From: Ara.T.Howard@...
Date: 2004-10-01T11:35:01+09:00
Subject: Re: Can you rewrite this in a better way?

On Fri, 1 Oct 2004, Bob Sidebotham wrote:

> I would like to define a method that can be used to compactly define some 
> items of interest (here called "fields") for subclasses. It's used like this:
>
> class B < A
>  fields :a, :b, :c
> end
>
> class C < A
>  fields :x, :y
> end
>
> p B.new.fields                => [:a, :b, :c]
> p C.new.fields                => [:x, :y]
>
> Here are two definitions for class A that support these semantics:
>
> ---#1------------------------------------------------
>
> class A
>  def self.fields(*fields)
>    @fields = fields
>    def fields
>      self.class.getFields
>    end
>  end
>
>  private
>  def self.getFields
>    @fields
>  end
> end
>
> ---#2------------------------------------------------
>
> class A
>  def self.fields(*fields)
>    eval "def fields
>      [#{fields.collect{|f| ':' + f.to_s}.join(',')}]
>    end"
>  end
> end
>
> -----------------------------------------------------
>
> Is there a better way (more efficient, shorter and/or clearer) to do this?
>
> Thanks!
> Bob Sidebotham

i don't know if it's any better, but:

   harp:~ > cat a.rb

     class A
       def self.fields(*f); @fields||=f; end
       def fields; self.class.fields; end
     end
     class B < A
       fields :a, :b, :c
     end
     class C < A
       fields :x, :y
     end
     p B.new.fields                # => [:a, :b, :c]
     p C.new.fields                # => [:x, :y]

   harp:~ > ruby a.rb

     [:a, :b, :c]
     [:x, :y]

this would be a great place for a mixin:

   harp:~ > cat a.rb

     module Fieldable
       def self.append_features klass
         class << klass
           def fields(*f); @fields||=f; end
         end
         super
       end
       def fields; self.class.fields; end
     end
     class B
       include Fieldable
       fields :a, :b, :c
     end
     class C
       include Fieldable
       fields :x, :y
     end
     p B.new.fields                # => [:a, :b, :c]
     p C.new.fields                # => [:x, :y]

   harp:~ > ruby a.rb

     [:a, :b, :c]
     [:x, :y]


it's not shorter - but it frees you from inheritence, which breaks
encapsulation.

kind regards.

-a
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