logger.rb

# coding: UTF-8

class Logger
  def self.write(out, log)
      out.write log
    rescue
      raise TypeError, "書き込めない何かだよ"
  end
end

File.open("hoge", "w") do |f|
  Logger.write(f, "log") # ファイルに書き込み
end

# Logger.write(Object.new, "hoge")

class MyClass
  def write(num)
    raise TypeError, "数値じゃないよ" unless num.is_a? Fixnum
  end
end

Logger.write(MyClass.new, "hoge") #=> TypeError, "書き込めない何だよ"