From: Robert Klemme <bob.news@...>
Date: 2005-11-10T18:32:49+09:00
Subject: Re: Numeric <=>

daz wrote:
> Robert Klemme wrote:
>> Christophe Grandsire wrote:
>>> Selon ChrisH:
>>>
>>>> Thanks Eric.  I just wonder why Numeric doesn't implement it
>>>> properly?
>>>
>>> Probably because it can't. I'm not completely up-to-date with the
>>> Numeric inheritance chain, but I guess for instance Complex numbers
>>> also inherit from Numeric. Being Numeric doesn't necessarily mean
>>> that there is a meaningful order among the objects.
>>
>> Although I agree with your analysis that Numeric can't properly
>> implement <=> the question remains why <=> is actually implemented
>> in Numeric.  ATM I cannot see what necessitates it.  Including
>> Comparable is not a reason IMHO because that will break either way
>> (i.e. with missing <=> and with incomplete implemented <=>).
>>
>>> Then again, I'm out on a limb here, and may be completely wrong.
>>
>> Not completely but I have the feeling we're still not there.  Someone
>> probably needs to take the time and have a look at the sources...
>>
>
> #-------------
> class Roo < Numeric; end
>
> a = Roo.new
> b = Roo.new
> c = a
>
> p a <=> b  #-> nil
> p a <=> c  #-> 0
> #-------------
>
> Well, that's helpful!
>
> Ruby (like me) hasn't a clue what a 'Roo' is,
> but it's able to tell that a == c which might
> be enough to keep a routine running.

Definitely not:

>> ar=(0..2).map { Roo.new }
=> [#<Roo:0x101957e8>, #<Roo:0x101957b8>, #<Roo:0x10195788>]
>> ar.sort
ArgumentError: comparison of Roo with Roo failed
        from (irb):14:in `sort'
        from (irb):14
        from :0

>> Roo.new < Roo.new
ArgumentError: comparison of Roo with Roo failed
        from (irb):21:in `<'
        from (irb):21
        from :0

This is what happens in the "standard cases".

>> class Bar;include Comparable; end
=> Bar
>> ar=(0..2).map { Bar.new }
=> [#<Bar:0x10182360>, #<Bar:0x10182330>, #<Bar:0x10182300>]
>> ar.sort
NoMethodError: undefined method `<=>' for #<Bar:0x10182360>
        from (irb):20:in `sort'
        from (irb):20
        from :0


>> Foo = Class.new
=> Foo
>> ar=(0..2).map { Foo.new }
=> [#<Foo:0x1018bc60>, #<Foo:0x1018bbe8>, #<Foo:0x1018bbb8>]
>> ar.sort
NoMethodError: undefined method `<=>' for #<Foo:0x1018bc60>
        from (irb):17:in `sort'
        from (irb):17
        from :0


> <Changelog>
> Wed Nov 20 01:52:21 2002  Yukihiro Matsumoto  <matz@ruby-lang.org>
>
>  * numeric.c (num_cmp): added to satisfy Comparable assumption.

Hm...  Doesn't sound as if the assumption was satisfied...

>
http://www.ruby-lang.org/cgi-bin/cvsweb.cgi/ruby/numeric.c.diff?r1=1.59;r2=1.60;f=h

Kind regards

    robert