From: James Edward Gray II <james@...>
Date: 2010-11-02T06:55:55+09:00
Subject: Re: the dark side of inherited methods

On Nov 1, 2010, at 4:32 PM, Daniel Berger wrote:

> On Oct 31, 5:49 pm, James Edward Gray II <ja...@graysoftinc.com>
> wrote:
>> On Oct 31, 2010, at 5:30 PM, timr wrote:
>> 
>>> Let's say I want to make a new class, Vector (that will function,
>>> eventually, like R vectors for mathematical operations), and I write
>>> this:
>> 
>>> class Vector < Array
>>>  def initialize(*arr)
>>>    super(arr.flatten)
>>>  end
>>> end
>> 
>>> v = Vector.new(1,2,3) # => [1, 2, 3]
>>> v.class # => Vector
>>> v.collect{|item| item * 3}.class # => Array
>>> v.collect!{|item| item * 3}.class # => Vector
>> 
>>> I noticed that I inherited the Array#collect and Array#collect!
>>> methods. Yeah for inheritance! But then look at the returned classes.
>>> Array#collect returns and array. Array.collect! returns a Vector.
>>> Is there a simple way to fix this for all similar methods? Or do I just have to live with the dark side of inherited methods?
>> 
>> It is a problem with inheritance and another reason why inheritance is almost never what we want.  Luckily, this is Ruby where anything is possible:
>> 
>> class Vector < BasicObject
>>   def initialize(*array)
>>     @array = array.flatten
>>   end
>> 
>>   def class
>>     ::Vector
>>   end
>> 
>>   def method_missing(meth, *args, &blk)
>>     result = @array.send(meth, *args, &blk)
>>     if result.object_id == @array.object_id
>>       self
>>     elsif result.class == ::Array
>>       self.class.new(result)
>>     else
>>       result
>>     end
>>   end
>> end
>> v = Vector.new(1,2,3)                # => [1, 2, 3]
>> v.class                              # => Vector
>> v.collect { |item| item * 3 }.class  # => Array
>> v.collect!{ |item| item * 3 }.class  # => Vector
>> 
>> That's a solution for Ruby 1.9, but a similar trick is possible with 1.8 using a touch more code.  It's probably not perfect yet, but you get the idea.
> 
> Eeep, method_missing.

My first reaction when I saw the original code was, "Eeep, inheriting from a core class."

I guess we both have some fear to get past.  :)

James Edward Gray II