From: Wilson Bilkovich <wilsonb@...>
Date: 2008-03-09T05:33:22+09:00
Subject: Re: Double colon vs module block

On Thu, Mar 6, 2008 at 1:51 PM, Daniel Mendler <dmendler@wurzelteiler.de> wrote:
> Trans schrieb:
>
>
> > On Mar 6, 1:21 pm, Daniel Mendler <dmend...@wurzelteiler.de> wrote:
>  >
>  >> Hi!
>  >>
>  >> I am new to ruby. I have a question concerning namespaces. Is there a
>  >> difference between using the double colon operator or a module block?
>  >> Example:
>  >>
>  >> module Library
>  >>   class Book
>  >>   end
>  >> end
>  >>
>  >> vs.
>  >>
>  >> class Library::Book
>  >> end
>  >>
>  >
>  > The later will give an error if Library isn't yet defined (personally
>  > I with is would just auto-instantiate a new module). Also the lookup
>  > of constants resolves differently. For example:
>  >
>  >   module Library
>  >     SOMETHING = "whatever"
>  >   end
>  >
>  >   class Library::Book
>  >     SOMETHING  #=> NameError: uninitialized constant
>  >   end
>  >
>  > But I _think_ this has been fixed in Ruby 1.9... Can someone confirm
>  > that?
>  >
>  > T.
>  >
>  >
>  That's exactly the exception I got. Can you explain why this happens?
>


module Foo::Baz directly 'opens' the Baz scope without opening the Foo
scope (for the purposes of constant resolution).

This code here illustrates the difference:

# contrived example.. ACTIVATE
module A
  A_CONSTANT = 1
  module B
    B_CONSTANT = 2
  end
end

module A # open A's scope
  module B # open B's scope under A
    p B_CONSTANT
    p A_CONSTANT
  end
end

module A::B # open B's scope without opening A
  p B_CONSTANT # B constants are resolved
  p A_CONSTANT # This throws a NameError unless A_CONSTANT is defined
in B or TOPLEVEL
end