From: Marcin 'Qrczak' Kowalczyk <qrczak@...>
Date: 2003-10-02T05:19:35+09:00
Subject: Re: Making == symmetric?

W li�cie z �ro, 01-10-2003, godz. 21:42, Sean O'Dell pisze:

> The notion of a symmetrical equality test is an abstract concept.  In 
> reality, whether it's Ruby or anything else, testing is NOT symmetrical.

The method used to determine the outcome is not symmetrical internally,
but the outcome is (or rather: should be).

> If you say:
> 
>    if (a == b) then
> 
> ... then you are accepting one pass.  If you say:
> 
>    if (a == b and b == a) then
> 
> ... then you are demanding a second pass and trying to get a better idea 
> if they are equal or not.

My concept of == requires that they give the same result, i.e. if they
differ then some definition of == must be insane (probably a programmer-
-supplied interpretation of == for a particular type).

I mean that if an algorithm relies on == being symmetrical but doesn't
explicitly check both directions and fails because of this, then the
fault is at the definition of ==, not in the algorithm which uses ==.

It wouldn't know what to do with conflicting answers anyway, except to
signal error. Why do you use "and" and not "or" for example?

Well, another, equally valid requirement is x==x, which is usually not
satisied by NaN. I say: it is indeed against the rules; sometimes it's
useful to make an exception. You shouldn't rely much on equality of
floats anyway.

> > I don't say that the language should enforce == to be symmetric. I say
> > that the language should allow to define a symmetric == with the desired
> > meaning.
> 
> But it does.  You can very easily override == just as I described in my 
> example code.  You can make == anything you want it to be, for any 
> class, existing or not.

How to make a new numeric type (say, long integer objects from another
language or library wrapped as Ruby objects) and make == between this
type and other numeric types compute the equality of numbers being
represented? You don't know all other numeric types.

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