Functional Core, Imperative Shell

The basic idea is that your core domain model implements business logic using functional techniques, with immutable values and copy-on-write. The functional core is wrapped by an imperative shell which drives the core models and interacts across the boundary with the outside world through “primitive” values. The immutable domain models can be easily converted to primitive data structures whose state has no affect on the immutable core.

nonobvious.models and nonobvious.fields provide a declarative modeling language, similar to Django’s model objects, but without an ORM. We can easily use validators and adaptors from the valideer project to ensure that the models are always in a valid state and can easily be constructed from standard python data structures.

By making use of concon.frozendict, concon.frozenlist, and concon.frozenset within model adaptors allows us to model complex data structures guaranteed to be correctly formed at all times.

As for the imperative shell, at present, you’re on your own!