Convert an Imperative Recommendation Service to Pure Functional Style
Overview
What this challenge is about.
Take the Python ranking module (around 900 lines). Identify side-effects (DB reads, cache lookups, logging, time) and lift them to the boundary. Rewrite the ranking core in Scala 3 as pure functions over immutable case classes. Express side-effecting dependencies as function parameters (Reader pattern) so tests need no mocks. Cover all ranking edge cases with unit tests that take no test doubles. Deliver Scala source, mock-free test suite, and a 6-page writeup on the refactor patterns applied (separation of pure core from imperative shell).
The Brief
What you'll do, and what you'll demonstrate.
Rewrite a recommendation-ranking module from imperative Python to pure-functional Scala with mock-free tests, demonstrating the functional-core/imperative-shell pattern.
Earning criteria — what you'll demonstrate
- Separate a pure functional core from imperative shell
- Apply the Reader pattern for dependency injection without DI frameworks
- Write a test suite that needs no mocks because the code has no hidden dependencies
- Communicate refactor patterns clearly to a Python-native team
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