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=head1 TITLE
Data: overloading via the SECOND operand if needed
=head1 VERSION
Maintainer: Ilya Zakharevich <ilya@math.ohio-state.edu>
Date: 15 September 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.