rack_env_array.rb

require 'rack'

class StackFrame
  def initialize app
    @app = app
  end

  def call env
    p [:call, self.to_s, env]
    _env = env.reduce(:merge)
    _env[:a] += 1
    ret = @app.call(env + [_env])
  ensure
    p [:return, self.to_s, ret]
  end
end

class App
  def call env
    p [:call, self.to_s, env]
    ret = env.reduce(:merge)[:a]
  ensure
    p [:return, self.to_s, ret]
  end
end

app = Rack::Builder.new do
  use StackFrame
  use StackFrame
  run App.new
end

stack = [{}, {a: 0}]
a = app.call stack
puts "a is: #{a}"

# [:call, "#<StackFrame:0x007fd2848f2f60>", [{}, {:a=>0}]]
# [:call, "#<StackFrame:0x007fd2848f2f88>", [{}, {:a=>0}, {:a=>1}]]
# [:call, "#<App:0x007fd2848f3190>", [{}, {:a=>0}, {:a=>1}, {:a=>2}]]
# [:return, "#<App:0x007fd2848f3190>", 2]
# [:return, "#<StackFrame:0x007fd2848f2f88>", 2]
# [:return, "#<StackFrame:0x007fd2848f2f60>", 2]
# a is: 2