F# Testing with Expecto

Philosophy: Test What Matters

Tests are not bureaucratic overhead—they're executable specifications that catch regressions and document behavior. Focus on testing the parts that matter: domain logic, validation, and state transitions.

Before writing tests, ask:

• What behavior am I verifying?
• Is this testing a pure function (easy) or I/O (harder)?
• What edge cases exist?
• What's the cost if this breaks in production?

Core Principles:

1. Test Pure Functions Thoroughly: Domain logic is pure and trivial to test. No mocking needed. These tests are your best ROI.

2. Test Boundaries, Not Internals: Validate inputs at API boundaries, not deep inside implementation. Test the contract, not the implementation.

3. Property Tests for Invariants: When a property should always hold (commutativity, idempotence), property-based testing finds edge cases you won't think of.