Feature #21311
Updated by tagomoris (Satoshi Tagomori) about 6 hours ago
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 App.val # "8080"
p ns1::App.port # 2048
p ns1::App.new.val ns1::App.val # "2048"
p ns2::App.port # 4096
p ns2::App.new.val ns2::App.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).
### 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