monkey patch This post is about limiting the scope of “monkey patches” with Ruby’s Refinements. This is certainly nothing new - there are numerous blogs posts and talks on this topic if you just Google around a bit. Despite Refinements having been around in Ruby since version 2.0, it’s not something we see very often. For that reason, it can help revist the topic in order to get re-acquainted with it, or learn something new.

The Problem with Ruby’s Open Classes

As usual, to understand a thing, we need to understand the problem that thing is meant to solve.

Open Classes

The Open Class technique, more commonly known as ‘monkey patching’, is a meta programming technique that lets the developer add new methods (or change existing ones) to a class at run-time. For example:

class String
  def randomize
    self.chars.shuffle.join
  end
end

In this example, the String class is re-opened and the randomize method is added. Doing this affects every existing instance of String and all new instances going forward. This works because instance methods are stored in the class object.

str = 'monkey'

begin
str.scramble 
rescue => e
  puts e.message
  #=> undefined method `scramble' for "monkey":String
end

class String
  def scramble
    self.chars.shuffle.join
  end
end

puts str.scramble 
#=> omknye

So, what’s the problem with this?

“Monkey patching” is global. The above change affects every String object in the entire application. Some things to consider:

  • Will it conflict with a 3rd party library?
  • Could we accidentally override an existing method?
  • Will our patch remain compatible with future versions of Ruby?

Refine

We can limit the scope of our “monkey patches” by calling refine inside a module definition:

module StringExtensions
  refine String do
    def scramble
      self.chars.shuffle.join
    end
  end
end

A Refinement is not active just by defining it. To activate a Refinement, it must be done explicitly with using.

Using

To activate Refinement call using:

using StringExtensions

We can do this inside a module or class so that our patch is only active inside a module or class definition.

class Scramble
  using StringExtensions
  def self.call(word)
    word.scramble
  end
end

Scramble.call('monkey')
#=> knmeyo

A Refinement is active is two places:

  1. Inside the refine block itself
  2. Starting at the place in the code where using was called until the end of the definition if inside a module or class.

Some Gotchas

  • Methods already called in a definition are not Refined after calling using. Here is an example:
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class Calculate
  def add1_to(num)
    num + one
  end

  def one
    1
  end
end

module CalculateExtensions
  refine Calculate do
    def one
      1.0
    end
  end
end

using CalculateExtensions
Calculate.new.one #=> 1.0
Calculate.new.add1_to(2) #=> 3

After we call using, it’s reasonable that one would expect Calculate.new.add1_to(2) to return 3.0 due to coercion, but we’re actually still adding 1 and not 1.0. That’s because the call to one inside add1_to(num) method on line 3 happens before the call to using on line 19.

  • calling using directly in IRB at the moment doesn’t work. You can learn more about this here.

Refinements are an easy way to limit the scope of our “monkey patches”, thereby making them much safer to implement. We can avoid unexpected results that can come with making global changes to our code, yet still take advantage of Open Classes.

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