From: Florian Frank <flori@...>
Date: 2004-10-07T00:10:17+09:00
Subject: Re: quality of error messages

On 06.10.2004, at 15:48, Joachim Wuttke wrote:
>    Example 2: NoMethodError: undefined method.
> -> Wrong, I defined the method. I just forgot to
> include it in the attr_reader list. The system
> shouldn't mislead me.

It doesn't. If you define the method with "def" yourself, you don't 
have to call attr_reader at all.

attr_reader :foobar

generates an instance method like

def foobar
   @foobar
end

that returns the instance variable @foobar.

You can actually define your own metaprogramming methods:

class Module
   def attr_writer_positive(*ids)
     ids.each do |id|
       define_method("#{id}=") do |value|
         raise "not positive" unless value > 0
         instance_variable_set("@#{id}", value)
       end
     end
     nil
   end
end
# => nil

class Klass

   attr_writer_positive :foo

end
# => nil

k = Klass.new
# => #<Klass:0x5de78>
k.foo = 5
# => 5
k.foo = -5
RuntimeError: not positive
         from foo.rb:5:in `foo='
         from foo.rb:4:in `foo='
         from foo.rb:21

Florian Frank