From: Robert Klemme <shortcutter@...>
Date: 2011-01-05T19:22:31+09:00
Subject: Re: Can't get class name

On Tue, Jan 4, 2011 at 6:29 PM, Samnang Chhun <samnang.chhun@gmail.com> wrote:
> I am learning how to design a DSL by using const_missing, but I face a
> problem on getting class name:

I am not sure what you are attempting.  But you should keep these facts in mind:

1. You cannot change inheritance of a class after creation.

2. A class has a name only after const assignment.

11:11:21 Temp$ ruby19 -e 'c=Class.new;p c, c.name;A=c;p c, c.name'
#<Class:0x1005309c>
nil
A
"A"

11:12:50 Temp$ ruby19 -e 'c=Class.new;p c,
c.name;Object.const_set("A",c);p c, c.name'
#<Class:0x1005304c>
nil
A
"A"

> class Drawing
>  def initialize(&block)
>    instance_eval(&block)
>  end
>
>  def self.const_missing(constant)
>    klass = Class.new do
>      def self.<(super_klass)
>        puts "#{self.name} inherits from #{super_klass.name}"
>      end

Bad idea IMHO: Class#< is defined already with particular semantics
(inheritance check):

11:13:59 Temp$ ruby19 -e 'p Array < Enumerable'
true

Also, as I said, you cannot change inheritance after creation of a class.

>      def self.-(association_klass)
>        puts "#{self.name} has association with
> #{association_klass.name}"
>      end
>    end
>
>    const_set(constant, klass)
>  end
> end
>
> Drawing.new do

You are creating an instance of Drawing while you probably rather want
constants Student, Person etc. set in class Drawing.

>  Student < Person
>  Student - Courses
> end
>
> Output:
>  inherits from
>  has association with
>
> Expected output:
> Student inherits from Person
> Student has association with Courses

Try this to see what's happening:

class Drawing
 def initialize(&block)
   instance_eval(&block)
 end

 def self.const_missing(constant)
   p [self, constant]

   klass = Class.new do
     def self.<(super_klass)
       # puts "#{self.name} inherits from #{super_klass.inspect}"
       printf "%p < %p\n", self, super_klass
     end

     def self.-(association_klass)
       # puts "#{self.name} has association with #{association_klass.inspect}"
       printf "%p - %p\n", self, association_klass
     end
   end

   def klass.x(cl)
     printf "%p x %p\n", self, cl
   end

   const_set(constant, klass)
 end
end

Drawing.new do
 Student < Person
 Student - Courses
 Student.x Foo
end

p Drawing.constants

class Drawing
 Student < Person
 Student - Courses
 Student.x Foo
end

p Drawing.constants

Kind regards

robert


-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/