principle-boundary-discipline

Validate data once at system boundaries such as CLI args, config parsing, and external APIs, then trust typed data internally. Return errors at the boundary and propagate them inside rather than adding redundant nil checks deep in call chains.

Place business logic in pure functions with no framework dependencies, and keep the outer shell thin and mechanical. Parse functions transform raw bytes to typed state, and scoring functions transform state to results, so all logic is testable in isolation.

No. App and UI modules should consume domain models and repository interfaces only. The data layer maps DTOs and entities to domain models and implements the repositories, so no new app-to-data model imports should be added.

Validation is redundant when the data is not crossing a system boundary at that point. If the boundary already parsed and validated the input into typed domain data, re-checking it inside the system adds noise without safety.

Expose domain concepts across the boundary, not the boundary's private representation. Do not re-export transport, storage, framework, or wire types through the public surface; keep general-purpose mechanism inside and policy at the edge.