Memoizing a method with an optional default value without receiving splat args

memoize_example.rb

module Memoize
  NOT_SET = Module.new
  private_constant :NOT_SET

  def memoize(method_name)
    meth = method method_name
    case meth.parameters.map(&:first)
    when [:opt]
      define_method method_name do |arg=NOT_SET|
        _memoizations.fetch arg do
          _memoizations[arg] = (arg == NOT_SET ? meth.call : meth.call(arg))
        end
      end
    else
      raise 'unoptimized path goes here'
    end
  end

  private def _memoizations
    @_memoizations ||= {}
  end
end

extend Memoize
memoize def greeting(name='world')
  name # => "world", "Josh"
  "Hello, #{name}"
end

greeting        # => "Hello, world"
greeting        # => "Hello, world"
greeting 'Josh' # => "Hello, Josh"
greeting 'Josh' # => "Hello, Josh"
_memoizations   # => {Memoize::NOT_SET=>"Hello, world", "Josh"=>"Hello, Josh"}