[ruby-core:59672] Re: [ruby-trunk - Feature #7688] Error hiding with rb_rescue() on Comparable#==, #coerce and others

On Fri, Jan 10, 2014 at 06:03:03AM +0900, Eregon (Benoit Daloze) wrote:
> 
> Issue #7688 has been updated by Eregon (Benoit Daloze).
> 
> 
> @tenderlove These are probably bugs then, is it not? I will try to have a look.

I can't say for sure whether or not it's bugs, but I can say I don't
really like this change.

Say you write a class like this:

class MyObject
  include Comparable
  def <=> other
    raise ArgumentError unless other.is_a?(MyObject)
    # Do some comparisons
  end
end

I raise an argument error because they are not comparable.  To me it
implies that `self` and `other` are also not equal, but not that `==`
should raise an exception.

I'd never expect this to raise an exception regardless of the
implementation of <=>:

MyObject.new != 10

IOW it seems like <=> is to ==, what respond_to? is to method_missing.
Anyway, I'm not a *huge* fan, but this does break our tests (though I
can fix them).  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).

Could we find a middle ground and just rescue ArgumentError?  Or some
sort of NonComparable error?

-- 
Aaron Patterson
http://tenderlovemaking.com/