Issue #18035 has been updated by ioquatix (Samuel Williams).
There is a PR here that mostly just works:
https://github.com/ruby/ruby/pull/4879
However, additional work would be required to:
1. Tidy up the implementation and
2. Mark some core classes as Immutable as outlined in
https://gist.github.com/eregon/bce555fa9e9133ed27fbfc1deb181573
----------------------------------------
Feature #18035: Introduce general model/semantic for immutability.
https://bugs.ruby-lang.org/issues/18035#change-105910
* 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.
Proposed PR:
https://github.com/ruby/ruby/pull/4879
--
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