I want this for my own :P

bench.rb

require "rubygems"
require "rbench"

RBench.run(500) do
  report("memoed") do
    Foo.memoed("hello")
  end
  report("unmemoed") do
    Foo.unmemoed("hello")
  end  
end

example.rb

class Foo
  class << self
    memoize do
      def memoed(baz)
        baz * 1000
      end
    end
    
    def unmemoed(baz)
      baz * 1000
    end
  end
  
  memoize do
    def memoed(baz)
      baz * 1000
    end
  end
  
  def unmemoed(baz)
    baz * 1000
  end
end

memoizer.rb

module Memoizer  
  def method_added(meth)
    @_methods_to_memoize ||= []
    @_methods_to_memoize << meth
  end
  
  def methods_to_memoize
    @_methods_to_memoize
  end
end

simple-works.rb

module Memoizable
  def self.extended(klass)
    klass.class_eval do
      def clear_memo!(name, args = nil)
        ivar = "@_memoized_#{name}"
        if args == nil
          instance_variable_set("@_#{name}_is_memoized", false)
          instance_variable_set(ivar, nil)
        else
          instance_variable_get(ivar).delete(args)
        end
      end
    end
  end
  
  def memoize(kind = Hash, &blk)
    memoizer = Module.new do
      extend Memoizer
    end
    
    memoizer.class_eval(&blk)
    self.send(:include, memoizer)
    
    memoizer.methods_to_memoize.each do |meth|
      arity = self.instance_method(meth).arity
      ivar = "@_memoized_#{meth}" 
      if arity == 0
        self.class_eval <<-RUBY
          def #{meth}                                # def formats
            return #{ivar} if @_#{meth}_is_memoized  #   return @_memoized_formats if @_formats_is_memoized
            #{ivar} = super                          #   @_memoized_formats = super
            @_#{meth}_is_memoized = true             #   @_formats_is_memoized = true
          end                                        # end
        RUBY
      else
        args = arity == 1 ? "args" : "*args"
        self.class_eval <<-RUBY
          def #{meth}(#{args})                       # def find_layout(*args)
            #{ivar} ||= #{kind}.new                  #   @_memoized_find_layout ||= Hash.new
            if memo = #{ivar}[args]                  #   if memo = @_memoized_find_layout[args]
              return memo                            #     return memo
            end                                      #   end
            #{ivar}[args] = super                    #   @_memoized_find_layout[args] = super
          end                                        # end
        RUBY
      end
    end
  end
end