[ruby-core:122157] [Ruby Feature#21311] Namespace on read (revised)

Issue #21311 has been updated by fxn (Xavier Noria).


@bughit I totally see what you mean, and I had myself this very objection up in the thread.

While my post had the aim of spreading the word and make namespaces accessible to a broader audience, I have personally still not reached a definitive opinion about these details.

Now we have only one namespace, and in the new paradigm, things are still as they are today _per namespace_ (mod the leaks, but that will go away, I think). The rules get tricky when we are in cross-namespace territory. But this is a new territory! We need to experiment and understand the new model.

Think about it, can today a single program have objects with the same ID that are not equal? Yes! Absolutely it can!

That may happen when you fork, and we do not perceive anything weird here because we are so used to think about processes as isolates that being _the same program_ does not suggest an inconsistency to us. Even though in the same program the processes can communicate and exchange object IDs.

My intuition is that we'll start to think about a lot of stuff "per namespace" from now on. Like globals, per namespace.

>>From my perspective, that is not right or wrong in an absolute sense of the word. There is no one single definition of OOP or FP. Heck, the only thing that exists for real are CPU instructions, everything else is an abstraction we invent. How many languages allow you to modify something as fundamental as `Object` or `Kernel`? Not so many! Ruby has a very unique culture.

In the end, a programming language is a formal language, and it is up to the language designers to set the rules. You can define INTERCAL if you wish, the rules are yours and you can depart from what is considered standard as much as you want and give shape to your own variant of a theme.

What we all expect is that the rules are consistent.

Not saying you have to agree with any of these points, please feel free to strongly disagree, but maybe that opens a bit the way we all can look at it and adapt to the new concepts.






----------------------------------------
Feature #21311: Namespace on read (revised)
https://bugs.ruby-lang.org/issues/21311#change-113307

* Author: tagomoris (Satoshi Tagomori)
* Status: Assigned
* Assignee: tagomoris (Satoshi Tagomori)
* Target version: 3.5
----------------------------------------
This replaces #19744 

## Concept

This proposes a new feature to define virtual top-level namespaces in Ruby. Those namespaces can require/load libraries (either .rb or native extension) separately from other namespaces. Dependencies of required/loaded libraries are also required/loaded in the namespace.

This feature will be disabled by default at first, and will be enabled by an env variable `RUBY_NAMESPACE=1` as an experimental feature.
(It could be enabled by default in the future possibly.)

### "on read" approach

The "on write" approach here is the design to define namespaces on the loaded side. For example, Java packages are defined in the .java files and it is required to separate namespaces from each other. It can be implemented very easily, but it requires all libraries to be updated with the package declaration. (In my opinion, it's almost impossible in the Ruby ecosystem.)

The "on read" approach is to create namespaces and then require/load applications and libraries in them. Programmers can control namespace separation at the "read" time. So, we can introduce the namespace separation incrementally.

## Motivation

The "namespace on read" can solve the 2 problems below, and can make a path to solve another problem:

* Avoiding name conflicts between libraries
  * Applications can require two different libraries safely which use the same module name.
* Avoiding unexpected globally shared modules/objects
  * Applications can make an independent/unshared module instance.
* Multiple versions of gems can be required
  * Application developers will have fewer version conflicts between gem dependencies if rubygems/bundler will support the namespace on read. (Support from RubyGems/Bundler and/or other packaging systems will be needed)

For the motivation details, see [Feature #19744].

## How we can use Namespace

```ruby
# app1.rb
PORT = 2048
class App
  def self.port = ::PORT
  def val = PORT.to_s
end

p App.port # 2048

# app2.rb
class Number
  def double = self * 2
end

PORT = 2048.double
class App
  def self.port = ::PORT
  def val = PORT.double.to_s
end

p App.port # 4096

# main.rb - executed as `ruby main.rb`
ns1 = Namespace.new
ns1.require('./app1') # 2048
ns2 = Namespace.new
ns2.require('./app2') # 4096

PORT = 8080
class App
  def self.port = ::PORT
  def val = PORT.to_s
end

p App.port # 8080
p App.new.val # "8080"

p ns1::App.port # 2048
p ns1::App.new.val # "2048"

p ns2::App.port # 4096
p ns2::App.new.val # "8192"

1.double # NoMethodError
```

## Namespace specification

### Types of namespaces

There are two namespace types, "root" and "user" namespace. "Root" namespace exists solely in a Ruby process, and "user" namespaces can be created as many as Ruby programmers want.

### Root namespace

Root namespace is a unique namespace to be defined when a Ruby process starts. It only contains built-in classes/modules/constants, which are available without any `require` calls, including RubyGems itself (when `--disable-gems` is not specified).

At here, "builtin" classes/modules are classes/modules accessible when users' script evaluation starts, without any require/load calls.

### User namespace

User namespace is a namespace to run users' Ruby scripts. The "main" namespace is the namespace to run the user's `.rb` script specified by the `ruby` command-line argument. Other user namespaces ("optional" namespaces) can be created by `Namespace.new` call.

In user namespace (both main and optional namespaces), built-in class/module definitions are copied from the root namespace, and other new classes/modules are defined in the namespace, separately from other (root/user) namespaces.
The newly defined classes/modules are top-level classes/modules in the main namespace like `App`, but in optional namespaces, classes/modules are defined under the namespace (subclass of Module), like `ns::App`.

In that namespace `ns`, `ns::App` is accessible as `App` (or `::App`). There is no way to access `App` in the main namespace from the code in the different namespace `ns`.

### Constants, class variables and global variables

Constants, Class variables of built-in classes and global variables are also separated by namespace. Values set to class/global variables in a namespace are invisible in other namespaces.

### Methods and procs

Methods defined in a namespace run with the defined namespace, even when called from other namespaces.
Procs created in a namespace run with the defined namespace too.

### Dynamic link libraries

Dynamic link libraries (typically .so files) are also loaded in namespaces as well as .rb files.

### Open class (Changes on built-in classes)

In user namespaces, built-in class definitions can be modified. But those operations are processed as copy-on-write of class definition from the root namespace, and the changed definitions are visible only in the (user) namespace.

Definitions in the root namespace are not modifiable from other namespaces. Methods defined in the root namespace run only with root-namespace definitions.

## Enabling Namespace

Specify `RUBY_NAMESPACE=1` environment variable when starting Ruby processes. `1` is the only valid value here.

Namespace feature can be enabled only when Ruby processes start. Setting `RUBY_NAMESPACE=1` after starting Ruby scripts performs nothing.

## Pull-request

https://github.com/ruby/ruby/pull/13226






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