This is a Valid Rack Syntac

application.rb

# This is a valid syntax
class App
  def call(env)
    [200, {'Content-Type' => 'text/plain'}, ['Hello World']]
  end
end


# This is a valid syntax (singleton)
class App
  def self.call(env)
    [200, {'Content-Type' => 'text/plain'}, ['Hello World']]
  end
end

# This is a valid syntax (lambda 1)
app = ->(env) { [200, {'Content-Type' => 'text/plain'}, ['Hello World']] }

# This is a valid syntax (lambda 2)
app = lamda { |env| [200, {'Content-Type' => 'text/plain'}, ['Hello World']] }

# This is a valid syntax (proc)
app = Proc.new do |env|
  [200, {'Content-Type' => 'text/plain'}, ['Hello World']] 
end