gistfile1.rb

namespace :app do
  namespace :error_pages do
    desc "Generates static pages 500, 403, 404, 422"
    task :generate => :environment do
      av = ActionView::Base.new
      av.class_eval do
        include ApplicationHelper
        include Rails.application.routes.url_helpers
      end

      file = Rails.root.join("app", "views", "layouts", "error_page.html.haml")
      [500, 403, 404, 422].each do |code|
        puts "Generate public/#{code}.html"
        page = File.join(Rails.public_path, "#{code}.html")
        File.open(page, "w") do |output|
          output.write av.render(file: file, locals: {code: code})
        end
      end
    end
  end
end