From: Simon Strandgaard <neoneye@...>
Date: 2004-05-19T23:20:58+09:00
Subject: Re: Arguments and formal variables

Angel Martin wrote:
> In first place I sorprised with te behavior of formal variables inside a
> metod or proc:
> 
> > irb(main):020:0> a=proc { |i| p i.id;i=7;p i.id}
> > => #<Proc:0x4028b604@(irb):20>
> > irb(main):021:0> b=5
> > => 5
> > irb(main):022:0> b.id
> > => 11
> > irb(main):023:0> a.call(b)
> > 11
> > 15
> > => nil
> > irb(main):024:0> b.id
> > => 11
> 
> I google this and in the faq warn about it, calling it formal variables.
> 
> Ok, if I want to change the value of a Array i can use Array#replace, 
> but is a way to change other types, Fixnum for example.
> Is any way to do this??

You will need to box the value.. there are many way to do this,
The most common quick and dirty solution is to use an Array,
but you can also do:
  
class Boxer
  def initialize(ref)
    @ref = ref
  end
  attr_reader :ref
  def set(ref)
    @ref = ref
  end
end
a=proc { |i| 
  p i.ref.object_id  # -> 11
  i.set(7)
  p i.ref.object_id  # -> 15
}
b=Boxer.new(5)
p b.ref.object_id  # -> 11
a.call(b)
p b.ref.object_id  # -> 15

-- 
Simon Strandgaard