From: headius@...
Date: 2016-09-09T01:20:33+00:00
Subject: [ruby-core:77226] [Ruby trunk Feature#12086] using: option for	instance_eval etc.

Issue #12086 has been updated by Charles Nutter.


> Yes, you'll get unexpected results in this case.

I think you'd get unexpected results in my original case too, wouldn't you? Both of those two blocks still have the same prev_cref, which is where the refinements collection comes from. Am I wrong?

In any case, even with the single Proc, it seems like a showstopper in this design. There's no way to avoid two threads refining the same block incompatibly.

> It's true that cache is invalidated by instance_eval(using:), but global method caching has been improved in MRI, and I don't know how different compared to JRuby.

What is the impact of that invalidation? I am not familiar with how MRI globally invalidates these days and how it reduces the impact.

> I don't know it should be applied to code used with the new features.

But if I have chosen not to use this feature, for the performance of my application, I'll also have to check every library I depend on to see if they use the feature. If I don't, such a library might impact the performance of my entire application. I don't think we want that.

So summarizing my concerns up to this point:

* The current design is not thread-safe. It might be possible to make it thread-safe at the cost of additional complexity, which may mean further reducing performance. (design issue)
* If any code in the system uses the current implementation of this feature, that impacts the performance unrelated code by invalidating global caches. (implementation issue)
* All blocks everywhere in the system will now be suspect; you will not be able to tell what method will be called unless you control everywhere that block will be passed (usability issue, in my opinion)

----------------------------------------
Feature #12086: using: option for instance_eval etc.
https://bugs.ruby-lang.org/issues/12086#change-60453

* Author: Shugo Maeda
* Status: Open
* Priority: Normal
* Assignee: 
----------------------------------------
Currently refinements can be activated only in toplevel or class/module definitions.
If they can be activated in block-level, it's useful to implement internal DSLs.

How about to add a new option using: for Kernel#instance_eval and Moule#{class,module}_eval?

```ruby
module FixnumDivExt
  refine Fixnum do
    def /(other)
      quo(other)
    end
  end
end

p 1 / 2 #=> 0
instance_eval(using: FixnumDivExt) do
  p 1 / 2 #=> (1/2)
end
p 1 / 2 #=> 0
```

Proof-of-concept implementation is available at <https://github.com/shugo/ruby/tree/eval_using>.

In my previous proposal before Ruby 2.0, refinements used in a class or module are
implicitly activated by instance_eval and class_eval, but now I think it's better to
explicitly specify refinements to be activated.

Considerations:

* In the PoC implementation, refined methods are not cached inline, and thus it decreases
  the performance of refined method call.
  If there is a way to guarantee that blocks never be evaluated in different environments,
  refined methods can be cached inline.
* {instance,class,module}_exec cannot be extended in the same way, because they take arbitrary
  arguments and there's no way to distinguish an option hash from the last argument hash.




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