From: Mike Stok <mike@...>
Date: 2004-04-25T01:44:15+09:00
Subject: Re: pushing hash

Joel VanderWerf <vjoel@PATH.Berkeley.EDU> writes:

> Dave Thomas wrote:
> > 
> > On Apr 23, 2004, at 21:49, Pe�a, Botp wrote:
> > 
> >> Is there a reason why "<<" isn't allowed for hashes?
> > 
> > 
> > Perhaps because hashes aren't ordered, and the common semantics of "<<" 
> > implies order.
> 
> Can't be the whole story...
> 
> irb(main):001:0> require 'set'
> => true
> irb(main):002:0> Set.new << 3
> => #<Set: {3}>

I don't think it's ordering so much as Arrays and Sets can have things
added to them "unambiguously."  The Array gets the new value added to
the end, and a set gets a new member added if it doesn't already
contain the member.

When adding a key/value pair to a hash there are two situations to consider:

1) It's a new key, so just add the key/value pair

2) It's an existing key.  What should we do?  Replace the old
   key/valur pair or somehow mutate the existing value? 

If you just want to replace existing key/value pairs then

  hash << ['key', 'value']

just means hash['key'] = 'value'.

Well, that's my take.

Hope this helps,

Mike

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