From: Marcin 'Qrczak' Kowalczyk <qrczak@...>
Date: 2003-10-02T03:56:09+09:00
Subject: Re: Making == symmetric?

W li�cie z �ro, 01-10-2003, godz. 20:34, Mark Wilson pisze:

> I think Ruby does allow one to define a symmetric ==.

Only if you agree to limit cases of "different class but equal" to those
where both classes know each other. Because otherwise you would have to
change the implementation of == in other classes which were already
written elsewhere.

For example there doesn't seem to be a way to add a numeric type which
plays well with other numeric types wrt. ==, including numeric types you
don't know about.

> As a general 
> matter, I think a language is only required to enforce a symmetric == 
> for objects of the same class (or values of the same type) in order to 
> have algebraic closure.

I prefer to trust the programmer in this case. The only sensible way I
imagine in which the language could enforce a symmetric == is to execute
the programmer-provided equality in both directions and throw error if
they disagree, and this is too little benefit (catching rare bugs) for
too much cost (performance of a common operation). Turning the
disagreement into false or true would mask an error, so it only
discovers bugs - it doesn't add functionality.

-- 
   __("<         Marcin Kowalczyk
   \__/       qrczak@knm.org.pl
    ^^     http://qrnik.knm.org.pl/~qrczak/