application_controller.rb

class ApplicationController < ActionController::Base
  # Creates an accessor which is exposed to the view
  def self.view_accessor(*names)
    attr_accessor *names
    helper_method *names
  end
end

articles_controller.rb

class ArticlesController < ApplicationController
  # List the variables/methods you want to expose to the view
  view_accessor :article, :articles
  
  def index
    # Assign the variable in the action
    self.articles = Article.all
  end
  
  def show
  end
  
  protected
  
  # Or implement directly as a method
  def article
    Article.find(params[:id])
  end
end

index.html.haml

%h1 Articles
-# Simply call the articles method instead of the instance variable.
- articles.each do |article|
  %article
    %h1.title= article.title

z_articles_controller_spec.rb

require 'spec_helper'

# Easy to test as well.

describe ArticlesController do
  describe "index" do
    let!(:articles) { create_list :article, 3 }
    
    it "assigns all the articles" do
      get :index
      controller.articles.should == articles
    end
  end
end