Example of making a Rails inspired minimalist web framework for Rack

config.ru

# ru means "rackup"

class Object
  private def erb(text)
    ERB.new(text).result(binding)
  end
end

class MyRails
  attr_accessor :routes
  def call(env)
    routes.each do |path, (controller, method)|
      regex = Regexp.new "^#{path}$".gsub(/:\w+/) { |name| "(?<#{name}>[^/]+)" }
      match = regex.match env['REQUEST_PATH']
      next unless match
      params = match.named_captures.transform_keys do |key|
        key[1..-1].to_sym
      end
      return controller.new(env, params).call(method)
    end

    [404, {'Content-Type' => 'text/html'}, [erb(<<~HTML)]]
    <h1>Not Found</h1>
    <p>Could not find a route for #{env['REQUEST_PATH'].inspect}</p>

    <h2>Known Routes</h2>
    <ul>
      <% routes.each do |path, _| %>
        <li><%= path.inspect %></li>
      <% end %>
    </ul>
    HTML
  end
end

User = Struct.new :name

class ApplicationController
  attr_reader :params

  def initialize(env, params)
    @env     = env
    @params  = params
    @status  = 200
    @headers = {'Content-Type' => 'text/html'}
  end

  def call(method)
    [@status, @headers, [erb(send(method))]]
  end
end

class UsersController < ApplicationController
  def index
    @users = [User.new("Josh"), User.new("Nurudeen")]
    <<~HTML
    <h1>users</h1>
    <ul>
      <% @users.each do |user| %>
        <li><%= user.name %></li>
      <% end %>
    </ul>
    HTML
  end

  def show
    @user = User.new(params[:name])
    <<~HTML
    <h1><%= @user.name %></h1>
    HTML
  end
end

rails = MyRails.new
rails.routes = {
  '/users' => [UsersController, :index],
  '/users/:name' => [UsersController, :show],
}


class RemoveTrailingSlashesMiddleware
  def initialize(app)
    @app = app
  end

  def call(env)
    env['REQUEST_PATH'].chomp! "/"
    @app.call(env)
  end
end

use RemoveTrailingSlashesMiddleware
run(rails)