We extend prior work, on a language-based approach to versioned software development, to add support for versioned programs with mutable state and evolving method interfaces.

Unlike the traditional approach of mainstream version control systems, where each evolution step is represented by a textual diff, we treat versions as programming elements.

Each evolution step, merge operation, and version relationship, is represented explicitly in code. This provides static guarantees for safe code reuse from previous versions, as well as forward and backwards compatibility between versions, allowing clients to use newly introduced code without needing to manually refactor their program.

By lifting the versioning to the language level, we pave the way for tools that interact with software repositories to have more insight regarding the evolution of a system’s behaviour.

We instantiate our work in the Python programming language and demonstrate its applicability in regards to common evolution and refactoring patterns found in different versions of popular Python packages.