Anonymous Classes In Ruby

Often times, I find myself wanting something anonymous. This occurs quite frequently in code when you need to define, pass or call some functionality which is usually very short and only useful in this moment. Many languages provide anonymous functions (usually called lambdas) for this sort of thing: haskell has \x y -> x + y and ruby has lambda {|x,y| x + y}, Proc.new and the new ->(x,y) syntax which I’m actually not very fond of.

Sometimes, in ruby, I find myself wanting an anonymous class for much the same reasons. At first, this seemed like a silly thing to do, so I didn’t expect it to be clean or easy – but in fact, it is. Ruby itself uses anonymous classes for all sorts of things, and the syntax we’ll use to do it is almost comically obvious.

Testing

Sometimes if you’re writing a test for a module, you need to include or extend it into something to accurately test it. Here’s one approach to doing that:

# assume MyModule is defined as a module, which you want to test

class ModuleTest < Test::Unit::TestCase
  def test_the_thing
    klass = MyClass.new

    # assert something about klass now that it's included your module
  end

  private

  class MyClass
    include MyModule

    # ...

  end
end

This is fairly contrived, but I think we all agree that sometimes you need a new class to test something (like modules). Putting in some private subclass for the purposes of testing seems fairly appropriate, albeit pretty smelly.

Let’s see how an anonymous class can help:

class ModuleTest < Test::Unit::TestCase
  def test_the_thing
    klass = Class.new do
      include MyModule

      # ...

    end.new

    # assert something about klass
  end
end

Not only is this a bit shorter, but I’d say it’s clearer too now that the object under test is made more prominent.

Rake tasks

I like to write rake tasks to do useful things. Sometimes one of those tasks wants to move files around. FileUtils is great for this, and it’s best used when mixed into a class.

I won’t bore you with the non-anonymous version, so here’s the one using Class.new, hopefully you can imagine it with more boilerplate:

require 'fileutils'

desc "do the damn thing"
task :run do
  Class.new do
    include FileUtils

    def run!
      mv this, that

      cp here, there

      rm the_thing
    end
  end.new.run!
end

So short!

This really speaks to ruby’s flexibility when it comes to “everything is an object” and hopefully illustrates that if you understand the benefits of anonymous functions, why not start thinking about how to use anonymous classes too?