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comp.lang.ruby

metaprogramming to generate list of class-specific attributes

Phlip

9/14/2007 8:05:00 PM

Rubyists:

Here's a test case illustrates the problem. Then we'll have a candidate fix
and some questions.

class Kozmik
def self.acts_as_frog(species)
@@frog_list << species
end
@@frog_list = []
cattr_accessor :frog_list
end

class Toad < Kozmik
acts_as_frog :bufo
acts_as_frog :kermit
end

class BullFrog < Kozmik
acts_as_frog :tree
acts_as_frog :riparian
end

def test_fun_with_metaclasses
assert_equal [:tree, :riparian], BullFrog.frog_list
assert_equal [:bufo, :kermit], Toad.frog_list
end

Suppose we need an 'acts_as_' system to generate some methods for our target
classes. And suppose one method is a list of all the arguments passed to
each acts_as_() in each specific class.

The above test fails because @@ occupies the entire inheritance chain. So
both frog_list() class-methods return [:tree, :riparian, :bufo, :kermit].
The fix seems to be to reach into the meta-class for each derived class:

class Kozmik
def self.acts_as_frog(species)
@frog_list ||= []
@frog_list << species
self.class.send :attr_accessor, :frog_list
end
end

Is that as clean and lean as it could be? Is there some way to do this with
even fewer lines?

Next question: Couldn't class Kozmik be a module instead?

--
Phlip
http://www.oreilly.com/catalog/9780...
^ assert_xpath
4 Answers

Joel VanderWerf

9/14/2007 9:22:00 PM

0

Phlip wrote:
...
> class Kozmik
> def self.acts_as_frog(species)
> @frog_list ||= []
> @frog_list << species
> self.class.send :attr_accessor, :frog_list
> end
> end
>
> Is that as clean and lean as it could be? Is there some way to do this with
> even fewer lines?
>
> Next question: Couldn't class Kozmik be a module instead?

Sure...

module Kozmik
def acts_as_frog(species)
@frog_list ||= []
@frog_list << species
self.class.send :attr_accessor, :frog_list
end
end

class Toad; extend Kozmik
...
end

Or is that cheating somehow?

--
vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407

Phlip

9/14/2007 9:27:00 PM

0

Joel VanderWerf wrote:

> class Toad; extend Kozmik

extend... different from include, right?

--
Phlip

Joel VanderWerf

9/14/2007 9:38:00 PM

0

Phlip wrote:
> Joel VanderWerf wrote:
>
>> class Toad; extend Kozmik
>
> extend... different from include, right?

If you need it to be #include, you can always call #extend from within
#append_features as in ruby-talk:35979.

--
vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407

Ari Brown

9/14/2007 9:45:00 PM

0


On Sep 14, 2007, at 5:00 PM, Phlip wrote:

> Rubyists:
Here is one answering
> class Kozmik
> def self.acts_as_frog(species)
> @frog_list ||= []
> @frog_list << species
> self.class.send :attr_accessor, :frog_list
> end
> end
>
> Is that as clean and lean as it could be? Is there some way to do
> this with
> even fewer lines?

I am not really sure if this works, but couldn't you do:

class Kozmik
def self.inherited
module_eval do
@frog_list = []
end
end

def self.acts_as_frog(species)
@frog_list << species
end
end

Also, does module_eval only work in a class method? Why can't I do
def something; module_eval do puts 5 end; end ?

Ari
-------------------------------------------|
Nietzsche is my copilot