From: Ilmari Heikkinen <kig@...>
Date: 2005-05-10T00:43:40+09:00
Subject: Re: An idiom I like... modifiable defaults


On 9.5.2005, at 17:42, Hal Fulton wrote:

> Just thought I'd share a little concept that I find
> useful. Your comments are welcome.
>
> Sometimes objects are created with certain defaults.
> One way to override them is with default values in
> the constructor (and often corresponding writer methods).
>
> But sometimes I "don't like" the default and want to
> change it (for this program/session).
>
> Often I use class-level accessors for that purpose.
>
> Here's a contrived example...
>
>
> Cheers,
> Hal
>

Hi,
I quite like this, but wonder if it works with inheritance?

On a related note, it seems a lot of people (you, me, Ara Howard, 
probably many others) are coming up with these configuration idioms, 
maybe we could find a way that fulfills most needs (and perhaps even 
try to get it into ruby core)?

Here's a quick list of features for discussion, feel free to add your 
own:
- named arguments (ie. hash argument way)
- auto-assigning configuration vars to instance variables
- works with inheritance (this is probably the most difficult -- and 
useful -- one)
- auto-generated accessors
- configuration by block
- maybe some way to co-operate with the regular positional params


Would look something like this in use:

class Image
   include TheAutoConfigModule
   config_accessor{
     name # defaults to nil
     width = 100
     height = 100
     depth = 32
   }
   def bytes
     @width * @height * @depth / 8
   end
end

class BWImage < Image
   config_accessor{ depth = 8 }
end

i = Image.new(:height => 200, :depth => 16)
i.width == 100 and i.height == 200 and i.depth == 16
#=> true
i.name
#=> nil
i.bytes
#=> 40000

bwi = BWImage.new( 'holiday shots', 10, 10 )
bwi.width == 10 and bwi.height == 10 and bwi.depth == 8
#=> true
bwi.name
#=> 'holiday shots'
bwi.bytes
#=> 100

Image.config
#=> {:name => nil, :width => 100, :height => 100, :depth => 32}

BWImage.config.height = 200
BWImage.config
#=> {:name => nil, :width => 100, :height => 200, :depth => 8}

Thoughts?


Cheers,
Ilmari

>
>   class Text
>
>     class << self
>       attr_accessor :color
>       Text.color = "black"
>     end
>
>     attr_accessor :color
>
>     def initialize(txt, color="black")
>   # Hint: You can improve this further by saying
>   # def initialize(txt, color=Text.color)
>       puts "#{color} text..."
>     end
>   end
>
>
>   # The old way...
>
>   a = Text.new("some")           # black
>   b = Text.new("random","blue")  # blue
>   c = Text.new("text")           # black
>   c.color = "blue"               # but now it's blue
>
>   # The new way...
>
>   Text.color = "blue"
>
>   e = Text.new("Ruby is cool")   # blue
>   f = Text.new("as dry ice")     # blue
>