From: Hugh Sasse <hgs@...>
Date: 2005-10-21T19:14:12+09:00
Subject: Re: Reflection, observ(er|able) instance vars?

On Fri, 21 Oct 2005, Daniel Sheppard wrote:

> Problem with the == method you defined is that hash equality != object
> equality - especially for your hashing method.

Yes, I'd spotted that and was going to take that into account.
Thanks
> 
> This module will provide you with the hash and eql? methods (alias those
> to other methods as appropriate). The hash function is cached, and reset
> whenever a var= method is called (make sure you don't go modifying
> hashed vars directly). 
> 
> This is probably not the only/best way to do this, but it made sense to
> me:
> 
> module FastHashEquals
>     def attr_hashing( *arr )
>         attr_reader *arr
>         hashers = class_eval "@@__hashers ||= []"
>         hashers.concat(arr)
>         arr.each do |var|
>             define_method(var) {
>                 instance_variable_get("@#{var}")
>             }
>             define_method("#{var}=") {|obj|
>                 instance_variable_set("@#{var}", obj)
>                 instance_variable_set(:@__equals_hash, nil)
>             }
>             define_method('eql?') { |other|
>                 hash === other.hash && hashers.all? { |x|
>                     send(x) == other.send(x)
>                 }                
>             }
>             define_method('hash') {
>                 @__equals_hash ||= hashers.inject(0) { |s,x|
>                     s + instance_variable_get("@#{x}").hash
>                 }              
>             }
>         end
>     end
> end
> 
> To use:
> 
> class HashTest
> 	extend FastHashEquals
> 	attr_hashing :one, :two, :three
> end

This "delivers us from eval" :-) but does define_method offer
anything else over the class_eval %{...} approach?  Maybe [only :-)]
speed?
> 
        Thank you,
        Hugh