From: Chad Perrin <code@...>
Date: 2011-12-06T04:59:26+09:00
Subject: Re: black magical hash element vivification

On Tue, Dec 06, 2011 at 01:34:39AM +0900, Adam Prescott wrote:
> On Mon, Dec 5, 2011 at 16:28, Chad Perrin <code@apotheon.net> wrote:
> 
> >    >> foo = Hash.new( Hash.new )
> >    => {}
> >    >> foo[3][2] = true
> >    => true
> >    >> foo
> >    => {}
> >
> 
> This is odd.
> 
> 
> >    >> foo[3]
> >    => {2=>true}
> >    >> foo[2]
> >    => {2=>true}
> >
> 
> This isn't. Hash.new(Hash.new) != Hash.new { |h, k| h[k] = Hash.new }. The
> left hand side uses the same object for each default access, hence f =
> Hash.new([]); f[1] << 1; f[2].include?(1) == true. The right hand-side
> behaves as you expect for this part.

I figured out that Hash.new(Hash.new) used the same object for all of
them, but found that quite surprising.  Now that I see it side-by-side
with the block argument form, though, I realize it should not have
surprised me as it did.

Thanks for helping me fix my code.


> 
> foo.rehash didn't fix anything. Looks like a bug to me?

That's what I was wondering -- whether I'd found a bug in there
somewhere.