lambda_fun.rb

# Simple user class
# We're pretending this is an ActiveRecord::Base model
class User
  attr_reader :name

  def initialize(name)
    @name = name
  end
end

# Controller class
# We're pretending this is an ActionController::Base
class Controller
  def self.before_action(lmb = ->{})
    @@before_actions ||= []
    @@before_actions << lmb
  end

  def run_before_actions!(context)
    @@before_actions.each do |a|
      context.instance_exec(&a)
    end
  end
end

# Our own controller
# This is like when we make a new controller in a Rails app
class UsersController < Controller
  before_action -> { puts @user.name }

  def create
    @user = User.new('Joe')
    run_before_actions!(self)
  end
end

# We make a new instance of our controller and call the #create action
controller = UsersController.new
controller.create