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Singletons in Ruby

Singletons in Ruby

Ruby provides a nice builtin module that lets easily turn your class into a singleton:

require 'singleton'

class MyClass
  include Singleton

  def say
    puts "Hello"
  end
end

MyClass.instance.say  # ---> "hello"

Problem

But I'm not quite happy with that. I'm not talking about the general wide overuse of the singleton pattern. What bothers me, is that the fact of MyClass being a singleton is just an implementation detail and it shouldn't change how I'm interfacing with this class.

For example, I might decide that instead of having a singleton I'd just like to have a class with static methods:

class MyClass
  def self.say
    puts "Hello"
  end
end

Nice. But now I have to go through all of my code and replace all the uses of MyClass.instance.say with MyClass.say.

That's not good. The rest of my code shouldn't care if MyClass is implemented as a singleton or not.

Additionally I like my lines short. For me MyClass.say looks much cleaner and simpler to understand than MyClass.instance.say.

Maybe in Java and friends that's the best you can do, but in Ruby there sure is some better way.

Solution

So, after fiddling around a bit with some ruby meta-programming, I came up with my own improved version of the Singleton module:

require 'singleton'

module MySingleton
  def self.included(base)
    base.class_eval do
      # Include the builtin Singleton module
      include ::Singleton

      # Redirect calls from MyClass.method to MyClass.instance.method
      def self.method_missing(meth, *args, &block)
        self.instance.send(meth, *args, &block)
      end
    end
  end
end

You use it just like the standard Singleton:

require 'my_singleton'

class MyClass
  include MySingleton

  def say
    puts "Hello"
  end
end

With the important difference of how you call the methods on your instance:

MyClass.say  # ---> "hello"

Oh, and if you need it, you can still use the old way of calling out the singleton. This will still work:

MyClass.instance.say  # ---> "hello"

Extra

I have always found static methods in ruby to be too painful to define. You need to prefix all the static methods with self.:

class MyClass

  def self.foo
  end

  def self.bar
  end

  def self.baz
  end

end

And then, when you decide to change them all into instance methods, you'll need to remove all the self. prefixes. Not good at all.

But with our new Singleton implementation we can just turn this same class into a singleton and make it behave as a class with static methods:

class MyClass
  include MySingleton

  def foo
  end

  def bar
  end

  def baz
  end

end

And when you change your mind, you just remove that one line and you've got a nice instantiable class again.

A win-win I would say.

Kirjutatud 10. septembril 2012.

Trinoloogialeht

Eesti Trinoloogide Maja. Eesti trinoloogiahuviliste avalik kogunemiskoht. info@triin.net

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