From: "Jesús Gabriel y Galán" <jgabrielygalan@...>
Date: 2008-06-09T21:15:15+09:00
Subject: Re: double assignation in a hash

On Sun, Jun 8, 2008 at 10:25 PM, nico Itkin <nicolas.papon@gmail.com> wrote:
> Thanks evebody for yours answers, helps a lot :) !
>
> Jesús : looks fantastic, but could you give me fex explanation how it
> works?
>> a = Hash.new {|h,k| h[k] = Hash.new(&h.default_proc)}
>
> is this a way to override the method new for the object a?
>
> You're doing so by passing the following block but then i'm lost, do you
> have any links to share which detail that ?

Hash.new receives a block, which is executed whenever you try to access
a non-existing key. In the block you can assign a value to the key in the hash.
The value that we assign above is a hash. If we did this:

a = Hash.new {|h,k| h[k] = Hash.new}

Then we would achieve a two-level hash, but the second level hash,
wouldn't be able to assign a next level hash to a non-existing key, because
a block wasn't provided.

Hash has a default_proc variable that contains the block (in proc form)
that was passed to the constructor, so if we assign this proc as the block
for the constructor, we get a hash that can assign a hash to a non-existing key,
in a recursive way, cause each hash will pass its default proc to the hash
created in the constructor block.

So many words that I don't know if I made myself clear...

Jesus.