Quick "Hello World" for running Grape and Sinatra together using Rack::Cascade.

config.ru

# Put this in a new directory, then run `rackup`

require 'sinatra'
require 'grape'

class Web < Sinatra::Base
  get '/' do
    "Hello world."
  end
end

class API < Grape::API
  get :hello do
    {hello: "world"}
  end
end

use Rack::Session::Cookie
run Rack::Cascade.new [API, Web]