From: Sean O'Halpin <sean.ohalpin@...>
Date: 2008-11-28T03:30:30+09:00
Subject: Re: Ruby's duck typing

On Thu, Nov 27, 2008 at 5:10 PM, stephan.zimmer
<stephan.zimmer@googlemail.com> wrote:
> I would like to represent certain data by a list; to this end I let
> class SomeData inherit from Array:
>
>    class SomeData < Array
>        def get_field
>            self[2]
>        end
>    end
>
> (I want to keep it simple, therefore, the example might look silly.)
> Since the "get_field" method is not universal I don't want to reopen
> class "Array". If I write
>
>    SomeData.new([1,2,3]).get_field
>
> everything is fine. If I, however, try to do
>
>    [1,2,3].get_field
>
> I get an exception "NoMethodError", which, of course, is not
> surprising.
>
> My question is: is there a way around this, that is, to simply write
> [1,2,3] to denote a constant of type "SomeData" instead of always
> writing "SomeData.new([1,2,3])" (without reopening Array)?
>
> Thanks a lot,
>
> Stephan
>
>
The short answer is no.

A technique that a (select ;) few of us favour is to define factory
methods like this:

def SomeData(*a, &b)
  SomeData.new(*a, &b)
end

so you can write:

  a = SomeData [1,2,3]

You might also want to have a look at Ara Howard's arrayfields:
http://www.codeforpeople.com/lib/ruby/arrayfields/. Latest version
docs here: http://www.codeforpeople.com/lib/ruby/arrayfields/arrayfields-4.6.0/README

Having said all that, subclassing core classes tends to throw up a
host of fiddly issues. Robert's suggestion to use delegation instead
is a good one where your class doesn't need to be considered as a kind
of Array (in this case).

Regards,
Sean