From: Trans <transfire@...>
Date: 2005-11-24T15:17:26+09:00
Subject: Re: string range membership

> I'm not sure what you meant here.  Range has no relation with
> sorting.  Can you elaborate?

#succ defines a sort order of sorts (pun intended ;-). But #<=> defines
a sort order too along with comparability. In most classes there's no
problem, but in String the two come into conflict --the orders are not
the same.

Then consider that Range is not a true interval because it uses #succ.
This is why I created a true Interval class that uses #+ instead.
Likewise Range shouldn't use #<=> either, but another method, lets call
it #cmp. This would fix the problem.

In general:

  module Comparable
    def cmp(o)
      self<=>o
    end
  end

That is to say, for anything comparable #cmp is the same as #<=>,
unless otherwise defined. (Alternately you could define #cmp as an
alias of #<=> directly in the classes it is needed --that would
probably be better.) Then in String define #cmp specially to confom to
the successive order as defined by #succ.

Thus having Range use #cmp instead of #<=> the issue is solved.

In summary, an object would then be "Rangeable" if it supports #succ,
but only fully so if is also supports #cmp too (instead of #<=>).

Does it make sense now? (Sorry if I'm not explaining well, it's a tad
subtle and it's been awhile since I worked on it too, so I have been
trying to recall it all myself too).

T.