From: Christopher Dicely <cmdicely@...>
Date: 2009-02-07T02:44:28+09:00
Subject: Re: Special Hash Constructors

On Fri, Feb 6, 2009 at 9:09 AM, Robert Klemme
<shortcutter@googlemail.com> wrote:
>
> Hi there,
>
> we frequently do
>
> ha = Hash.new {|h,k| h[k] = []}
>
> so we can later do
>
> ha[any_key] << anything
>
> Since the idiom is so common, what does everybody think of putting it into
> the standard library:
>
> def Hash.with_arrays
>  new {|h,k| h[k] = []}
> end
>
> While we're at it, we might as well add
>
> def Hash.with_hashes
>  new {|h,k| h[k] = Hash.new(&h.default_proc)}
> end
>
> Kind regards
>
>        robert

Rather than making class methods on Hash to create special hashes like
this, why not make subclasses, so you can attach specialized instance
methods that make sense only for these kinds of hashes. For instance,
for the Hash-of-Hashes form, you might want a method that "rotates"
the Hash into a new Hash-of-Hashes like this:

def rotate
  res = HashOfHashes.new
  each { |ok, ov| ov.each { |ik, iv| res[ik][ok] = iv } }
  res
end