A simple demo example of the Functional Core, Imperative Shell pattern in TypeScript. For more details about this pattern, please check out my blog post, The Functional Core, Imperative Shell Pattern.

The Functional Core (core.ts) implements the Blog Post Workflow shown in my blog post, How to Model Workflows in REST APIs, in a functional way using immutable values and pure functions.

Each state in the workflow is implemented as an immutable type and uses the tagged union pattern (with the status property) to easily differentiate between the different states.

The reason for representing each state in the workflow as a type (rather than having one type with a status property) is to make illegal workflow transition impossible and to spot them at compile time rather than at runtime.

Each workflow transition is implemented as a pure function.

The Imperative Shell (shell.ts) do the side-effectful stuff like writing to the console. In a more realistic implementation, this would also be where web services call, database interactions, and UI interactions would happen, as those are all non-functional activities by their very nature.

To run the example, just type npm run dev.