From: Gavin Sinclair <gsinclair@...>
Date: 2003-01-13T16:38:03+09:00
Subject: Re: Array_Of(AClass)

On Monday, January 13, 2003, 5:37:21 PM, Tom wrote:

> i wonder, a coding challenge of sorts, what would it talk to create an 
> Array_Of(AClass) constructor? such that an array would be created that can 
> only contain objects of the class specified.

Sorry, I give up.  I can't see how to do it elegantly.  I guess it's
just a poor fit for Ruby.

My approach was like this:

  def ArrayOf(klass, *args)
    the_array = Array.new(*args)
    class << the_array
      #
      # Redefine ... here
      #
    end
    return the_array
  end

Where "..." is

  - +(ary)
  - []=(range, ary)

  - <<(obj)
  - []=(idx, obj)
  - []=(start, length, obj)
  - push(obj)
  - unshift(obj)

The methods are grouped like that because you need to typecheck the
last argument always.  In the first two cases that argument will be an
array (and you need to check the type of all the elements).  For the
rest you need to typecheck a single object.

[]= is a special case.  The last argument can be an object OR an
array.

I think the approach of using a singleton class is good, because you
can access the underlying Array method with "super".  My first though
was delegation, or even a blank class with just #method_missing.
However, in these cases, the resulting object wouldn't pass the "is_a?
Array" test.

I want to highlight this: from a singleton class, you can use "super"
like in normal inheritance.  Keep this in mind if you are redefining
methods anytime.  I find the pattern

  alias_method :old_xxx, :xxx

  def xxx
    ...
    old_xxx
    ...
  end

rather smelly.  Much better is

  class << obj
    def xxx
      ...
      super
      ...
    end
  end

I just learned that through this exercise, so I'm glad some good came
of it.

Gavin