[ruby-core:96583] [Ruby master Feature#12086] using: option for instance_eval etc.

Issue #12086 has been updated by Eregon (Benoit Daloze).


Should we close this?
I agree with @headius here.

`using` with a lexical block (i.e., `using(refinement) do ... end`, not `using(&proc)`) might be a good alternative (#12281).

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Feature #12086: using: option for instance_eval etc.
https://bugs.ruby-lang.org/issues/12086#change-83550

* Author: shugo (Shugo Maeda)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
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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