[ruby-core:105381] [Ruby master Feature#18035] Introduce general model/semantic for immutable by default.

["tenderlovemaking (Aaron Patterson)" <noreply@ruby-lang.org>]

Issue #18035 has been updated by tenderlovemaking (Aaron Patterson).


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
```

----------------------------------------
Feature #18035: Introduce general model/semantic for immutable by default.
https://bugs.ruby-lang.org/issues/18035#change-93795

* 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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