McCarthy's `amb` operator using exceptions

amb.rb

require 'continuation'

# Example implementation using call/cc, 
# stolen in part from randomhacks.net blog
class CallCCAmb 
  def initialize
    @backtrack_points = []
  end

  def backtrack
    if @backtrack_points.empty?
      raise "Can't backtrack"
    else
      @backtrack_points.pop.call
    end
  end

  def amb *choices
    backtrack if choices.empty?
    callcc {|cc|
      @backtrack_points.push cc

      return choices[0]
    }

    amb(*choices[1...choices.length])
  end

  def example
    a = amb(1, 2, 3)
    b = amb(2, 3, 4)
    backtrack unless a*b == 6
    puts a, b
  end
end

puts "Call/cc example: find numbers that * to 6:"
CallCCAmb.new.example
puts "\n"

class ExceptionAmb
  Backtrack = Class.new(StandardError)

  def initialize
    @answers = []
  end

  def backtrack
    raise Backtrack
  end

  def amb *choices, &block
    backtrack if choices.empty?

    block.call choices[0]

    # We've made it without backtracking!
    @answers << choices[0]
  rescue Backtrack
    backtrack if choices.empty? # Backtrack *up a level*
    amb(*choices[1..choices.length], &block) # Try the rest
  end

  def example
    amb(1, 2, 3) do |a|
      amb(2, 3, 4) do |b|
        backtrack unless a*b == 6
      end
    end
    puts @answers
  end
end

puts "Exception-backtracking example: find numbers that * to 6:"
ExceptionAmb.new.example