From: "Jesús Gabriel y Galán" <jgabrielygalan@...>
Date: 2008-05-08T01:16:21+09:00
Subject: Re: Reading from file, create a class with variables

On Tue, May 6, 2008 at 11:33 AM, Pelle Strul <aardtwig@msn.com> wrote:
> Hi, I'm trying to load a file with specifications like:
>
>  title     :Person
>  attribute :name, String
>  attribute :age,  Fixnum
>  constraint :name, 'name != nil'
>  constraint :name, 'name.size > 0'
>  constraint :name, 'name =~ /^[A-Z]/'
>  constraint :age, 'age >= 0'
>
>  After which I want to create a class Person, with variables name being a
>  string, variable age being a fixed number and also the constraints for
>  them.

>  Any ideas how these specifications can be read and declared?

This is my first try at solving this problem. It might not be very good,
and I would also like people's comments on my solution, since I would
like to learn how to do these things better:

class ClassGenerator
	def initialize
		@constraints = {}
		@attr_names = []
	end
	
	def title the_title
		@class_name = the_title
	end	
	
	def attribute attr, klazz
		@attr_names << attr
		# First constraint is to check the class
		@constraints[attr] = ["#{attr}.is_a? #{klazz}"]
	end
	
	def constraint attr, constr
		@constraints[attr] << constr
	end
	
	def generate data
		instance_eval data
		klazz = Class.new
		klazz.class_eval "attr_reader #{@attr_names.map{|k| ":#{k}"}.join(",")}"	
		init_params = @attr_names.join(",")	
		initialize_body = ""
		@attr_names.each do |attr|
			@constraints[attr].each do |constraint|
				initialize_body <<  "raise ArgumentError.new('#{constraint}')
unless #{constraint};"
			end
			initialize_body << "@#{attr}=#{attr};"
		end
		klazz.class_eval "def initialize(#{init_params}); #{initialize_body}; end"	
		Object.const_set @class_name, klazz
	end
end

# The data could be read from a file, obviously
data =<<EOD
title     :Person
attribute :name, String
attribute :age,  Fixnum
constraint :name, 'name != nil'
constraint :name, 'name.size > 0'
constraint :name, 'name =~ /^[A-Z]/'
constraint :age, 'age >= 0'
EOD

# This should create a Person class, with
# an initialize method with a param for each attribute
# which checks the constraints raising ArgumentError
# if not passed, and assigning to an instance variable
# It also creates attr_readers for the attributes.

ClassGenerator.new.generate data

# So now we can do:
a = Person.new "A", 3
puts a.name
puts a.age

# These should fail with ArgumentError
# The msg of the error contains the constraint
Person.new "A", -3
Person.new "a", 3
Person.new '', 3

Hope this helps and I would appreciate any comment on my code.

Jesus.