cached_def.rb

module CachedDef
  def cached(name)
    meth = instance_method(name)

    raise ArgumentError.new("cached can be used only for method with no arguments") if meth.arity != 0

    alias_method "__cached_def_original__#{name}", name
    class_eval <<-EOS
      def #{name}
        @__cached_def_cache__#{name} ||= __cached_def_original__#{name}
      end
    EOS
  end
end


class Foo
  extend CachedDef

  def bar
    puts "computing bar"
    sleep 0.5
    1
  end

  cached def cached_bar
    puts "computing cached_bar"
    sleep 0.5
    2
  end
end


foo = Foo.new

3.times { puts foo.bar }
3.times { puts foo.cached_bar }

out.sh

⌘ ~/scratch λ ruby cached_def.rb
computing bar
1
computing bar
1
computing bar
1
computing cached_bar
2
2
2