From: Austin Ziegler <halostatue@...>
Date: 2007-05-18T09:55:19+09:00
Subject: Re: Simulating single assignment in Ruby

On 5/17/07, Daniel Berger <djberg96@gmail.com> wrote:
> Is there a way to freeze instance variables in Ruby in order to make
> them single assignment? I'm just wondering if there's a way to
> simulate Erlang in this regard as a way to avoid side effects.
>
> I don't want to simply not define (or undefine) a setter method
> because you could still get at the instance variable via
> instance_variable_set. Redefining instance_variable_set won't work
> either, because that method is apparently not called when performing
> direct assignment of instance variables. I tried calling the freeze
> method on the instance variables directly but that didn't seem to work
> either.

Do it differently:

require 'digest/md5'

  class Module
    def erl_accessor(*names)
      names.each do |name|
        var = "@_#{Digest::MD5.hexdigest(rand(65536).to_s + name.to_s)}"
        define_method(name) { || instance_variable_get(var) }
        define_method("#{name}=") { |v|
          instance_variables.include? var and raise "Cannot change @#{name}."
          instance_variable_set(var, v)
        }
      end
      nil
    end
  end

  class Foo; erl_accessor :bar, :baz; end

  foo = Foo.new
  foo.bar = 5

Since your actual instance variables aren't related to the names, you
can only access through accessors.

-austin
-- 
Austin Ziegler * halostatue@gmail.com * http://www.halostatue.ca/
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