Question

Just for fun, again, but is it possible to take a block that contains method definitions and add those to an object, somehow? The following doesn’t work (I never expected it to), but just so you get the idea of what I’m playing around with.

I do know that I can reopen a class with class << existing_object and add methods that way, but is there a way for code to pass that information in a block?

I guess I’m trying to borrow a little Java thinking here.

def new(cls)
  obj = cls.new
  class << obj
    yield
  end
  obj
end

class Cat
  def meow
    puts "Meow"
  end
end

cat = new(Cat) {
  def purr
    puts "Prrrr..."
  end
}

cat.meow
  # => Meow

# Not working
cat.purr
  # => Prrrr...

EDIT | Here’s the working version of the above, based on edgerunner’s answer:

def new(cls, &block)
  obj = cls.new
  obj.instance_eval(&block)
  obj
end

class Cat
  def meow
    puts "Meow"
  end
end

cat = new(Cat) {
  def purr
    puts "Prrrr..."
  end
}

cat.meow
  # => Meow

cat.purr
  # => Prrrr...

Answer 1

You can use class_eval(also aliased as module_eval) or instance_eval to evaluate a block in the context of a class/module or an object instance respectively.

class Cat
  def meow
    puts "Meow"
  end
end

Cat.module_eval do
  def purr
    puts "Purr"
  end
end

kitty = Cat.new
kitty.meow #=> Meow
kitty.purr #=> Purr

kitty.instance_eval do
  def purr
    puts "Purrrrrrrrrr!"
  end
end

kitty.purr #=> Purrrrrrrrrr!