[Ruby] def文によらずに特異メソッドを定義する To define singleton methods without `def' sentence

define_singleton_method.rb

# ---------- definition ----------
class Object
  def define_singleton_method(name, method = nil, &block)
    unless !!(method) ^ !!(block)
      raise ArgumentError, "Only one method should be given"
    end
    
    source_object = self
    Module.new do
      define_method(name, method || block)
      source_object.extend(self)
    end
  end
end

# ---------- usage ----------
if $0 == __FILE__
  def print_hoge(obj)
    begin
      puts obj.hoge
    rescue NoMethodError
      puts "[Method `hoge' not found]"
    end
  end
  
  str = "hoge"
  print_hoge(str) # => "[Method `hoge' not found]"
  
  str.define_singleton_method(:hoge) do
    "HOGEHOGE"
  end
  
  print_hoge(str) # => "HOGEHOGE"
  
  str.define_singleton_method(:hoge, ->{ "PIYOPIYO" } )
  
  print_hoge(str) # => "PIYOPIYO"
end