howto-program-functionally-ish

To separate pure logic from side effects, apply the Gather → Process → Persist pattern to isolate calculations from I/O, file access, and external calls within a thin orchestrator. This enforces pure function boundaries to improve testability and maintainability.

The functional core pattern identifies pure versus side-effecting code and provides a checklist to guide refactoring. It extracts pure logic into isolated functions, leaving only thin orchestrators to handle non-deterministic external calls and data persistence.

Refactor a small function to extract pure logic and isolate I/O in a thin orchestrator, then run tests. This Gather → Process → Persist approach separates data gathering, processing, and persistence to ensure determinism and minimal side effects.

Nondeterministic behavior during testing often occurs because calculations intermix with side effects like I/O or external calls. Separating pure logic from side effects enforces pure function boundaries and minimizes external dependencies to ensure deterministic test results.

Yes, this approach works for software projects that intermix calculations with I/O, file access, or external calls during development or refactoring. It guides testing practices to minimize nondeterminism and external dependencies without requiring prior structural overhauls.

You should avoid the Gather → Process → Persist pattern when refactoring overhead outweighs maintainability gains, such as in simple scripts with minimal calculations or trivial I/O where separating pure logic adds unnecessary architectural complexity.