From: Ken Bloom <kbloom@...>
Date: 2008-05-01T14:10:06+09:00
Subject: Re: How to create "def method(item)= (value)" ?

On Wed, 30 Apr 2008 09:56:04 -0500, I単aki Baz Castillo wrote:

> 2008/4/30, Ken Bloom <kbloom@gmail.com>:
>> def initialize
>>   @params={}
>>  end
>>
>>  def param
>>   @params
>>  end
> 
> But I don't want that since it allows any self modification way (.strip!
>  .chomp! ...) and I just want to allow []=
> 
> Thanks.

In that case, do as I wrote further on and have the param function return 
a proxy that edits the params array in only the ways you want.

class ParamsProxy
  def initialize hash
    #thanks to duck typing hash can be an Array instead, if you'd like
    @hash=hash
  end
  def [] key
    #with .dup here, mutating operations on the returned value
    #will not affect the values stored in the @hash
    #note that even attr_reader isn't this careful about the
    #objects that it returns, and so if you want behavior
    #like attr_reader, then omit the .dup here
    @hash[key].dup
  end
  def []= key,value
    @hash[key]=value
  end
end

class MyClass
  def initialize
    @params={}
  end

  def param
    ParamsProxy.new @params
  end
end


I presented this in rough outline before, without code, because there are 
many ways one could want to dress this up -- having some kind of 
validation code, or having the object.param[key]=value update something 
else entirely, or shoving the whole thing behind a nice attr_accessor-
like interface. I don't have the time now to think through all of the 
possibilities involved to come up with a general solution, nor do I have 
a clear idea of exactly what your other constraints are. But start here, 
be creative, and let us know exactly what you do come up with.

--Ken

-- 
Ken (Chanoch) Bloom. PhD candidate. Linguistic Cognition Laboratory.
Department of Computer Science. Illinois Institute of Technology.
http://www.iit.edu/~kbloom1/