From: "jeremyevans0 (Jeremy Evans)" <noreply@...>
Date: 2021-09-22T21:55:31+00:00
Subject: [ruby-core:105383] [Ruby master Feature#18035] Introduce general model/semantic for immutable by default.

Issue #18035 has been updated by jeremyevans0 (Jeremy Evans).


tenderlovemaking (Aaron Patterson) wrote in #note-12:
> jeremyevans0 (Jeremy Evans) wrote in #note-11:
> > @maciej.mensfeld alluded to this already, but one thing to consider is that no object in Ruby is truly immutable unless all entries in `object.singleton_class.ancestors` are also frozen/immutable.
> 
> Are they not?  It seems like for Arrays they are (I haven't checked other types), so maybe there's some precedent:
> 
> ```ruby
> x = [1, 2, 3].freeze
> 
> Mod = Module.new { def foo; end }
> 
> begin
>   x.extend(Mod)
> rescue FrozenError
>   puts "can't extend"
> end
> 
> begin
>   def x.foo; end
> rescue FrozenError
>   puts "can't def"
> end
> 
> begin
>   y = x.singleton_class
>   def y.foo; end
> rescue FrozenError
>   puts "can't def singleton"
> end
> ```

Apologies for not being more clear.  Freezing an object freezes the object's singleton class.  However, it doesn't freeze the other ancestors in `singleton_class.ancestors`:

```ruby
c = Class.new(Array)
a = c.new
a << 1
a.first # => 1
c.define_method(:first){0}
a.first # => 0
```

As this shows, an instance of a class is not immutable unless its class and all other ancestors of the singleton class are immutable. 

----------------------------------------
Feature #18035: Introduce general model/semantic for immutable by default.
https://bugs.ruby-lang.org/issues/18035#change-93797

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
----------------------------------------
It would be good to establish some rules around mutability, immutability, frozen, and deep frozen in Ruby.

I see time and time again, incorrect assumptions about how this works in production code. Constants that aren't really constant, people using `#freeze` incorrectly, etc.

I don't have any particular preference but:

- We should establish consistent patterns where possible, e.g.
  - Objects created by `new` are mutable.
  - Objects created by literal are immutable.

We have problems with how `freeze` works on composite data types, e.g. `Hash#freeze` does not impact children keys/values, same for Array. Do we need to introduce `freeze(true)` or `#deep_freeze` or some other method?

Because of this, frozen does not necessarily correspond to immutable. This is an issue which causes real world problems.

I also propose to codify this where possible, in terms of "this class of object is immutable" should be enforced by the language/runtime, e.g.


```ruby
module Immutable
  def new(...)
    super.freeze
  end
end

class MyImmutableObject
  extend Immutable

  def initialize(x)
    @x = x
  end
  
  def freeze
    return self if frozen?
    
    @x.freeze
    
    super
  end
end

o = MyImmutableObject.new([1, 2, 3])
puts o.frozen?
```

Finally, this area has an impact to thread and fiber safe programming, so it is becoming more relevant and I believe that the current approach which is rather adhoc is insufficient.

I know that it's non-trivial to retrofit existing code, but maybe it can be done via magic comment, etc, which we already did for frozen string literals.



-- 
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