From: eregontp@...
Date: 2018-02-22T13:42:03+00:00
Subject: [ruby-core:85754] [Ruby trunk Bug#14541] Class variables have broken semantics, let's fix them

Issue #14541 has been updated by Eregon (Benoit Daloze).


shevegen (Robert A. Heiler) wrote:
> I'd rather just remove them altogether. :P

That might be possible in Ruby 3, but unlikely in Ruby 2.x.
Even then, I don't think we want to break compatibility too much for Ruby 3.
I would rather see this fixed before Ruby 3.

There are currently 228 instances of "@@" in the standard library alone, so it seems that breaking those and many gems would be unbearable.

Moreover, manually defining class-level instance variables with

~~~ruby
class MyClass
  @classvar = :initial_value
  class << self
    attr_accessor :@classvar
  end

  def some_use_of_classvar
    MyClass.classvar ||= ...
  end
end
~~~

is quite cumbersome, verbose and error-prone (to define the accessors on the singleton class).

So I think having the current class variables (@@) but with simple semantics would be convenient.
And I believe that would incidentally achieve what most of class variables usages in the wild expect (no inheritance, just state on the specific class).

----------------------------------------
Bug #14541: Class variables have broken semantics, let's fix them
https://bugs.ruby-lang.org/issues/14541#change-70596

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.6.0dev (2018-01-29 trunk 62091) [x86_64-linux]
* Backport: 2.3: UNKNOWN, 2.4: UNKNOWN, 2.5: UNKNOWN
----------------------------------------
Class variables have the weird semantics of being tied to the class hierarchy and being inherited between classes.
I think this is counter-intuitive, dangerous and basically nobody expects this behavior.

To illustrate that, we can break the tmpdir stdlib by defining a top-level class variable:

    $ ruby -rtmpdir -e '$SAFE=1; @@systmpdir=42; p Dir.mktmpdir {}'
    -e:1: warning: class variable access from toplevel
    Traceback (most recent call last):
    	3: from -e:1:in `<main>'
    	2: from /home/eregon/prefix/ruby-trunk/lib/ruby/2.6.0/tmpdir.rb:86:in `mktmpdir'
    	1: from /home/eregon/prefix/ruby-trunk/lib/ruby/2.6.0/tmpdir.rb:125:in `create'
    /home/eregon/prefix/ruby-trunk/lib/ruby/2.6.0/tmpdir.rb:125:in `join': no implicit conversion of Integer into String (TypeError)

Or even simpler in RubyGems:

    $ ruby -e '@@all=42; p Gem.ruby_version'
    -e:1: warning: class variable access from toplevel
    Traceback (most recent call last):
    	3: from -e:1:in `<main>'
    	2: from /home/eregon/prefix/ruby-trunk/lib/ruby/2.6.0/rubygems.rb:984:in `ruby_version'
    	1: from /home/eregon/prefix/ruby-trunk/lib/ruby/2.6.0/rubygems/version.rb:199:in `new'
    /home/eregon/prefix/ruby-trunk/lib/ruby/2.6.0/rubygems/version.rb:199:in `[]': no implicit conversion of String into Integer (TypeError)

So defining a class variable on Object removes class variables in all classes inheriting from Object.
Maybe @@systmpdir is not so prone to conflict, but how about @@identifier, @@context, @@locales, @@sequence, @@all, etc which are class variables of the standard library?

Moreover, class variables are extremely complex to implement correctly and very difficult to optimize due to the complex semantics.
In fact, none of JRuby, TruffleRuby, Rubinius and MRuby implement the "setting a class var on Object removes class vars in subclasses".
It seems all implementations but MRI print :foo twice here (instead of :foo :toplevel for MRI):

~~~ ruby
class Foo
  @@cvar = :foo
  def self.read
    @@cvar
  end
end

p Foo.read
@@cvar = :toplevel
p Foo.read
~~~


Is there any library actually taking advantage that class variables are inherited between classes? I would guess not or very few.
Therefore, I propose to give class variable intuitive semantics: no inheritance, they behave just like variables of that specific class, much like class-level instance variables (but separate for compatibility).

Another option is to remove them completely, but that's likely too hard for compatibility.

Thoughts?



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