From: Robert Klemme <bob.news@...>
Date: 2004-08-31T15:40:25+09:00
Subject: Re: Flexible operations for a collection class


"Edgardo Hames" <ehames@gmail.com> schrieb im Newsbeitrag
news:478c16ae040830134175316839@mail.gmail.com...
> On Tue, 31 Aug 2004 05:25:26 +0900, Robert Klemme <bob.news@gmx.net>
wrote:
> >
> > >
> > > but I would like to implement the #<=> method in my objects, so I can
> > > delegate the #sort method to @elements. I came up with this,
> > >
> > > def sortable_by=(element_attribute)
> > >   @sortable_by=element_attribute
> > >   @elements.each{|o| o.sortable_by=@sortable_by}
> > > end
> >
> > Putting the comparison criteria into the elements is very bad design.
What
> > will you do if your elements are in two different collections at the
same
> > time?  The sorting criterium belongs into the collection.
>
>
> From The Pickaxe Book, in the Enumerable Module
>
> "Mix in Enumerable, and suddenly your class supports things such as
> map, include?, and find_all?. If the objects in your collection
> implement meaningful ordering semantics using the <=> method, you'll
> also get min, max, and sort."
>
> That's the reason behind my design.

Yeah, but <=> just gives you the default ordering (e.g. lexical for strings
and numeric for fixnum).  As far as I can see, that's not what you wanted:
you wanted to be able to define the order yourself.  Changing <=> all the
time is not a good idea because of the issue with multiple collections as
well as other pieces of code relying on the natural ordering.

Btw, see also Comparable.
http://www.rubycentral.com/book/ref_m_comparable.html

> But, the Set solution satisfies
> most of my needs. I just need to add a few methods to it.

Fine!

> Thanks for your clear response.

You're welcome.

    robert