Avoiding class_eval in Ruby metaprogramming

Question

I want to have a return_empty_set class method in Ruby, similar to the attr_reader methods. My proposed implementation is

class Class
  def return_empty_set *list
    list.each do |x|
      class_eval "def #{x}; Set.new; end"
    end
  end
end

and example usage:

class Foo
  return_empty_set :one
end
Foo.new.one  # returns #<Set: {}>

but resorting to a string seems like quite a hack. Is there a cleaner or better way to write this, perhaps avoiding class_eval? Or is this the best way to go?

Marc-André Lafortune · Accepted Answer · 2010-05-22 19:37:35Z

8

Use define_method:

class Module
  def return_empty_set *names
    names.each do |name|
      define_method(name){Set.new}
    end
  end
end

edited May 22, 2010 at 19:37

answered May 22, 2010 at 18:44

Marc-André Lafortune

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2 Comments

Peter Over a year ago

excellent, thanks. and you're right, it should be under Module, since then I can include this functionality.

Marc-André Lafortune Over a year ago

Not sure if that's what you mean, but the reason to put it under Module is that you can use it when inside a Module definition (to be included later) as well as a Class'.

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