From: Gavin Sinclair <gsinclair@...>
Date: 2004-06-18T16:06:32+09:00
Subject: Re: How to make generic #== method?

Zakaria wrote:
>
> I remove the eql? and hash method because
> 1) I don't need it

You probably need hash, and should define it (see below).  About eql?, I'm
under the understanding that eql? and == should always return the same
thing.

> 2) It doesn't work if the class has hash attribute because
>    {'a' => 1}.hash != {'a' => 1}.hash

Now that is weird.  I've a faint recollection of it being mentioned on
ruby-talk before though, so perhaps its not a bug.

> 3) I'm not really understand the functionality to test it
>
> Could someone enlighten me why Hash#hash doesn't result the same
> and where .hash and .eql? used ?

#hash is used to generate a hash code, which is used to put an object into
a hash (i.e. a Hash object).  If two objects have the same #hash value,
then a Hash thinks they're probably the same, but it's not guaranteed, so
it double-checks with #eql?.  However, if two objects have *different*
#hash values, then a Hash considers them to be definitely different.

Bottom line, if you (re)define #== in an object, you really really should
(re)define #hash as well, to make sure they are consistent.

> PS: Is there any place where I could post this snippet,
>     so others could use it?

As it happens, I've already written a detailed consideration of Joel's
code for the not-yet-formally-announced project 'addlib', so that people
can reuse it without having to manage the code themselves.

Until then, however, you might take a look at the snippet repository on
RubyForge.

Joel's code is better, however.  What yours adds is the implicit
definition of #contents (in Joel's terminology).  So why not:

  module AttrsEquality
    include ContentEquality
    def contents
      res = {}
      instance_variables.each {|n| res[n.intern] = instance_variable_get(n) }
      res
    end
  end

Gavin