From: Ezra Zygmuntowicz <ezmobius@...>
Date: 2008-02-20T03:40:02+09:00
Subject: Re: Object#freeze as a basis for caching of method results?

Hi~

On Feb 19, 2008, at 8:01 AM, Shot (Piotr Szotkowski) wrote:

> Hello, ruby-talk.
>
> After profiling my code I figured out I might want to attempt some
> caching of results obtained in ‘heavy’ methods. Is there an idiomatic
> way to do method-results caching in Ruby?
>
> The first thing that came to my mind as a mean to ensure the cache’s
> ‘freshness’ is to freeze the object in question. Does this make sense?
>
> For example, assume that a Blanket is a Set of Blocks, and
> that there’s a computation-intesive method ‘separations’:
>
>
>
> class Blanket < Set
>
>  def separations
>    seps = Set[]
>    # some ‘heavy’ code that builds seps
>    seps
>  end
>
> end
>
>
>
> Would the below make sense?
>
>
>
> class Blanket < Set
>
>  def freeze
>    each { |block| block.freeze }
>    super
>  end
>
>  def separations
>    return @seps if @seps
>    @seps = Set[]
>    # the ‘heavy’ code from that builds @seps this time
>    freeze
>    @seps
>  end
>
> end
>
>
>
> I guess my question boils down to what does exactly Object#freeze
> prevent from being modified – simply all the properties?
>
> If so, does it mean I can only have one method like the above (because
> if some other method did any freezing, I couldn’t initialise the @seps
> cache when the first call to separations occurs)?
>
> Is there any ‘best practice’ in Ruby
> with regards to caching method results?
>
> Is there any obvious other approach to such caching? (I thought about
> having a @cache instance variable that would get reset on object
> changes, but I’m not sure how to obtain all the list of methods that
> can change an object – i.e., methods that throw error when a given
> object is frozen.)
>
> Thanks in advance for any replies/suggestions/insights!
>


	The usual idiom for caching heavy results is something like this:

def foo
    @foo ||=  some_long_calculation
end

Cheers-


- Ezra Zygmuntowicz
-- Founder & Software Architect
-- ezra@engineyard.com
-- EngineYard.com