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Extension Methods

An extension method is a new language feature of C# starting with the 3.0 specification, as well as Visual Basic.NET starting with 9.0 and Oxygene with 2.0. Extension methods enable you to "add" methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type. Extension methods are a special kind of static method, but they are called as if they were instance methods on the extended type. For client code written in C# and Visual Basic, there is no apparent difference between calling an extension method and the methods that are actually defined in a type. --Wikipedia

With RewriteRails, you can now write Extension Methods for your Ruby on Rails projects. Just like in C#, you can "add" methods to existing classes or module (like Enumerable) without modifying the original class or module. The benefit is that your changes never conflict with other code. The headaches of opening core classes simply go away.

How to Write an Extension Method

First, you add a new class or module to the RewriteRails::ExtensionMethods module that mimics the class or module of the objects that will handle the method:

module RewriteRails
  module ExtensionMethods
  
    module Enumerable
    end
    
  end
end

This tells RewriteRails that we want to work with the standard library's Enumerable module. Since or new Enumerable module is nested inside RewriteRails::ExtensionMethods, our work will not conflict with the existing Enumerable module in any way. Next, we add class methods to our new class. Here's an example:

module RewriteRails
  module ExtensionMethods
  
    module Enumerable
    
      def self.sum(arr, identity = 0, &block)
        return identity unless arr.size > 0
        
        if block_given?
          arr.map(&block).sum
        else
          arr.flatten.inject(&:+)
        end
      end
      
    end
    
  end
end

(This definition should be placed somewhere that ActiveSupport::Dependencies can find it before you use it, like in lib/rewrite_rails/extension_methods/enumerable.rb.) The method we have just written recursively sums an enumerable. Since it is written as a class method, we could use it by hand as follows:

RewriteRails::ExtensionMethods::Enumerable.sum([[1,2], [3,4], [5,6]])
 => 21

The advantage of putting our own definition in a separate module is that if someone else monkey-patches the core library to add their own #sum method, we will not have a conflict. Why does this matter? Imagine they wrote this:

module Enumerable

  def sum(identity = 0, &block)
    return identity unless size > 0

    if block_given?
      map(&block).sum
    else
      inject { |sum, element| sum + element }
    end
  end
  
end

Your version and their version give different results for an array of arrays. While your version computes the sum of [[1,2], [3,4], [5,6]] as 21, their version does something else entirely:

[[1,2], [3,4], [5,6]].sum
  => [1, 2, 3, 4, 5, 6]

Having two versions of the same method that give different results is obviously a conflict. Had you written your version by opening up the Enumerable module and adding #sum, you might have overwritten their version, breaking their code that expects it to work their way. On the other hand, what if Rails loaded their version after yours? Then their code would work but your code would be mysteriously broken.

There are two problem with this 'safe' approach. The first is that it is tedious to write RewriteRails::ExtensionMethods::Enumerable.sum in place of plain "sum." The second is that sometimes our code will be more readable when we use infix notation (like instance method calls) instead of prefix notation (like class method calls). So what we want is to write:

[[1,2], [3,4], [5,6]].sum
  => 21

In our code, but we do not want to break any code that relies on the other definition of Enumerable#sum. Well, you are reading the docs for RewriteRails, so you know the answer already: You want it such that every time we write:

[[1,2], [3,4], [5,6]].sum

We want RewriteRails to rewrite our code as:

RewriteRails::ExtensionMethods::Enumerable.sum([[1,2], [3,4], [5,6]])

Notice how the receiver of our fake instance method is transformed into the first parameter of our helper class method. And in fact, this is almost exactly what happens. So you get the readability of code that looks like it is defining an instance method on a core class or module, but you get the safety of actually using a new helper module that won't conflict with other people's mucking about.

Major Caveat

Rewrite has to know which extension methods you've defined before it processes your .rr files. In practice, this means you want to include an initializer file with contents similar to this:

RewriteRails::ExtensionMethods::Enumerable

You want to force Rails to load each of your definitions before it processes your files, so you want one such lien for each class or module you have. Another way to do it is to put the entire body of the definition in an initializer, it's your choice.

The Fine Print

Here are a few considerations for those who want to "look under the hood." RewriteRails actually throws a little type discrimination into the rewritten code, so the actual rewritten code will look like this:

__124040247470700__ = [[1,2], [3,4], [5,6]]
if __124040247470700__.kind_of?(Enumerable)
  RewriteRails::ExtensionMethods::Enumerable.sum(__124040247470700__)
else
  __124040247470700__.sum
end

And if you write a #sum method on two different classes, the code will look something like this:

__124040247470700__ = [[1,2], [3,4], [5,6]]
if __124040247470700__.kind_of?(Enumerable)
  RewriteRails::ExtensionMethods::Enumerable.sum(__124040247470700__)
elsif __124040247470700__.kind_of?(LegalArgument)
  RewriteRails::ExtensionMethods::LegalArgument.sum(__124040247470700__)
else
  __124040247470700__.sum
end

One obvious limitation of this approach is that it unexpected things will happen when you define the same method on two different modules included in one object, or on two different classes in the same inheritance hierarchy. When you need to handle complicated things like this, you really should use an actual method and live with any conflicts that arise: Extension methods are not intended to replace instance methods, merely provide a low-pain syntactic substitute for simple cases.

Another consideration is that the actual method we write is not a real instance method. So you can't access instance variables or private methods. Our helper methods are just that, helpers.

A third consideration is that this is just syntactic sugar. If we didn't have somebody else helpfully defining Enumerable#sum elsewhere, the following might surprise you:

[[1,2], [3,4], [5,6]].respond_to?(:sum)
  => false
[[1,2], [3,4], [5,6]].methods.include?('sum')
  => false
[[1,2], [3,4], [5,6]].send(:sum)
  => NoMethodError
[[1,2], [3,4], [5,6]].sum
  => 21

These things look like instance methods, but please remember they are nothing more than syntactic sugar that are transformed at run time into calls to our helper methods. If you are doing any kind of meta-programming, they are not going to work for you. If you need to define methods on your classes that might be called with Object#send or queried, they are not going to work for you.

Instead, use these to write simpler, easier to read code with fewer compatibility headaches. That's what they're for.

This may work for Larry, but what about Curly and Moe?

An area man asked:

What if we all used rewrite_rails? You can still get two definitions for RewriteRails::ExtensionMethods::Enumerable.sum

Good catch! The long answer is that Rewrite Rails is specifically written to make it simple for Rails developers to insulate themselves from the vagaries of gem and plugin authors. So my first priority is protecting your code from ordinary Ruby metaprogramming conflicts.

That being said, I do have it in my mind that one day you will be able to use this technology to write gems that are likewise insulated from every other gem. That wasn't an immediate goal, so I initially left out some stuff that would add complexity to the implementation.

But since I was chided by a fellow kind enough to link to Rewrite Rails on his blog... Steps have been taken.

Scoped Extension Methods

In addition to RewriteRails::ExtensionMethods, you can place your extension methods somewhere in the parent hierarchy above the current class or module. Let me be specific. If you are writing ::Foo, there is no parent and you must place all of your extension methods in ::RewriteRails::ExtensionMethods. You can run into conflicts.

If you are writing ::Admin::LoginController, you can place your extension methods in either ::RewriteRails::ExtensionMethods or ::Admin::ExtensionMethods.

If you really love your lexical scope, you might be writing ::Raganwald::TwitterTools::Admin::LoginController. Now you can place your extension methods in ::RewriteRails::ExtensionMethods, ::Raganwald::TwitterTools::Admin::ExtensionMethods, ::Raganwald::TwitterTools::ExtensionMethods, or ::Raganwald::ExtensionMethods as you please.

It's up to you to use modules and classes to create scopes where you can place your extension methods. Note that you cannot place your extension methods in the current scope. The following will not work:

class Foo
  module ExtensionMethods
    class Numeric
      def self.ata(depth_in_feet)
        depth_in_feet.to_f / 33.0 # FSW
      end
    end
  end
  
  def bar
    # ...
    depth.ata
    # ...
  end
  
end

This won't work because Foo::ExtensionMethods is defined in Foo. In the current implementation, the reason for this is that the module containing your various extension method definitions must be available to the rewriter before it rewrites a file. It might be possible to fidjit with things such that Foo::ExtensionMethods is processed in one file and the body of Foo in another, but the current implementation will ignore Foo::ExtensionMethods when processing Foo, period.

However, go wild with:

# lib/seawater/extension_methods/number.rb

class Seawater::ExtensionMethods::Numeric
  def self.ata(depth)
    depth_in_feet.to_f / 33.0
  end
end

# lib/seawater/extension_methods/number.rb

class Freshwater::ExtensionMethods::Numeric
  def self.ata(depth)
    depth_in_feet.to_f / 34.0
  end
end

# app/controllers/conversion_controller.rr

class Seawater::ConversionController

  def show
    # ...
    @pressure = @depth.ata 
      # => something like Seawater::ExtensionMethods::Numeric.ata(@depth) 
    # ...
  end
  
end

# app/models/nitrox.rr

class Freshwater::Nitrox

  def validate
    # ...
    @ppo2 = self.depth.ata * self.fo2
      # => something like Freshwater::ExtensionMethods::Numeric.ata(self.depth) * self.fo2
    # ...
  end
  
end

# config/initializers/extension_methods.rb

Freshwater::ExtensionMethods::Numeric
Seawater::ExtensionMethods::Numeric

Don't forget that like extension methods defined in RewriteRails::ExtensionMethods, you need to make sure that Rails loads them before processing your .rr files.


RewriteRails