[ruby-core:76316] [Ruby trunk Feature#12086] using: option for instance_eval etc.

Issue #12086 has been updated by Shugo Maeda.


Nobuyoshi Nakada wrote:
> I'm against `instance_eval` under the hood by libraries.

I used to be against it too, but it's common now whether `using:` is available or not.

Let me talk about a use case.

I wrote a packrat parser library called radd_djur (https://github.com/shugo/radd_djur) before.
radd_djur uses refinements to implement an internal DSL to define grammar rules.

```ruby
require "radd_djur"

using RaddDjur::DSL

calc = RaddDjur::Grammar.new(:expr) {
  define :expr do
    [:int, "+", :int].bind { |x, *, y|
      ret x + y
    } /
    [:int, "-", :int].bind { |x, *, y|
      ret x - y
    }
  end

  define :int do
    (?0..?9).one_or_more.bind { |xs|
      ret xs.foldl1(&:+).to_i
    }
  end
}
p calc.parse("123+456")
```

`define` and `ret` are provided by `instance_eval`, and `bind`, `/`, and `one_or_more`
are provided by refinements.

It looks cool, but there are two problems here:

1. We have to write `using RaddDjur::DSL` explicitly.
2. Refinements for DSL is available out of the grammar rules.

If `instance_eval(using: refinement)` is introduced, RaddDjur::DSL can be activated
only in the block given to `RaddDjur::Grammar.new`, and these two problems will be solved.

Note that `instance_eval` and refinement activation have to be done atomically in this case.
That's why I proposed this feature as a new option of `instance_eval`.


----------------------------------------
Feature #12086: using: option for instance_eval etc.
https://bugs.ruby-lang.org/issues/12086#change-59552

* 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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