From: matz@... (Yukihiro Matsumoto)
Date: 2002-11-15T06:12:26+09:00
Subject: Re: Array#next - Enumerable#next ?

Hi,

In message "Array#next - Enumerable#next ?"
    on 02/11/15, ahoward <ahoward@fsl.noaa.gov> writes:

|does anyone know a way to implement this generically for all enumerable
|objects in an efficient manner?

Scheme 1)

  Use "to_a" to convert Enumerable into Array first.
  Easiest way.

Scheme 2)

  Use thread to save/restore "each" iteration.

Scheme 3)

  Use continuation to save/restore "each" iteration.

Hint, here's my "generator.rb" which uses scheme 3).  You can start
from this sample.

							matz.
------
class Generator
  def initialize(method, *args)
    @cont_l = nil
    method.call(*args) {|v|
      callcc {|c|
	@cont_i = c
	@value = v
	return unless @cont_l
	@cont_l.call
      }
    }
    @cont_i = nil
    @cont_l.call
  end
  def next
    unless @cont_i
      raise "no more element"
    end
    v = @value
    callcc{|cc|
      @cont_l = cc
      @cont_i.call
    }
    v
  end
  def more?
    if @cont_i then true else false end
  end
end

class Object
  def generator(method, *args)
    Generator.new(self.method(method), *args)
  end
end

if $0 == __FILE__
  def zip(*args)
    generators = args.collect{|x| x.generator(:each)}
    ary = []
    while generators.any?{|x| x.more?}
      ary.push(generators.collect{|x| x.next rescue nil})
    end
    ary
  end

  a = 1..10
  b = "a".."z"
  for x,y in zip(a,b)
    p [x,y]
  end
end