http://rubylearning.com/blog/2010/04/29/rpcfn-interactive-fiction-9/

game.rb

require 'yaml'

class Game
  Room   = Struct.new :title, :description, :exits, :objects
  Object = Struct.new :terms, :description
  Action = Struct.new :terms, :code

  attr_reader :blackboard

  def initialize filename, options={}
    @input      = options.fetch(:input)  { $stdin  }
    @output     = options.fetch(:output) { $stdout }

    # game state
    @actions    = {}
    @actsyns    = {}
    @objects    = {}
    @objsyns    = {}
    @rooms      = {}
    @seen       = {}
    @inventory  = []
    @blackboard = {}

    data = File.read filename
    load data
    # yaml hash will be unordered
    @room = @rooms[data[/^Room (@\w+):$/, 1]]

    # set up action terms as synonyms, then rekey off primary term
    @actions.values.each { |act| @actsyns[act.terms[0]] = act.terms[1..-1] }
    @actions = @actions.inject({}) { |h, (k, v)| h.update v.terms[0] => v }

    # install the default actions
    %w[enter exit north south east west].each { |dir| @actions[dir] = Action.new [dir], proc { || move dir } }
    %w[look inventory take drop].each { |name| @actions[name] = Action.new [name], method(name) }

    @actsyns.each { |k, v| v.each { |syn| @actions[syn] = @actions[k] } }
    @objects.each { |k, v| v.terms.each { |n| @objsyns[n.downcase] = k } }

    # longest matches first so that more specific actions wins
    @inputrx = /^(#{@actions.keys.sort_by { |k| k.length }.reverse.map { |s| Regexp.quote s }.join('|')})(?:\s+(.+))?$/
  end

  def load data
    # offload parsing to YAML by convert to a valid YAML document.
    # regexps rely on indentation to avoid restricting contents of code blocks
    # (eg allow a string containing '}}}' in a code block)
    yaml = data.gsub(/^  Code:/, "\\0 |-").gsub(/^  (\S+):\n((?:    .*\n)+)/) do
      next "  #$1: >\n#$2" if $1 == 'Description'
      "  #$1:\n" + $2.map { |l| l.sub(/^    (?= )/, "\\0  ").sub(/^    (?=\S)/, "\\0- ").sub(/^    \S.* guarded by:$/, "\\0 |-") }.join
    end
    doc = YAML.load yaml
    # could use YAML aliases to build the full object graph,
    # but for now do it separately
    doc.each do |key, value|
      case key
      when /^(Room) (@\w+)$/
        value['Objects'] ||= []
        value['Exits'] = value['Exits'].inject({}) do |h, s|
          s, code = Hash === s ? [s.keys[0].sub(/ guarded by$/, ''), s.values[0]] : [s, nil]
          code = eval code.sub(/\A\{\{\{\s*\n/, "proc do\nThread.new do\n$SAFE = 4\n").sub(/\n\}\}\}\s*\Z/, "\nend.value\nend") if code
          k, room = s.match(/^(\w+) to (@\w+)$/)[1..-1]
          h.update k => [room, code]
        end
      when /^(Object) (\$\w+)$/
        value['Terms'] = value['Terms'].split(/,\s+/)
      when /^(Action) (\!\w+)$/
        value['Terms'] = value['Terms'].split(/,\s+/)
        value['Code'] = eval value['Code'].sub(/\A\{\{\{\s*\n/, "proc do || text, updates = Thread.new do\n$SAFE = 4\n").sub(/\n\}\}\}\s*\Z/, "\nend.value\nputs text unless text.empty?\nblackboard.update updates\nend")
      when 'Synonyms'
        @actsyns = value.inject({}) { |h, (k, v)| h.update k => v.split(/,\s+/) }
        next
      else
        raise 'invalid construct - %p' % key
      end
      type, name = key.split ' ', 2
      klass = self.class.const_get type
      instance_variable_get("@#{type.downcase}s").update name => klass.new(*value.values_at(*klass.members.map { |s| s.capitalize }))
    end
  end

  def player_in? room
    @rooms[room] == @room
  end

  def player_has? obj
    @inventory.include? obj
  end

  def look
    puts [@room.description, *@room.objects.map { |obj| @objects[obj].description }]
    @seen[@room] = true
  end

  def move dir
    dest, guard = @room.exits[dir]
    return puts('There is no way to go in that direction.') unless dest
    ok, text = guard ? guard.call : [true, '']
    puts text unless text.empty?
    return unless ok
    @room = @rooms[dest]
    @seen[@room] ? puts("You're #{@room.title}.") : look
  end

  def inventory
    return puts("You're not carrying anything") if @inventory.empty?
    puts ['You are currently holding the following:', *@inventory.map { |obj| @objects[obj].terms.first }]
  end

  def take obj
    return puts("There is no `#{obj}' here.") unless found = @room.objects.delete(@objsyns[obj.downcase])
    puts 'OK'
    @inventory << found
  end

  def drop obj
    return puts("You don't have a `#{obj}'.") unless found = @inventory.delete(@objsyns[obj.downcase])
    @room.objects << found
  end

  def play!
    start!
    execute_one_command! until ended?
  end

  def execute_one_command!
    if @output == $stdout
      @output.print '> '
      @output.flush
    end
    case s = @input.gets
    when nil
      puts
      exit
    when /^\s*$/
    when @inputrx
      act = @actions[$1]
      unless ($2 ? 1 : 0) == act.code.arity
        puts 'what you sayin?'
      else
        act.code.call(*[$2].compact)
      end
    else
      puts 'wtf are you talking about? (%s)' % s
    end
  end

  alias start! look

  def ended?
    false
  end

private

  def puts(*args)
    @output.puts(*args)
  end
end

if $0 == __FILE__
  unless story_path = ARGV[0]
    warn "Usage: #{File.basename $0} STORY_FILE"
    exit 1
  end
  Game.new(story_path).play!
end