From: Robert Klemme <bob.news@...>
Date: 2004-03-18T18:19:33+09:00
Subject: Re: Can subclass have superclass method return instance of subclass


<walter@mwsewall.com> schrieb im Newsbeitrag
news:4058461D.22531.E498AB@localhost...
>
> > walter@mwsewall.com wrote:
> >
> > > class QArray < Array
> > >   def sum_e
> > >     inject(0){|sum, item| sum+=item.e}
> > >   end
> > > end
> >
> > Why not just put sum_e into Array, instead of creating a subclass?
> > That would then allow it to be accessed from any array object.
> >
> >    class Array
> >      def sum_e
> >        ...
> >      end
> >    end
> >
> >    ...
> >    sum = a.find_all {...}.sum_e
> >
>
> That would work in this simple example...  What I find is that I tend
> to perform similar tasks on groups of Objects.

If it's general enough module Enumerable is the place to put it:

irb(main):001:0> module Enumerable
irb(main):002:1> def sum_e; inject(0){|sum,i|sum+i}; end
irb(main):003:1> end
=> nil
irb(main):004:0> (1..10).to_a.sum_e
=> 55
irb(main):005:0>

> We have classes that represent different objects in our domain ie
> Customers, invoices, deliveries, etc... and we tend to perform
> similar tasks on groups of these classes.  Sum all invoices,  find
> invoices that are overdue, find most recent delivery, etc.

If it's that special I'd do any of these:

(i) Create a module of your own and extend collections:

irb(main):001:0> module MyEnum
irb(main):002:1> def sum_e; inject(0){|sum,i|sum+i}; end
irb(main):003:1> end
=> nil
irb(main):004:0> a=(1..10).to_a
=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
irb(main):005:0> a.extend MyEnum
=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
irb(main):006:0> a.sum_e
=> 55

(ii) Create a module (or multiple modules) of your own with the functions
you need:

irb(main):001:0> module EnumAlgorithms
irb(main):002:1> def self.sum_e(enum); enum.inject(0){|sum,i|sum+i}; end
irb(main):003:1> end
=> nil
irb(main):004:0> EnumAlgorithms.sum_e( (1..10).to_a )
=> 55

> I have created CustomerList, InvoiceLists, etc, so I can embed the
> logic of the domain in one area and not have the same logic spread
> throughout the application and ui layers.  I really don't want to put
> all of that logic in Array, plus then I'd have to do a lot of type
> checking in many cases.

In that case I'd probably choose option (ii) and put the algorithmic
functions into the classes on which they operate, i.e.

class InvoiceLists
  def self.sum(invoices)
    invoices.inject(0){|sum,inv| sum += inv.amount}
  end
end

etc.

> Ruby has such beautiful Array functionality that I find I always end
> up wanting to use it in my customized subclasses.

Trying to convince methods that return Array to return something else will
not work.  Extending Array itself does work but should be done with care
because IMHO only the most general functionality that applies to Arrays
should be there.  There are other better ways to solve this (see above).

Kind regards

    robert