From: "J. Cooper" <nefigah@...>
Date: 2008-04-10T04:46:01+09:00
Subject: Re: My error or bug in "attr_reader" ?

Well, it's true; the instance variable isn't writable. Doing

showcase.entries = [3,2,1,7]

will throw an error, because entries is read-only. (There is no entries= 
method or attr_writer :entries). There is a difference between a 
read-only attribute that is a collection, and an attribute that is a 
read-only collection. In the former, you can't up and assign a different 
collection to the attribute, but you can certainly modify the contents 
of the collection itself. In the latter, the collection itself 
(independent of the fact that it's an attribute of a class) isn't 
modifiable.

Here is how to do the latter:

class AttrReaderErr
   attr_reader :entries
   def initialize()
     @entries = [0,1,2].freeze  # This makes the collection unmodifiable
   end
end

showcase = AttrReaderErr.new
showcase.entries[1] = "foo"   # TypeError: can't modify frozen array
showcase.entries = ["foo"]  # NoMethodError: undefined method 
`entries='...
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