Decorating with metaprogramming

decorator.rb

class Decorator
  attr_reader :klass, :config, :decorator

  def initialize(klass, config, decorator)
    @klass = klass
    @config = config
    @decorator = decorator
  end

  def >(method_name)
    method = klass.instance_method(method_name)
    # Only local variables are accessible from within `define_method`
    decorator = self.decorator
    config = self.config

    # Redefine the method wrapping it with the decorator
    klass.define_method(method_name) do |*args|
      # `self` here is an instance of the class using the decorator.
      # Both `method` and `decorator` are unbound and we can now bind them to
      # this instance.
      bound_method = method.bind(self)
      bound_decorator = decorator.bind(self)

      bound_decorator.call(config, bound_method, args)
    end
  end
end

module Decoratable
  def decorate(method_name)
    method = instance_method(method_name)

    define_method(method_name) do |*config|
      Decorator.new(self, config, method)
    end
  end
end

demo.rb

class Bar
  extend Memoizable

  memoize >
  def bar
    puts 'first time!'
    'bar'
  end
end

b = Bar.new # => #<Bar>
b.bar       # => puts 'first time!' and returns 'bar'
b.inspect   # => #<Bar @bar="bar">
b.bar       # => only returns 'bar'

require 'benchmark/ips'

class TraditionalBar
  def bar
    @bar ||= begin
      puts 'first time!'
      'bar'
    end
  end
end

Benchmark.ips do |x|
  meta = Bar.new
  traditional = TraditionalBar.new

  x.report('meta') { meta.bar }
  x.report('traditional') { traditional.bar }

  x.compare!
end

# Warming up --------------------------------------
#                 meta
#     50.895k i/100ms
#          traditional
#    275.834k i/100ms
# Calculating -------------------------------------
#                 meta    590.188k (± 3.7%) i/s -      2.952M in   5.009063s
#          traditional      8.173M (± 3.8%) i/s -     40.823M in   5.003081s
#
# Comparison:
#          traditional:  8173312.4 i/s
#                 meta:   590187.6 i/s - 13.85x  slower

memoizable.rb

module Memoizable
  extend Decoratable

  # `config` is not used here, but could be used for configuring the decorator.
  # Example:
  #   # `config` here would be `[{ times: 5 }]`
  #   retry(times: 5) >
  decorate def memoize(config, method, method_args)
    var_name = "@#{method.name}"

    if instance_variable_defined?(var_name)
      instance_variable_get(var_name)
    else
      result = method.call(*method_args)
      instance_variable_set(var_name, result)
      result
    end
  end
end