#!/usr/bin/env ruby

class Amb
  class ExhaustedError < RuntimeError; end
  
  def initialize
    @back = Array(lambda { raise ExhaustedError, "amb tree exhausted" })
  end
  
  def choose(*choices)
    choices.each do |choice|
      callcc do |current_continuation|
        @back << current_continuation
        return choice
      end
    end
  end
  
  def assert(cond)
    failure unless cond
  end
  
private
  
  def failure
    @back.pop.call
  end
end

require 'rubygems'; require 'ruby-debug'; debugger;
amb = Amb.new
x = amb.choose(1,2,3,4)
amb.assert((x % 2) == 0)
amb.assert x > 3

puts x