ugh.rb

####
# It must output this:
#
#  hello one two
#  world one two
#
class Filter
  def initialize name
    @name = name
  end

  def call app, request
    app.call(request) do |body, status, headers|
      yield ["#{body} #{@name}", status, headers]
    end
  end
end

class App
  def call request
    yield ["hello", 200, {}]
    yield ["world", nil, nil]
  end
end

class Context
  def initialize(app, filters)
    @chain = filters.inject(app) do |a, filter|
      lambda do |request, &block|
        filter.call(a, request, &block)
      end
    end
  end

  def serve request
    @chain.call(request) do |triple|
      write triple.first
    end
  end

  def write str
    puts str
  end
end

app     = App.new
filters = [Filter.new("one"), Filter.new("two")]
ctx     = Context.new(app, filters)
ctx.serve({})