From: Farrel Lifson <farrel.lifson@...>
Date: 2006-09-14T00:51:43+09:00
Subject: Re: What does it take to make an object hashable, no really...

On 13/09/06, Tim Pease <tim.pease@gmail.com> wrote:
> On 9/13/06, Just Another Victim of the Ambient Morality
> <ihatespam@rogers.com> wrote:
> >     Suppose I want to define a class and be able to use this class as a key
> > to a hash.  How does one do this?  ...And please test your response before
> > posting it.
>
> Chapter 22 of the Programming Ruby, The Pragmatic Programmer's Guide
>
> A key must respond to "hash" and the value returned must not change.
>
> What is not in the book is that you must also provide an eql? method
> that evaluates to true when two objects are equivalent.
>
> class C
>   def hash() "C".hash end
> end
>
> h = {}
> h[C.new] = 1
> h[C.new]
>
> => nil
>
> class C
>   def eql?(other) true end
> end
>
> h[C.new]
>
> => 1
>
>
>
> >     Secondly, if an objects works in a hash, is it guaranteed to work in a
> > set?
>
> No idea about this one.  Have not used the Ruby Set class thus far.
>
> TwP
>
>

Is there a reason why an implementation of eql? is needed instead of
just being able to define <=>?

Farrel