost if the second argument is does not override anything;

=item

almost no cost if the first argument is not overriden by anything;

=item

in the worst case: costs a hash lookup in a hash.

=back

Given this, the additional overhead of the hash lookup incurs only if needed,
this may be ignored.

Overriding does not chain automatically, but if C<:override_chain> is used
instead of C<:override>, the overrider will also override packages overriden
by the package it overrides.

A method to query overriding info should be provided.

If C<A> I<isa> C<B>, and C<B> overrides C<C>, then C<A> overrides C<C>.
Similarly, if C<C> I<isa> C<D>, then C<A> overrides C<D>.

=head1 MIGRATION ISSUES

None.

=head1 IMPLEMENTATION

Keep a global flag whether override was ever requested.  Keep a flag in
each package whether it is overrided by anything.  Keep a hash of packages
which the given package overrides.  Keep these data in the overloading
table of the package.

=head1 REFERENCES

None.