[ruby-core:105384] [Ruby master Feature#18035] Introduce general model/semantic for immutable by default.

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


jeremyevans0 (Jeremy Evans) wrote in #note-13:
> 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.

Ah right.  I think your example is missing a `freeze`, but I get it.  If freezing an instance were to freeze all ancestors of the singleton, wouldn't that extend to `Object` / `BasicObject`?  I feel like we'd have to stop freezing *somewhere* because it would be pretty surprising if you can't define a new class or something because someone did `[].freeze`.  Simple statements like `FOO = [1].freeze` wouldn't work (as Object would get frozen before we could set the constant).

Maybe we could figure out a cheap way to copy things so that a mutation to the `Class.new` from your example wouldn't impact the instance `a`.

But regardless it seems like gradual introduction would be less surprising.  IOW maybe the goal would be to make *all* references immutable, but that really isn't practical. Instead expand the frozen horizon as much as we can without breaking existing code?

----------------------------------------
Feature #18035: Introduce general model/semantic for immutable by default.
https://bugs.ruby-lang.org/issues/18035#change-93798

* 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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