[ruby-core:93436] [Ruby trunk Feature#15966] Introducing experimental features behind a flag, disabled by default

Issue #15966 has been updated by shevegen (Robert A. Heiler).


I was about to write too much, but then I reconsidered - I think it is better to write just a bit,
so here goes:

I do not necessarily have the same opinion as benoit, so I do not reach the same conclusion either.
Some of it is understandable; I think a bit of this is to be blamed on japanese-centric discussion
and possibly japanese culture. But I may be wrong too; it just seems as if people who complain tend
to be non-japanese more than japanese ;P - but it is also possibly true that there is a lot of 
information in japanese, such as communication, and for those who only speak english this information
is not easily accessible (unless a heroic japanese person translates it of course, which also 
has happened).

The only part that I would like to see changed/improved is that new features are documented/explained
eventually.

It's perfectly fine that matz experiments with new ideas. And matz often explains ideas and changes
in his presentations in english, too. :) But before that, it may be really helpful to explain what or
why something has been added. I know that you (mame) did so before e. g. with pattern matching (I refer
to the blog post of how other languages approach or have approached pattern matching historically),
but ideally it would be great if we could have this either at the bug tracker first; and/or (optionally)
at the wiki. The pipeline syntax was actually one of the very few that did not really have a whole
lot of discussion behind it - just about every other feature had quite a lot of discussion and
explanation here - refinements, startless range, pattern matching, all had quite a lot of information.

It would be great if this could be "uniform", even for new ideas that may be experimental. There are
of course blog posts that explain the context, but to me it feels a bit strange when blog posts
contain more information than the ruby bug tracker.

----------------------------------------
Feature #15966: Introducing experimental features behind a flag, disabled by default
https://bugs.ruby-lang.org/issues/15966#change-78981

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
* Target version: 2.7
----------------------------------------
I feel frustrated with some recent experimental features being added in `trunk`. These features, in my opinion:
* did not have enough discussion before being introduced.
* are going to be almost impossible to remove or even improve.
* got criticisms from hundreds of Rubyists in a couple days, illustrating the feature has major issues.

I think some (or maybe all?) experimental features being added to the language should be disabled by default, and enabled by a flag.
For instance, we could use the features system and have `--enable-experimental=...`.

For instance I'm thinking to 2 experimental features which have many problems reported by many people:
* #4475 / #15723 / #15897 Numbered parameters (`@1`, `@2`, ...)
  I think nobody realized matz would say "yes" after 1 month on https://bugs.ruby-lang.org/issues/4475#note-14, and so there was *zero* discussion about `@1`, `@2` before it was merged, and now we might get stuck with it.
  Many people shared their opinions after, and I think we should consider them carefully, which means not releasing `@1`, `@2` "accidentally" in 2.7.

* #15799 The pipeline operator (`|>`).
  Very little discussion before it was introduced also (the bug was only opened 2 weeks).
  The discussion on the bug tracker seems to have been mostly ignored, the feature was added without a response to concerns on the bug tracker.

Even though both these feature are documented as experimental, if they are released in 2.7, they will not be experimental in any way: there is no warning, nothing special to use it.
So there is no hope to change them if they get in Ruby 2.7.

With the new flag, these experimental features would then be disabled by default, until the core team decides they are stable enough (e.g., there are no major issues with it anymore),
and could still be tried as soon as implemented using e.g., `--enable-experimental=nbargs,pipeline`.

This would allow experimenting with the feature, and discuss about it,
and yet not having the feature set in stone and still be able to change it or remove it.

When such a feature is introduced in Ruby, there is very little chance for it to change or be removed.
In fact, it requires many people in the community to  invest significant time to express the problems of the feature and yet sometimes it's not enough.
If by December, no agreement was reached or no decision by matz was taken, the feature will stay (basically forever) whether it's good or has many issues.
This is not a good process. It requires massive efforts from many people in the community and might result in nothing changed.

By massive efforts from many people, I mean for example:
* Going to Japan in person to discuss with Japanese MRI committers and get a better idea of motivations, thoughts, concerns.
  MRI committers outside Japan and Rubyists in general cannot attend or express their opinion at developer meetings where many decisions are taken, and so have to rely on Japanese MRI committers to relay their opinions.

* Summarizing threads of 100+ comments, and preparing slides or a summary for discussion at a developer meeting (it takes hours).
* Trying to get the motivation for the feature written down on the issue tracker, since often it is not well explained on the issue tracker.
* Writing blog posts sharing thoughts and how to improve the feature/why it should be removed.
* Posting comments on the MRI bug tracker (inconvenient for many people, requires extra account, we see more responses on Twitter or GitHub).
* Writing [blog posts](http://katafrakt.me/2019/06/15/im-worried-about-ruby-future/) or issues like this one to express the general problems of the process for introducing experimental features.

Also, people sometimes respond rather aggressively (me too, I am sorry about that),
because I think people feel it's already too late to share opinions and have an impact on the decision,
the feature might have many problems and yet it's extremely unlikely to change.
I can say it's not a comfortable situation at least, I feel ignored, watching things go wrong and not being able to do much about it.

So I propose `--enable-experimental=...` to make the introduction of experimental features more smooth and allow discussion before the feature is basically written in stone.
What do you think?



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