Illustrates the outer middleware in a Rack script can be a lambda.

rack_of_lambdas.rb

# Illustrates that a Rack outer app can be a lambda.
# This code is stored at https://gist.github.com/keithrbennett/b178fe4b9910bfcc4ed6d76dc5411feb.

# See another approach at https://gist.github.com/keithrbennett/08aa675f04353b7c3a9faef2c53cddc8.

require "rack"
require "thin"

# Using a StringIO in case strings will be frozen.
BODY = StringIO.new

INNER_APP = -> (env) do
  BODY << "Hello World, from the inner middleware.\n"
  [ 200, { "Content-Type" => "text/plain" }, BODY ]
end

OUTER_APP = -> (env) do
  status, headers, body = INNER_APP.call(env)
  BODY << "Hello, World, from the outer middleware.\n"
  [status, headers, BODY.string]
end

Rack::Handler::Thin.run(OUTER_APP)

# Outputs:
#
# Hello World, from the inner middleware.
# Hello, World, from the outer middleware.