policy.rb

require 'virtus'

class User
  include Virtus.model

  attribute :confirmed_at, Time
  attribute :subscribed, Boolean, default: false
end

class Screencast
  include Virtus.model

  attribute :published_at, Time
  attribute :public, Boolean, default: false

  def private?
    !public
  end
end

class Policy
  def self.call(*args)
    new(*args).call
  end
end

class IsConfirmed < Policy
  include Virtus.model

  attribute :user

  def call
    !user.confirmed_at.nil?
  end
end

class IsSubscribed < Policy
  include Virtus.model

  attribute :user

  def call
    user.subscribed?
  end
end

class IsPublished < Policy
  include Virtus.model

  attribute :screencast

  def call
    !!screencast.published_at
  end
end

class IsPublic < Policy
  include Virtus.model

  attribute :screencast

  def call
    screencast.public?
  end
end

class IsPrivate < Policy
  include Virtus.model

  attribute :screencast

  def call
    screencast.private?
  end
end

def check(*args)
  params = args.pop
  policies = args

  results = {}

  policies.each do |policy|
    results[policy] = policy.call(params)
  end

  results
end

CheckFailed = Class.new(StandardError)

def check!(*args)
  results = check(*args)

  failures = results.select { |k, v| v != true }

  if failures.any?
    raise CheckFailed, "unmet conditions: #{failures.keys}"
  else
    true
  end
end

chuck = User.new(confirmed_at: Time.now, subscribed: true)
public_screencast = Screencast.new(published_at: Time.now, public: true)
private_screencast = Screencast.new(published_at: Time.now)
draft_screencast = Screencast.new(published_at: nil)

p check(IsConfirmed, user: chuck)
# => {IsConfirmed=>true}

p check(IsConfirmed, IsSubscribed, user: chuck)
# => {IsConfirmed=>true, IsSubscribed=>true}

p check(IsPublished, IsPublic, user: nil, screencast: public_screencast)
# => {IsPublished=>true, IsPublic=>true}

p check(IsPublished, IsPublic, user: nil, screencast: private_screencast)
# => {IsPublished=>true, IsPublic=>false}

p check(IsConfirmed, IsPublished, IsSubscribed, IsPrivate, user: chuck, screencast: private_screencast)
# => {IsConfirmed=>true, IsPublished=>true, IsSubscribed=>true, IsPrivate=>true}

p check!(IsConfirmed, IsSubscribed, IsPublished, user: chuck, screencast: draft_screencast)
# => policy.rb:98:in `check!': unmet conditions: [IsPublished] (CheckFailed)