an idea

rufus-rules.rb

class RuleBook

  def initialize (&block)
    @rules = []
    add_rules(&block)
  end

  def add_rules (&block)
    instance_eval(&block) if block
    self
  end

  def eval (facts)
    facts = Facts.new(facts) unless facts.is_a?(Facts)
    @rules.each do |r|
      if facts.instance_eval(&r[0])
        facts.instance_eval(&r[1])
        break
      end
    end
    facts
  end

  protected

  def rule (conditional, consequence)
    @rules << [ conditional, consequence ]
  end

  # for the sake of sparing 5 chars
  #
  def l (&block)
    lambda &block
  end
end

class Facts
  def initialize (h)
    @hash = h
  end
  def method_missing (mname, *args)
    mname = mname.to_s
    @hash[mname] || mname
  end
  def set (k, v)
    @hash[k] = v
  end
end

my_rules = RuleBook.new do

  rule(
    l { color == 'blue' },
    l { set(nose, green) })
  rule(
    l { color == red },
    l { set(count, 2) })
  rule(
    l { color == purple and size == medium },
    l { puts 'murasaki no ue' })
end

#
# if h['color'] = 'blue' then h['nose'] = 'green'
# if h['color'] = 'red' then h['count'] = 2
#
# :(
#

p my_rules.eval('color' => 'blue')
p my_rules.eval('color' => 'red')
p my_rules.eval('color' => 'white')
p my_rules.eval('color' => 'purple')
p my_rules.eval('color' => 'purple', 'size' => 'medium')