Issue #18035 has been updated by ioquatix (Samuel Williams).


I would like us to define a model for immutability that has real world use cases and applicability - i.e. useful to developers in actual situations rather than theoretically sound and impossible to implement or impossible to use. Not that I'm saying theoretically sound is not important, just that we have to, as was said above, stop somewhere. Since this proposal includes a new module, it's totally optional. But that module is semantically independent from how we actually implement some kind of `deep_freeze`. The point of the module is to enforce it in a visible way - as in, this class will always be frozen. Such a design can then be used by the interpreter for constant propagation, etc.

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Feature #18035: Introduce general model/semantic for immutable by default.
https://bugs.ruby-lang.org/issues/18035#change-93799

* 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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