Object evolution is a monotonic approach to typestate and object reclassification, enforcing that objects may gain, but not lose properties, to permit aliasing. We present a formalization and prototype implementation of our new language May , featuring inheritance-based evolution that changes the run-time class of an object to a subclass. To statically guarantee evolution succeeds, we introduce a simple affine permission system for ensuring evolvable references match the run-time type of an object. Furthermore, we demonstrate that our system provides an effective and type-safe way of expressing staged operations and complex initialization procedures.

@article{paperbot3786,
  title = {Type-Safe Monotonic Object Evolution},
  author = {Alexandra Mirrlees-Black and Haoyu Wu and Gregor Richards and Fabian Muehlboeck},
  journal = {Proceedings of the ACM on Programming Languages},
  volume = {10},
  number = {OOPSLA1},
  year = {2026},
  doi = {10.1145/3798252}
}