Including a Module. With Parameters!

Posted on March 11, 2016 by Carsten Zimmermann

The Ruby include statement only allows for one type of argument: the module constant(s) to include. Oftentimes I wish it would be possible to pass arguments that the included module receives as optional parameters to its included callback like so:

# Note: This would be nice … but this code doesn't compile
module MyMixin
  def self.included(base, *here_be_more_args)
    # Have access to the additional args passed
    # in the include statement below
  end
end

class MyClass
  include MyMixin, :param1, :param2 # ...
end

Module#included is the method that gets called when the module is included somewhere. Unlike in my would-be-nice example, it only receives one argument: the constant of the including class.

Additional arguments to include are expected to be modules, allowing you to include more than one module on one line.

A way to parameterize the behaviour of included modules is to add a singleton method in the included class and then have that class method accept parameters to do whatever (e.g. set class-wide variables or dynamically define methods).

module MyMixin
  def self.included(base)
    base.define_singleton_method :define_greeter do |method_name, output|
      # A dynamically defined method that dynamically defines another
      # method. What a crazy world!
      define_method(method_name) { output }
    end
  end
end

class MyClass1
  include MyMixin
  define_greeter :morning, 'Good morning, sir/madam'
end

class MyClass2
  include MyMixin
  define_greeter :afternoon, 'Good afternoon! Would you like some tea?'
end

MyClass1.new.morning   # => 'Good morning, sir/madam!'
MyClass2.new.afternoon # => 'Good afternoon! Would you like some tea?'

This is obviously a contrived example, but what I dislike about it is the two lines of meta programming that serve the same purpose: dynamically adding something to the including class. Also, the define_greeter class method is probably not needed anywhere else in the class and this is against my sense of aesthetics.

Now, how about dynamically creating a module and then including that one? Like … a module factory? How do we even start? It’s actually quite easy: the include statement only accepts arguments of type Module. So all we have to make sure that whatever we pass to include returns a proper Ruby Module. I give you this:

class ModuleFactory # Yes, it's a class
  def self.new(greeter_name, greeter_output) # overriding the constructor
    Module.new do
      define_method(greeter_name) { greeter_output }
    end
    # Important: We don't return an instance of ModuleFactory here. The
    # (implicit) return value is the anonymous module.
  end
end

class MyClass
  include ModuleFactory.new(:evening, 'Good evening!')
end

MyClass.new.evening # => 'Good evening!'

Voila! ModuleFactory.new(…) returns a module which can be included, thus combining the include statement with the dynamic meta-programming. The provided examples are all a bit abstract but I will provide a more concrete (Rails 5!) example in a follow-up post.

Including a Module. With Parameters!

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