Aspect-Oriented Programming in Ruby using Combinator Birds

In Combinatory Logic, the bluebird is one of the most important and fundamental combinators, because the bluebird composes two other combinators. Although this is usually discussed as part of functional programming style, it is just as valuable when writing object-oriented programs. In this post, we will develop an aspect-oriented programming (or "AOP") module that adds before methods and after methods to Ruby programs, with the implementation inspired by the bluebird.

As explained in Kestrels, the practice of nicknaming combinators after birds was established in Raymond Smullyan's amazing book To Mock a Mockingbird. In this book, Smullyan explains combinatory logic and derives a number of important results by presenting the various combinators as songbirds in a forest. Since the publication of the book more than twenty years ago, the names he gave the birds have become standard nicknames for the various combinators.

The bluebird is written Bxyz = x(yz). In Ruby, we can express the bluebird like this:

bluebird.call(proc1).call(proc2).call(value)
	=> proc1.call(proc2.call(value))

If this seems a little arcane, consider a simple Ruby expression (x * 2) + 1: This expression composes multiplication and addition. Composition is so pervasive in programming languages that it becomes part of the syntax, something we take for granted. We don't have to think about it until someone like Oliver Steele writes a library like functional javascript that introduces a compose function, then we have to ask what it does.

Before we start using bluebirds, let's be clear about something. We wrote that bluebird.call(proc1).call(proc2).call(value) is equivalent to proc1.call(proc2.call(value)). We want to be very careful that we understand what is special about proc1.call(proc2.call(value)). How is it different from proc1.call(proc2).call(value)?

The answer is:

proc1.call(proc2.call(value))
	=> puts value into proc2, then puts the result of that into proc1

proc1.call(proc2).call(value)
	=> puts proc2 into proc1, getting a function out, then puts value into the new function

So with a bluebird you can chain functions together in series, while if you didn't have a bluebird all you could do is write functions that transform other functions. Not that there's anything wrong with that, we used that to great effect with cardinals and quirky birds.

giving methods advice

We're not actually going to Greenspun an entire aspect-oriented layer on top of Ruby, but we will add a simple feature, we are going to add before and after methods. You already know what a normal method is. A before method simply specifies some behaviour you want executed before the method is called, while an after method specifies some behaviour you want executed after the method is called. In AOP, before and after methods are called "advice."

There is an unwritten rule that says every Ruby programmer must, at some point, write his or her own AOP implementation --Avdi Grimm

Ruby on Rails programmers are familiar with method advice. If you have ever written any of the following, you were using Rails' built-in aspect-oriented programming support:

after_save
validates_each
alias_method_chain
before_filter

These and other features of Rails implement method advice, albeit in a very specific way tuned to portions of the Rails framework. We're going to implement method advice in a module that you can use in any of your classes, on any method or methods you choose. We'll start with before methods. Here's the syntax we want:

def something(parameter)
	# do stuff...
end

before :something do |parameter|
	# stuff to do BEFORE we do stuff...
end

before :something do |parameter|
	# stuff to do BEFORE stuff to do BEFORE we do stuff...
end

As we can see, the before methods get chained together before the method. To keep this nice and clean, we are going to make them work just like composable functions: whatever our before method's block returns will be passed as a parameter up the chain. We also won't fool around with altering the order of before methods, we'll just take them as they come.

This is really simple, we are composing methods. To compare to the bluebird above, we are writing before, then the name of a method, then a function. I'll rewrite it like this:

bluebird.call(something).call(stuff_to_do_before_we_do_stuff).call(value)
	=> something.call(stuff_to_do_before_we_do_stuff.call(value))

Now we can see that this newfangled aspect-oriented programming stuff was figured out nearly a century ago by people like Alonzo Church.

Okay, enough history, let's get started. First, we are not going to write any C, so there is no way to actually force the Ruby VM to call our before methods. So instead, we are going to have to rewrite our method. We'll use a trick I found on Jay Fields' blog:

module NaiveBeforeMethods

  module ClassMethods

    def before(method_sym, &block)
      old_method = self.instance_method(method_sym)
      if old_method.arity == 0
        define_method(method_sym) do
          block.call
          old_method.bind(self).call
        end
      else
        define_method(method_sym) do |*params|
          old_method.bind(self).call(*block.call(*params))
        end
      end
    end

  end

  def self.included(receiver)
    receiver.extend         ClassMethods
  end

end

As you can see, we have a special case for methods with no parameters, and when we have a method with multiple parameters, our before method must answer an array of parameters. And the implementation relies on a "flock of bluebirds:" Our before methods and the underlying base method are composed with each other to define the method that is actually executed at run time.

Using it is very easy:

class SuperFoo

  def one_parameter(x)
    x + 1
  end

  def two_parameters(x, y)
    x * y
  end

end

class Foo < SuperFoo

  include NaiveBeforeMethods

  before :one_parameter do |x|
    x * 2
  end

  before :two_parameters do |x, y|
    [x + y, x - y]
  end

end

Foo.new.one_parameter(5)
	=> 11

Foo.new.two_parameters(3,1)
	=> 8

This could be even more useful if it supported methods with blocks. Adventurous readers may want to combine this code with the tricks in cardinal.rb and see if they can build a version of before that supports methods that take blocks.

the super keyword, perhaps you've heard of it?

Of course, Ruby provides a means of 'decorating' methods like this by overriding a method and calling super within it. So we might have written:

class Foo < SuperFoo

  def one_parameter(x)
    super(x * 2)
  end

  def two_parameters(x, y)
    super(x + y, x - y)
  end

end

On a trivial example, the two techniques seem equivalent, so why bother with the extra baggage? The answer is that using super is a little low level. When you see a method definition in a language like Ruby, you don't know whether you are defining a new method, overriding an existing method with entirely new functionality, or "decorating" a method with before advice. Using advice can be useful when you want to signal exactly what you are trying to accomplish.

Another reason to prefer method advice is when you want to share some functionality:

class LoggingFoo < SuperFoo

	def one_parameter(x)
		log_entry
		returning(super) do
			log_exit
		end
	end

	def two_parameters(x, y)
		log_entry
		returning(super) do
			log_exit
		end
	end

end

This could be written as:

class LoggingFoo < SuperFoo

  include NaiveBeforeMethods

  before :one_parameter, :two_parameters do # see below
    log_entry
  end

  after :one_parameter, :two_parameters do
    log_exit
  end

end

This cleanly separates the concern of logging from the mechanism of what the methods actually do

Although this is not the main benefit, method advice also works with methods defined in modules and the current class, not just superclasses. So in some ways it is even more flexible than Ruby's super keyword.

the queer bird

That looks handy. But we also want an after method, a way to compose methods in the other order. Good news, the queer bird combinator is exactly what we want.

Written Qxyz = y(xz), the Ruby equivalent is:

queer_bird.call(something).call(stuff_to_do_after_we_do_stuff).call(value)
	=> stuff_to_do_after_we_do_stuff.call(something.call(value))

Which is, of course:

def something(parameter)
	# do stuff...
end

after :something do |return_value|
	# stuff to do AFTER we do stuff...
end

The difference between before and after advice is that after advice is consumes and transforms whatever the method returns, while before advice consumes and transforms the parameters to the method.

We could copy, paste and modify our bluebird code for the before methods to create after methods. But before you rush off to implement that, you might want to think about a few interesting "real world" requirements:

If you define before and after methods in any order, the final result should be that all of the before methods are run before the main method, then all of the after methods. This is not part of combinatory logic, but it's the standard behaviour people expect from before and after methods.
You should be able to apply the same advice to more than one method, for example by writing after :foo, :bar do ... end
If you declare parameters for before advice, whatever it returns will be used by the next method, just like the example above. If you do not declare parameters for before advice, whatever it returns should be ignored. The same goes for after advice.
If you override the main method, the before and after methods should still work.
The blocks provided should execute in the receiver's scope, like method bodies.

One implementation meeting these requirements is here: before_and_after_advice.rb. Embedded in a lot of extra moving parts, the basic pattern of composing methods is still evident:

# ...
define_method(method_sym) do |*params|
  composition.after.inject(
    old_method.bind(self).call(
      *composition.before.inject(params) do |acc_params, block|
        self.instance_exec(*acc_params, &block)
      end
    )
  ) do |ret_val, block|
    self.instance_exec(ret_val, &block)
  end
end
# ...

That is why we looked at supporting just before methods first. If you are comfortable with the naïve implementation of before advice discussed above, the mechanism is easy to understand. The complete version is considerably more powerful. As mentioned, it supports before and after advice. It also uses instance_exec to evaluate the blocks in the receiver's scope, providing access to private methods and instance variables. And it works properly even when you override the method being advised.

Please give it a try and let me know what you think.

p.s. If the sample code gives an error, it could be a known bug in Ruby 1.8. Try declaring your advice with an empty parameter list, e.g. do || ... end.