Functional core, imperative shell

FCIS emerged as an architectural pattern over time in the functional programming community. It is a design principle that advocates separating the immutable core business logic from the mutable aspects, such as persistent storage or external system integration. It aligns and encourages functional programming principles with the use of stateless functions and immutable data structures.

Stateless functions are sometimes called pure functions. They can be executed without causing side effects, in other words, for a given input, the function always produces the same output. Immutable data structures never change their content.

The FCIS principle has been influenced by various software development paradigms and architectural patterns. It shares similarities with the hexagonal architecture, which also promotes separating the core logic from the infrastructure concerns.

There are two major components in FCIS. The functional core contains stateless...