application.html.haml

%body
  - flash.each do |key, value|
    - key = "info" if key == "notice"
    - key = "danger"  if key == "alert"
    = content_tag :div, value, class: "alert alert-#{key}"

application_controller.rb

class ApplicationController < ActionController::Base
  # フラッシュメッセージのキーを追加
  add_flash_types :success, :info, :warning, :danger
end

samples_controller.rb

class SamplesController < ApplicationController
  def create
    review = current_user.reviews.new(review_params)
    if review.save
      redirect_to books_url, success: "Successfully created your prototype."
    else
      redirect_to root_url,  warning: "Unfortunately failed to create."
    end
  end
end