Deterministic Core, Non-Deterministic Shell

Fourteen years ago, Gary Bernhardt coined the term Functional Core, Imperative Shell . Like most good ideas in computing it was not entirely new, but his conception had great clarity, and it forms an excellent basis for talking about testing and determinism in existing systems.

Briefly, Functional Core/Imperative Shell architecture divides the code into two parts. The Functional Core is purely functional - that is no IO, and no destructive state updates. It is concerned with the business logic of the application. The Imperative Shell has comparatively little pathing, but maintains state, coordinates external dependencies, and deals with the outside world - that is to say IO. Its job is to query the core with values, receive values back as the result of some blackbox decision, and use that to interact with the outside world; whether that's writing to a database, sending a request, or updating a GUI.