Defining attr like methods

ruby-talk
1

Here is what I'm trying to do

class List
  # Method item
  # Pushes an item into an array @items
  # Method to_s
  # returns elements within @items into a string
end

class Groceries < List
  item :lettuce
  item :potato
  item :ham
end

print Groceries.new

The trouble I'm having is figuring out how to go about adding an
instance variable, which is an array, and will be updated using these
attr like methods (I don't know what these are called which is giving me
some trouble). Is this possible to pull off?

···

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2

Here is what I'm trying to do

class List
  # Method item
  # Pushes an item into an array @items
  # Method to_s
  # returns elements within @items into a string
end

class Groceries < List
  item :lettuce
  item :potato
  item :ham
end

print Groceries.new

The trouble I'm having is figuring out how to go about adding an
instance variable, which is an array, and will be updated using these
attr like methods (I don't know what these are called which is giving me
some trouble).

You are looking for class methods.

Is this possible to pull off?

Yes, but I doubt it is what you want: you are asking for class methods to add items but you create an instance (Groceries.new). Where is the point in defining a list of items on class level and instantiate it multiple times?

If you describe what you want to achieve, i.e. what (business) problem you are trying to solve we can come up with other suggestions that may be more appropriate.

For example: this looks like a case for inheritance:

class Grocery
end

class Lettuce < Grocery
end

class Potato < Grocery
end
...

With a little bit of meta programming you can then get all the subclasses of Grocery.

Kind regards

  robert

···

On 20.04.2008 23:37, Chad Murphy wrote:

3

Robert Klemme wrote:

  item :lettuce
  item :potato
  item :ham
end

print Groceries.new

The trouble I'm having is figuring out how to go about adding an
instance variable, which is an array, and will be updated using these
attr like methods (I don't know what these are called which is giving me
some trouble).

You are looking for class methods.

Thanks.

Is this possible to pull off?

Yes, but I doubt it is what you want: you are asking for class methods
to add items but you create an instance (Groceries.new). Where is the
point in defining a list of items on class level and instantiate it
multiple times?

If you describe what you want to achieve, i.e. what (business) problem
you are trying to solve we can come up with other suggestions that may
be more appropriate.

For example: this looks like a case for inheritance:

class Grocery
end

class Lettuce < Grocery
end

class Potato < Grocery
end
...

With a little bit of meta programming you can then get all the
subclasses of Grocery.

I saw some code like this and tried playing around it. I think the
problem was that I had no idea whether it was something you could do in
order to abstract things from the sub class or something completely
different, but I realized you could do that with a method.

Kind regards

  robert

Thanks again.

···

On 20.04.2008 23:37, Chad Murphy wrote:

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Posted via http://www.ruby-forum.com/\.

4

Hi,

You might want to check out the Doodle rubygem at http://doodle.rubyforge.org/ .

Here's an example of its use:

daniel@daniel-desktop:~$ cat /tmp/doodle.rb
require 'rubygems'
require 'doodle'

class List < Doodle::Base
        has :items, :collect => :item
end

class GroceryList < List
        # nothing added here.
end

my_list = GroceryList do
        item "Chunky bacon"
        item "Lettuce"
        item "Tomato"
end

p my_list.items

daniel@daniel-desktop:~$ ruby /tmp/doodle.rb
["Chunky bacon", "Lettuce", "Tomato"]
daniel@daniel-desktop:~$

Dan

···

On Sun, Apr 20, 2008 at 7:09 PM, Chad Murphy <floppyformattingfrenzy@gmail.com> wrote:

Robert Klemme wrote:
> On 20.04.2008 23:37, Chad Murphy wrote:

>> item :lettuce
>> item :potato
>> item :ham
>> end
>>
>> print Groceries.new
>>
>> The trouble I'm having is figuring out how to go about adding an
>> instance variable, which is an array, and will be updated using these
>> attr like methods (I don't know what these are called which is giving me
>> some trouble).
>
> You are looking for class methods.

Thanks.

>> Is this possible to pull off?
>
> Yes, but I doubt it is what you want: you are asking for class methods
> to add items but you create an instance (Groceries.new). Where is the
> point in defining a list of items on class level and instantiate it
> multiple times?
>
> If you describe what you want to achieve, i.e. what (business) problem
> you are trying to solve we can come up with other suggestions that may
> be more appropriate.
>
> For example: this looks like a case for inheritance:
>
> class Grocery
> end
>
> class Lettuce < Grocery
> end
>
> class Potato < Grocery
> end
> ...
>
> With a little bit of meta programming you can then get all the
> subclasses of Grocery.

I saw some code like this and tried playing around it. I think the
problem was that I had no idea whether it was something you could do in
order to abstract things from the sub class or something completely
different, but I realized you could do that with a method.

> Kind regards
>
> robert

Thanks again.

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