principle-boundary-discipline

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, environment variables, external APIs, network protocols, RPC and IPC payloads, JSON, database rows): 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.
• Everything observable is a contract. With a sufficient number of users of an API, all observable behaviors of your system will be depended on by somebody, regardless of what you promise in the contract. Published names are compatibility identifiers. Renaming one is a migration, not an edit.

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.