hacking rubys syntax

February 3, 2010

What?

In Ruby you have basically two ways of defining private methods:

	# this is the first way to do it
	class Test
	def say_hello
	puts "I'm a private method"
	end
	private :say_hello
	end

	# and this is the second way
	class Test
	#other methods this class might have
	private
	def say_hello
	puts "I'm a private method"
	end
	end

view raw test.rb hosted with ❤ by GitHub

I see a small problem with both approaches. In the first one, and the most obvious, is that you need to duplicate the method name as well as add an extra method call - private - just to change its visibility.

The second approach avoids this but adds the risk of accidentally putting a method that is intended to be public under the private section of the source file, which can render an annoying debugging session.

Why?

Personally, I like to have a smooth reading flow in my source files. That means that if the public method_a makes use of the private method_b, I want method_b defined right below its caller, which is possible - but verbose - using the private method call:

	class Test
	def method_a
	method_b
	end

	def method_b
	puts "I'm a private method"
	end
	private :method_b
	end

view raw test.rb hosted with ❤ by GitHub

But can be somewhat harder to accomplish if you decide to split your source file in sections:

	class Test
	def method_a
	method_b
	end

	#just another public method. We might have several
	def method_c
	method_b
	end

	private

	def method_b
	puts "I'm a private method"
	end
	end

view raw test.rb hosted with ❤ by GitHub

I wanted to be able to define a private method with a single reserved keyword...

How?

What if I could define a private method using this new syntax:

	class Test
	def_p say_hello
	puts "I'm a private method"
	end
	end

view raw test.rb hosted with ❤ by GitHub

It turns out I can.

Notice the def_p keyword? This is a new keyword I created by changing ruby's parser and that behaves mostly like the def keyword, except that it defines a private method instead.

If you wanna read the code that allows this behavior and try it yourself, download the patch I wrote and apply it to the ruby source code - I patched version 1.9.1-p376.

After applying the patch, just build ruby as usual:

	$ ./configure
	$ make

view raw gistfile1.eclass hosted with ❤ by GitHub

And then try running this script:

	class Test
	def_p say_hello
	puts "I'm a private method"
	end
	end
	Test.new.say_hello

view raw test.rb hosted with ❤ by GitHub

You should see the following output:

pvt.rb:10:in `': private method `say_hello' called for # (NoMethodError)

view raw gistfile1.eclass hosted with ❤ by GitHub

Happy hacking :)

Ruby