From: Ben Giddings <bg-rubytalk@...>
Date: 2003-10-01T12:49:21+09:00
Subject: Re: Making == symmetric?

Nathan Weston wrote:
> def compare(a,b)
>    if a.can_compare_to?(b)
>       return a.compare_to(b)
>    elsif b.can_compare_to?(a)
>       return b.compare_to(a)
>    else
>      return false
>    end
> end

How about

def compare(a, b)
   retval = false
   if a.can_compare_to(b)
     retval = a.compare_to(b)
   end
   if retval and b.can_compare_to(a)
     retval = b.compare_to(a)
   end

   return retval
end

This way if both of them know how to compare themselves to the other one, 
the comparison returns true only if both agree they're equal.

Aside from the code though, I'm not sure if I like the idea.  While in 
principle I agree that equality tests should be symmetrical, I also think 
that it is more natural to have an equality operator which you can 
overload.  I think you'd still need "can_compare_to?" to know if 'a' knows 
how to compare itself to an instance of 'b'...

Maybe instead:

def compare(a, b)
   retval = false
   if a.can_compare_to?(b)
     retval = a.==(b)
   end
   if retval and b.can_compare_to?(a)
     retval = b.==(a)
   end

   return retval
end

Interesting food for thought though.

Ben