From: Robert Klemme <shortcutter@...>
Date: 2010-04-06T02:15:11+09:00
Subject: Re: Nested hash

On 04/05/2010 04:34 PM, Samuel Smith wrote:
> Hello All.
> 
> Sample code:
> 
> class NestedHash < Hash
>   attr_accessor :val
>   alias :to_str :to_s
>   @val=0
>   def initialize
>     blk = lambda {|h,k| h[k] = NestedHash.new(&blk)}
>     super(&blk)
>   end
>   def []= (key,value)
>     if value.is_a?(NestedHash)
>       super(key,value)
>     else
>       self[key].val=value;
>     end
>   end
> end
> 
> test=NestedHash.new();
> test['1']='11'
> test['1']['2']='12'
> test['1']['3']='13'
> test['2']['4']='24'
> 
> s=test['1'].val #s='11'
> 
> Work as expected. In C++ by using typecast overloading it is possible to
> implement  class that will work that way:
> s=test['1']# s='11'
> 
> Is it possible to implement same functionality in Ruby?

No.  Reason: you cannot override variable assignment - you can only 
override assignment method like you did with []=.

> I've tried:
> 
> alias :to_s :to_str
> def to_str
>   @val
> end
> 
> But first of all result values are wrong and i can't understand why.
> Secondly it works only in constructions like that:
> s=" "+test['1'] # ie i must tell Ruby that i need string result
> but not in
> s=test['1'] # if it were possible to tell Ruby that it use string result
> by default
> 
> Any other suggestions?

I would disallow mixing nesting and values.  Compare it to a filesystem: 
something is either a directory or not.  If it is a directory you can 
put arbitrary things in it but there is no special file for a certain 
directory (keeping resource forks and other file system specialties 
aside for a moment).  This is conceptually simpler - and it also makes 
the implementation easier because you do not need an extra class for this:

robert@fussel:~$ ruby19 -e 'ha=Hash.new {|h,k| 
h[k]=Hash.new(&h.default_proc)};ha[1][2][3]=4;p ha'
{1=>{2=>{3=>4}}}
robert@fussel:~$

Depending on the context a totally different approach might be even more 
reasonable: if the types you want to nest are known you could create a 
set of Struct classes that exactly represent what you want to model.  Of 
course this approach does not work with a generic mechanism (e.g. 
storing a different representation of any XML document).

Kind regards

	robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/