Question

I am writing an internal DSL in Ruby. For this, I need to programmatically create named classes and nested classes. What is the best way to do so? I recon that there are two ways to do so:

1. Use Class.new to create an anonymous class, then use define_method to add methods to it, and finally call const_set to add them as named constants to some namespace.
2. Use some sort of eval

I’ve tested the first way and it worked, but being new to Ruby, I am not sure that putting classes as constants is the right way.

Are there other, better ways? If not, which of the above is preferable?

Answer 1

If you want to create a class with a dynamic name, you’ll have to do almost exactly what you said. However, you do not need to use define_method. You can just pass a block to Class.new in which you initialize the class. This is semantically identical to the contents of class/end.

Remember with const_set, to be conscientious of the receiver (self) in that scope. If you want the class defined globally you will need to call const_set on the TopLevel module (which varies in name and detail by Ruby).

a_new_class = Class.new(Object) do
  attr_accessor :x

  def initialize(x)
    print #{self.class} initialized with #{x}"
    @x = x
  end
end

SomeModule.const_set("ClassName", a_new_class)

c = ClassName.new(10)

...