Ruby object that has "functions" rather than methods

fun.rb

class Function
  attr_reader :name, :fn

  class Composed
    attr_reader :left, :right

    def initialize(left, right)
      @left = left
      @right = right
    end

    def call(input)
      left.(right.(input))
    end
  end

  def initialize(name, fn)
    @fn = fn
  end

  def *(other)
    Composed.new(self, other)
  end

  def call(input)
    fn.(input)
  end
end

class FunctionObject
  def self.method_added(name)
    return if @busy
    meth = instance_method(name)
    defn(name, meth)
  end

  def self.defn(name, meth)
    @busy = true

    undef_method(name)

    define_method(name) do
      Function.new(name, meth.bind(self))
    end

    @busy = false
  end
end

RSpec.describe FunctionObject do
  it 'has functions' do
    things = Class.new(FunctionObject) do
      def sort(data)
        data.sort
      end

      def upcase(data)
        data.map(&:upcase)
      end
    end.new

    data = %w(foo bar)

    expect((things.sort * things.upcase).(data)).to eql(%w(BAR FOO))
  end
end