From: Mike Cahill <mike.cahill@...>
Date: 2009-02-11T13:01:25+09:00
Subject: redefining @my_attr=

I want to write a generic validator that ensures an attr value is in a
list before assigning, and if not, assigns a default.  It works for
self.attrib='value', and Dummy.new.attrib='value', but not
@attrib='value' within the class.  Any way to do that?  Here's the code:

module Validator

  def validate_is_member_of( attrib, list, default )
    original_method = instance_method( "#{attrib}=".to_sym )
    define_method( "#{attrib}=".to_sym ) do |val|
      instance_variable_set( "@#{attrib}", (list.include?( val ) ? val :
default ))
    end
  end

end

class Dummy
  extend Validator

  attr_accessor :name, :type
  validate_is_member_of :type, [ :fruit, :veggie, :dairy ], :fruit

  def initialize( name, type )
    @name = name
    @type = type
  end
end

good = Dummy.new('good', :veggie )      # => ... @type=:veggie
good.type = :ice_cream                              # => ...
@type=:fruit.  i know i overwrote a valid value. it's ok.
bad = Dummy.new('bad', :chocolate )     # => ... @type=:chocolate; I
want :fruit here.
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