Functional Core Imperative Shell (FCIS) Architecture FAQs

Question 1

Can I use logging in a Functional Core file?

Accepted Answer

Yes, this skill explicitly allows logging as the only permitted side effect in the Functional Core, as it generally does not interfere with the determinism of unit tests.

Question 2

Why does this skill require mandatory file comments?

Accepted Answer

The pattern comments (e.g., // pattern: Functional Core) serve as a structural enforcement tool, ensuring every file's architectural intent is explicitly declared and visible to both developers and AI.

Question 3

What is the Functional Core, Imperative Shell (FCIS) pattern?

Accepted Answer

It is an architectural pattern where business logic is kept in 'pure' functions (the Core) that have no side effects, while I/O operations like database calls or API requests are isolated in thin wrappers (the Shell).

Question 4

How does this improve unit testing?

Accepted Answer

Because the Functional Core contains no I/O, you can test complex business rules with simple assertions and zero mocks. The Imperative Shell is then tested via integration tests.

Question 5

What files are exempt from classification?

Accepted Answer

Files without runtime behavior, such as type-only files, constants, configuration files (JSON/YAML), and documentation, do not require the pattern classification comment.

Functional Core Imperative Shell (FCIS) Architecture

Functional Core Imperative Shell (FCIS) Architecture

主な機能

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主な機能

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