class Puma::Rack::Builder

def run(app)

run Heartbeat

end
end
[200, { "Content-Type" => "text/plain" }, ["OK"]]
def self.call(env)
class Heartbeat

However this could also be a class:

run lambda { |env| [200, { "Content-Type" => "text/plain" }, ["OK"]] }

The simplest form of this is a lambda object:
Takes an argument that is an object that responds to #call and returns a Rack response. 
def run(app)
  @run = app
end