From: Robert Klemme <shortcutter@...>
Date: 2010-11-19T18:37:36+09:00
Subject: Re: Descending ranges in Ruby

On Fri, Nov 19, 2010 at 5:49 AM, Harry Spier <harryspier@hotmail.com> wrote:
> Thanks for this work-around but my question was not so much how to get
> around this restriction but rather "why I'm restricted from iterating
> over a descending range if descending Ranges are legal?".  For example a
> Range returned by a function in my program that I may not know whether
> it is ascending or descending?

IIRC a Range finds the next value via #succ.  And Fixnum#succ always
yields the next higher number.  If you want to use a Range you must
use a type which reverses #succ and #<=>, example:

irb(main):001:0> RFN = Struct.new :value do
irb(main):002:1* def <=>(o) o.value <=> value end
irb(main):003:1> def succ; self.class.new(value - 1) end
irb(main):004:1> end
=> RFN
irb(main):005:0> (RFN.new(5) .. RFN.new(1)).to_a
=> [#<struct RFN value=5>, #<struct RFN value=4>, #<struct RFN
value=3>, #<struct RFN value=2>, #<struct RFN value=1>]

I'd rather stick with the other approaches presented or use Array#reverse.

Kind regards

robert

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