From: "Molitor, Stephen L" <Stephen.L.Molitor@...>
Date: 2006-06-28T03:59:00+09:00
Subject: Re: Ruby and Java equality usage

>>>> Jacob Fugal wrote:

> Is it true then that a.eql?(b) only if a.hash == b.hash?

Yes.  But not necessarily the vice versa.  If a.eql?(b) is true, then
a.hash == b.hash must also be true.  And if a.hash != b.hash, then
a.eql?(b) is necessarily false.  However if a.hash == b.hash, then
a.eql?(b) may be true or false.  (The above is all assuming proper
implementations of hash and eql?.)

> What's the damage in a.eql?(b) returning true when in different
> buckets?

That will never happen.  If a and b have different hashes then they go
in different buckets.  And if their hashes are different they are not
equivalent (not eql?).  


> Why doesn't the default implementation of Object#eql? just use #==
>  internally?

Good question!  Or to put it another way why do we need separate == and
eql? methods?  The only rationale I can think of is to completely
separate the hash related methods (eql? and hash) from ==.  So I can
override == knowing that I won't be changing hash behavior at all.
Except that if I'm overriding == I would usually want to change the hash
related behaviors to be consistent with my new concept of equivalence!

Steve