greeter.rb

class Greeter

  def greet(name)
    "Hello #{name}"
  end

end

class User

  def first_name
    "Foo"
  end

  def greet_with(&block)
    Greeter.new.instance_exec(&block)
  end

  def greet_first_name
    p = Proc.new { first_name }
    greet_with { greet(p.call) }
  end

end

puts User.new.greet_first_name