From: "Ara.T.Howard" <Ara.T.Howard@...>
Date: 2005-10-02T04:22:05+09:00
Subject: Re: Creating a Class factory

On Sun, 2 Oct 2005, Gavin Kistner wrote:

> As noted in another thread, I'm looking at ways of creating a Struct-like 
> class; one that is a factory for creating classes. I did it one way by using 
> Struct itself, but I'm not wild about it. I'd like to write my own, that sets 
> true instance variables instead of struct's members thing. Is Struct using C 
> magic during its initialize to cause "Struct.new" to return class objects, or 
> is it just doing the equivalent of the following:
>
> class Foo
>    def self.new
>        klass = Class.new( self )
>        # setup klass here
>        klass
>    end
> end
>
> Is there a better way to make a class that creates 'instances' which are also 
> classes?
>
> Erps, except that the above doesn't work, because if you ask the new class to 
> inherit from the factory, then 'new' is overridden and doesn't call 
> initialize. For example:
>
> class KeywordClass
>    def self.new( property_hash )
>        klass = Class.new( self )
>        code = <<-ENDCODE
>            attr_accessor #{property_hash.keys.map{|k| 
> k.to_s.intern.inspect}.join(', ')}
>            def initialize( values={} )
>                #{property_hash.map{ |prop,default|
>                    "@#{prop} = values[#{prop.inspect}] || # 
> {default.inspect}"
>                }.join("\n")}
>            end
>        ENDCODE
>        klass.class_eval code
>        klass
>    end
>    def foo
>        "whoa"
>    end
> end
>
> People = KeywordClass.new :cat=>true
> puts People.new.cat
> #=> ArgumentError: wrong number of arguments (0 for 1)

traits let's you do this easily:

   harp:~ > cat a.rb
   require 'traits'

   class People
     trait 'cat' => true
   end
   p People::new.cat

   # or

   module KeywordClass
     def self::new *a, &b
       klass = Class::new { a.each{|t| trait t} }
       klass.module_eval &b if b
       klass
     end
   end
   People2 = KeywordClass::new 'hat', 'cat' => true
   p People2::new.cat
   p People2::new.hat

   People3 = KeywordClass::new(%w(cat in the hat)){ trait 'striped' => true }
   person = People3::new
   People3.reader_traits.each{|t| p t => person.send(t)}


   harp:~ > ruby a.rb
   true
   true
   nil
   {"cat"=>nil}
   {"in"=>nil}
   {"hat"=>nil}
   {"the"=>nil}
   {"striped"=>true}

hth.  traits also inherit their default values up the general inheritence
hierarchy - even for class traits.

-a
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