cursor 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. 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:
- Validate config at parse time (the boundary), not inside business logic
- Parse raw data into domain types at the boundary
- Do not re-export transport, storage, framework, or wire types through the public surface
- No redundant nil checks deep in call chains if the boundary already validated
Code organization:
- Business logic in pure functions with no framework dependencies
- Parse functions: pure transforms from raw bytes to typed state
- Prompt construction: structured state in, string out
- Scoring and assessment: pure transforms from state to results
The tests:
- "Is this data crossing a system boundary right now?" If not, validation is redundant.
- "Can this be a pure function that the shell just calls?" If yes, extract it.