From: shugo@...
Date: 2016-09-08T23:27:18+00:00
Subject: [ruby-core:77223] [Ruby trunk Feature#12086] using: option for	instance_eval etc.

Issue #12086 has been updated by Shugo Maeda.


Thomas Enebo wrote:
> What is the scope of instance_eval here?  Can I do:

The answer is yes, in my original proposal.  But It may be possible to prohibit these uses.

If we add such a restriction, the following way suggested by Charles might be better:

```
require "radd_djur"

calc = RaddDjur::Grammar.new(:expr) {
  using RaddDjur::DSL

  define :expr do # refined call to "define"
    [:int, "+", :int].bind { |x, *, y|  # refined "bind"
      ret x + y # refined "ret" and "+"
    } / # refined "/"
    [:int, "-", :int].bind { |x, *, y| # etc
      ret x - y
    }  
...
```

> Either of these essentially makes a lexically defined feature into a non-lexical one.  It also means absolutely any code in the system may potentially be refined.

Yes.


----------------------------------------
Feature #12086: using: option for instance_eval etc.
https://bugs.ruby-lang.org/issues/12086#change-60450

* 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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