From: Nicholas Van Weerdenburg <vanweerd@...>
Date: 2004-12-08T06:45:53+09:00
Subject: Re: Freezing Variable Assignment

On Wed, 8 Dec 2004 04:57:32 +0900, Florian Gross <flgr@ccan.de> wrote:
> Nicholas Van Weerdenburg wrote:
> 
> > Is there a feature to freeze variable assignment?
> >
> > e.g.
> > a="hello"
> > a.assignfreeze
> > a="goodbye"  # ===> generates exception
> >
> > Or, in a related vein, a type freeze, so that only similar objects can be added.
> 
> The proposed Syntax confuses the object a variable is referring to with
> the variable itself. It would likely be  assign_freeze :var  instead.

Yes. My bad.

> But I don't think this is general enough to include it directly in Ruby.
> 
> Let me suggest an idea for a revised request instead: Request a way of
> specifying a callback for local variable assignment. trace_var already
> can be used for doing that with global variables. It would be a
> generalization to also allow this with local ones. The interface would
> be this:
> 
> trace_var(:string) do |arg|
>    raise(TypeError, "Can't assign #{arg.inspect} to variable 'string'," +
>          " because it is not a String.") unless arg.is_a?(String)
> end
> 
> Using that you could implement assign_freeze() by yourself. But please
> make sure to think about performance issues. It might be a good idea to
> implement it and benchmark it and send the patch with statistics to
> ruby-core. Note that slowdown that is only caused by *using* the new
> feature (e. g. does not make old code slower) is not much of an issue.
> 
> Sorry if this sounds like a lot of work, but I think that if you do it
> like that it would be a very nice feature that would be generally useful
> to the community without having too much of a downside. It would allow
> for very powerful meta programming.
> 
> 

Those are good ideas. Maybe over Christmas I can take my ruby up an
notch and start working with c files.

Thanks,
Nick