From: "Martin J. Dürst" <duerst@...>
Date: 2014-01-10T16:16:13+09:00
Subject: [ruby-core:59679] Re: [ruby-trunk - Feature #7688] Error hiding with rb_rescue() on Comparable#==, #coerce and others

On 2014/01/10 7:42, marcandre (Marc-Andre Lafortune) wrote:
>
> Issue #7688 has been updated by marcandre (Marc-Andre Lafortune).
>
>
> tenderlovemaking (Aaron Patterson) wrote:
>>   It seems like I would have to implement `==` with
>>   exactly the same logic as `<=>`, except return nil (to indicate it isn't
>>   comparable) instead of raise an exception (which is exactly what == does
>>   before this change).
>
> Not sure I follow... You don't have to implement `==` at all in your example.
> Moreover `==` should not return `nil`, it is `<=>` that should return `nil`.
> Neither `==`, `!=` nor `<=>` should ever raise exceptions.

I agree with Marc. The negative/0/positive convention of the <=> 
operator effectively comes from C functions such as strcmp. For such 
functions, there's no case of two arguments being incomparable (and 
therefore not equal) because the types are checked at compile time.

So we have to extend the three cases of negative/0/positive to four if 
we want to handle both comparable and incomparable cases. Here's a table 
of how it may/should work:

         comparable		incomparable
	neg	0	pos	nil
<	true	false	false	exception?
<=	true	true	false	exception?
==	false	true	false	false
!=	true	false	true	true
 >=	false	true	true	exception?
 >	false	false	true	exception?

Regards,    Martin.