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 als 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.