rack_of_lambdas_method_returning_lambda.rb

# Like the previous gist (https://gist.github.com/keithrbennett/b178fe4b9910bfcc4ed6d76dc5411feb)
# but illustrates a use case more similar to the class based approach
# illustrated in https://thoughtbot.com/upcase/videos/rack.
# In this code, a method is called to return a lambda that calls the passed inner app.

# This code is stored 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

# Returns a lambda that, when called with the environment,
# calls the inner_app parameter.
def fn_outer_app(inner)
  ->(env) do
    status, headers, body = inner.call(env)
    BODY << "Hello, World, from the outer middleware.\n"
    [status, headers, BODY.string]
  end
end

Rack::Handler::Thin.run(fn_outer_app(INNER_APP))

# Outputs:
#
# 2019-07-05 16:56:36 -0400. Using partial application...
# Hello World, from the inner middleware.
# Hello, World, from the outer middleware.