visitor_bob.rb

class Bob
  def reply_to(statement)
    statement.reply(self)
  rescue
    default_reply
  end

  def reply_to_silence
    "Fine. Be that way!"
  end

  def reply_to_yelling
    "Woah, chill out!"
  end

  def reply_to_question
    "Sure."
  end

  def default_reply
    "Whatever."
  end
end

class Statements
  def self.all
    [Question, Yelling, Silence, NullStatement]
  end
end

Statement = Struct.new(:statement)

class Question < Statement
  def self.match?(statement)
    statement.end_with?("?")
  end

  def reply(recipient)
    recipient.reply_to_question
  end
end

class Yelling < Statement
  def self.match?(statement)
    statement.upcase == statement && statement.downcase != statement
  end

  def reply(recipient)
    recipient.reply_to_yelling
  end
end

class Silence < Statement
  def self.match?(statement)
    statement.strip.empty?
  end

  def reply(recipient)
    recipient.reply_to_silence
  end
end

class NullStatement < Statement
  def self.match?(statement)
    true
  end

  def reply(recipient)
    recipient.default_reply
  end
end

class StatementFactory
  def self.build(statement)
    Statements.all.detect { |s| s.match?(statement) }.new(statement)
  end
end

y = StatementFactory.build("HELLO")
q = StatementFactory.build("HELLO?")
s = StatementFactory.build("")
d = StatementFactory.build("hello")

Bob.new.reply_to(y)
#=> "Woah, chill out!"
Bob.new.reply_to(q)
#=> "Sure"
Bob.new.reply_to(s)
#=> "Fine. Be that way!"
Bob.new.reply_to(d)
#=> "Whatever."