From: Xavier Noria <fxn@...>
Date: 2007-09-23T02:36:59+09:00
Subject: Re: (SUMMARY) Idiomatic Ruby for Array#extract / Range#length?

I think this thread deserves a summary.

Sammry Larbi asks three questions in the post that opens this thread,  
the first one asks whether it would make sense to define Range#length  
as the difference between its endpoints.

The short answer is that objects in ranges do not necessarily respond  
to #-.

Furthermore, according to the docs (Ruby docs and Pickaxe), objects  
in ranges respond to #succ, and so one may wonder whether you can  
define Range#length as an element count implemented as a loop from  
left endpoint to right endpoint via #succ. There's a warning, though,  
because #<=> and #succ do not guarantee ranges are finite, so such a  
method is not guaranteed to terminate. That's a claim at that point,  
and some people ask for further details.

There's a split here in the thread (mixed in practice):

1) The docs say so, but there's enough evidence to say they are  
wrong, objects in ranges are only required to respond to #succ if you  
want to iterate over them. If you want to implement Range#length that  
way you have to assume objects respond to #succ anyway.

2) A few examples of classes that give infinite ranges are described  
or implemented, the most simple of them being the ZSquared class  
copied below.

The other two questions of the original post remain unanswered by now  
(at least in the mailing list).

Best,

-- fxn



   class ZSquared
     include Comparable

     attr_reader :n, :m

     def initialize(n, m)
       @n, @m = n, m
     end

     def succ
       self.class.new(n+1, m)
     end

     def <=>(zs)
       [self.m, self.n] <=> [zs.m, zs.n]
     end
   end

   a = ZSquared.new(0, 0)
   b = ZSquared.new(0, 1)

   puts a < b # true

   length = 0
   (a .. b).each do |c|
     length += 1
     puts length
   end

   # be prepared to send Ctrl-C