From: Ralph Shnelvar <ralphs@...32.com>
Date: 2010-02-04T12:11:10+09:00
Subject: Re: hooking subscript operations in a hash

RD> On Wed, Feb 3, 2010 at 9:38 PM, Ralph Shnelvar <ralphs@dos32.com> wrote:
>> In order to help debug something, I'd like to hook the hash subscript operation.

>> I've tried this (and several variants) in irb ... but it doesn't do what I want it to.


>> module RalphMod
>>  def []=(index, value)
>>    p index
>>    p value
>>    if index == 'PATH_INFO' and value == '/undefined'
>>      puts 'found it!'
>>    end
>>    super index, value
>>  end
>> end

>> class Hash
>>  extend RalphMod
>> end

>> env = {}
>> env['X'] = 'Y'
>> env['PATH_INFO'] = '/undefined'


>> The values are not displayed and 'found it!' is also not displayed.

RD> A few problems with this code.

RD> First

RD>    class Hash
RD>        extend RalphMod
RD>     end

RD>    makes the method in RalphMod a method of the Hash class object not
RD> hash instances.

RD> The way to do that would be

RD>   class Hash
RD>      include RalphMod
RD>   end

RD> but this wouldn't work either because methods in included modules come
RD> after methods defined in the class, so you'll still invoke the
RD> original []= method.

RD> What you want is something like this:

RD> module RalphMod
RD>  def []=(index, value)
RD>    p index
RD>    p value
RD>    if index == 'PATH_INFO' and value == '/undefined'
RD>      puts 'found it!'
RD>    end
RD>    super index, value
RD>  end
RD> end
RD> env = {}
RD> env.extend RalphMod
RD> env['X'] = 'Y'
RD> env['PATH_INFO'] = '/undefined'

RD> This effectively inserts your module between the instance of Hash
RD> referenced by the variable env and it's class.

Yeah ... tried that ... but apparently env is going in and out of scope.

Is there any way to do it for all hash subscript operations?