[ruby-core:21330] Re: Proc-as-Binding is out again?

On Wed, 14 Jan 2009 10:51:35 +0900, Charles Oliver Nutter wrote:

> Yukihiro Matsumoto wrote:
>> One possible idea is making "binding" read-only, or noting that eval
>> with binding may not modify the original binding.
> 
> That would be a reasonable compromise. So binding would make a copy of
> the captured scope, rather than reference it. It seems like there are
> few use cases for being able to capture a binding and mutate captured
> values.
> 
> - Charlie

You can't use read-only bindings, because then a trick like the following 
fails:

#start by cluttering up the current scope with all kinds of local 
#variables, all to load code into the variable literate_program

def empty_scope
   self.class.class_eval {remove_method :empty_scope}
   binding
end

eval literate_program, empty_scope


I don't like the idea of even the copying restriction. While you may 
think that local variables are sacred, I think that sacredness should be 
overrideable just like everything else, and when a binding is explicitly 
passed, it should have access to change things in the binding. Maybe the 
solution in your world is that Proc#binding needs to be changed somehow 
to provide more of a guarantee.

I'm not convinced that local variables are so sacred that a few useful 
conveniences aren't in order. We expect people to use all kinds of other 
power responsibly (including the power to overwrite all of the operators 
on core classes), so why couldn't we have Proc as binding or 
Binding.of_caller as a convenience that rivals the simplicity of C macros.

I certainly don't want to see Proc#binding go away completely. Consider 
particularly the s-expression generator in my SqlStatement gem. For it to 
operate as expected (using RubyNode in Ruby 1.8), I need to have access 
to the caller's local variables so that I can explicitly substitute in 
the local variable's value when the appropriate reference to the local 
variable is found. I'm doing this with the Binding from a Proc. If you 
eliminate that feature, and I have to pass in an explicit binding, then 
you've doubled the work involved in using an otherwise really nice and 
clean convenience.

--Ken

-- 
Chanoch (Ken) Bloom. PhD candidate. Linguistic Cognition Laboratory.
Department of Computer Science. Illinois Institute of Technology.
http://www.iit.edu/~kbloom1/