From: Florian Gross <flgr@...>
Date: 2004-12-08T04:57:32+09:00
Subject: Re: Freezing Variable Assignment

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. 
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.