logger.rb

# coding: UTF-8

class Logger
  def self.write(out, log)
    raise TypeError, "書き込めない何かだよ" unless out.respond_to? :write
    out.write log
  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")