Explicit state without polluting the domain model
Previously, we made state explicit by assigning state evolution to the core and state persistence and mutation to the shell. This made state visible at the highest level, and changing it required explicit passing between shell and core.
So far, we looked at state that had meaning at the domain level. The shell knew about snakes and food, and passed them around intentionally. But some state is not part of the domain story: a parser may need to remember where it is in an input stream, or a cache may need to remember previous lookups.
Handling such internal details the same way as regular domain state would pollute the domain model. Worse, unrelated logic could start depending on these details unintentionally. The system would still be explicit, but internal implementation details would start leaking into the surrounding domain model.
In object-oriented C++, we would usually solve this by putting the state behind a class interface. But if we want to keep the functional mechanics from the previous posts, the question becomes more specific:
