Prefer Types That Always Represent Valid States

Overview

Design types so that only valid states are representable.

If your types allow invalid combinations of values, you'll end up with code that's harder to write, harder to read, and more prone to bugs.

When to Use This Skill

• Designing state types for applications
• Creating types with related fields
• Modeling states that have dependencies
• Debugging impossible state errors
• Refactoring confusing type definitions

The Iron Rule