Jay Fields' Thoughts

Neal Ford and I were recently looking at some code that used metaprogramming to define similar methods. The example below isn't the code, nor would I suggest managing state in this way; however, it does provide an example of metaprogrammatically defined methods.

class Ticket
  attr_accessor :status
  
  [:planned, :pre_sale, :on_sale].each do |method_name|
    define_method :"#{method_name}?" do
      status == method_name
    end
  end
end

The above code works, but I find it painful to maintain. When I begin to read [:planned, :pre_sale, :on_sale].each do |method_name| I'm not expecting to find methods being defined. Often, I define class methods that give me a more descriptive way to generate the similar methods. In this case I would consider creating a create_status_boolean_methods method. If I went down that path the code could look like the example found below.

class Ticket
  attr_accessor :status
  
  def self.create_status_boolean_methods(*methods)
    methods.each do |method_name|
      define_method :"#{method_name}?" do
        status == method_name
      end
    end
  end
  
  create_status_boolean_methods :planned, :pre_sale, :on_sale
end

This solution is decent, but there are a few things I don't like about it. Firstly, it's usually not important to understand how the create_status_boolean_methods method does it's job. Second, the definition of create_status_boolean_methods is often not near the actual usage of create_status_boolean_methods; therefore, on the rare occasion that you do need to understand how the methods are being defined you'll have to do a little searching within the class. Third, and perhaps the most obvious, it would take less code to define each method on it's own.

The above option is a good solution in some cases; however, Neal and I were looking for something simpler that didn't suffer from the above list of limitations. We came up with the following generalized solution to defining similar methods.

class Class
  def def_each(*method_names, &block)
    method_names.each do |method_name|
      define_method method_name do
        instance_exec method_name, &block
      end
    end
  end
end

Given the def_each method defined on Class the Ticket class becomes easier to read in my opinion. The reason it's easier (in my opinion) is because when I'm scanning the file and I see def_each :planned?, :pre_sale?, :on_sale? do |method_name| I know that similar methods are being defined. Below is the full code now required to define the Ticket class.

class Ticket
  attr_accessor :status
  
  def_each :planned?, :pre_sale?, :on_sale? do |method_name|
    :"#{status}?" == method_name
  end
end

You may have noticed the instance_exec method call in the implementation of the def_each method. I've previously written about instance_exec, but I'll include the implementation here for completeness.

class Object
  module InstanceExecHelper; end
  include InstanceExecHelper
  def instance_exec(*args, &block)
    begin
      old_critical, Thread.critical = Thread.critical, true
      n = 0
      n += 1 while respond_to?(mname="__instance_exec#{n}")
      InstanceExecHelper.module_eval{ define_method(mname, &block) }
    ensure
      Thread.critical = old_critical
    end
    begin
      ret = send(mname, *args)
    ensure
      InstanceExecHelper.module_eval{ remove_method(mname) } rescue nil
    end
    ret
  end
end

Labels: metaprogramming