From: Martin Duerst <duerst@...>
Date: 2007-12-24T16:29:02+09:00
Subject: Re: Why is Array not Comparable

At 10:40 07/12/23, Rick DeNatale wrote:
>On Dec 22, 2007 8:17 PM, Martin Duerst <duerst@it.aoyama.ac.jp> wrote:
>> I just bumped into this:
>> Why does Array not include Comparable?
>> Array defines <=>, and so including Comparable should be easy.
>>
>> It looks to me like a big oversight, or there must have been
>> some very deep reasons to not do this, which I'm completely ignorant of.
>>
>
>Good question.
>
>You can of course either open Array and include comparable, or extend
>an individual array instance.
>
>But some subtleties.

Good points, but they only make good reasons for why Array
might not even want to support <=>, or why users have to
be careful about <=> (and therefore <, <=, >, >=, ==, !=,
if availale), not for why <=> is available but not the rest.

Regards,    Martin.

>1) Just because Array implements <=> doesn't mean that it will work as
>you expect for all arrays.   Array basically implements <=> by
>comparing elements with the argument until either an element
>comparison returns a non-zero or either the receiver or argument 'runs
>out' of elements.
>
>So if elements aren't comparable with <=> pairwise ...
>
>2) Comparable not only requires that <=> be implemented but that it
>return -1, 0, or 1.  Some classes implement it to return nil if the
>argument isn't of the right class, so even if Array included
>comparable things like ["a] < [1] wouldn't work.



#-#-#  Martin J. Du"rst, Assoc. Professor, Aoyama Gakuin University
#-#-#  http://www.sw.it.aoyama.ac.jp       mailto:duerst@it.aoyama.ac.jp