Boundary Discipline

Place validation, type narrowing, and error handling at system boundaries. Trust internal code unconditionally. Business logic lives in pure functions; the shell is thin and mechanical.

Why: Scattered validation is noisy, redundant, and gives a false sense of safety. Validate data once at the boundary. Keep logic out of framework wiring so it can be tested without the framework.

The pattern:

• At boundaries (CLI args, config files, external APIs, network protocols): validate, return errors, handle defensively.
• Inside the system: typed data, error propagation, no re-validation. Trust the types.
• Across the boundary. Expose domain concepts, not the boundary's private representation. Keep general-purpose mechanism inside and special-purpose policy at the edge.

Applications:

Validation and error handling: