Boundary Discipline

Place validation, type narrowing, and error handling at the boundaries that own their contracts, including public framework APIs. Trust invariants established by types or earlier checks. Preserve operation-specific checks required by the contract when types or earlier checks do not establish the condition. Core logic lives in pure functions. The shell is thin and mechanical.

Why: Repeating established validation is noisy and redundant. Keep core rules separate from effectful adapter wiring so they can be tested independently.

The pattern:

• At boundaries (CLI args, config files, external APIs, network protocols, public framework APIs): validate and handle errors according to the contract.
• Inside the system: typed data and error propagation. Do not repeat validation of established invariants. Preserve operation-specific checks required by the contract when the types or earlier checks do not establish the condition.
• 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 leak private transport, storage, or wire representations through the public API. Framework types and intentional interoperability types may be part of the public contract.
• No redundant nil checks deep in call chains if the boundary already validated

Code organization:

• Core rules in pure functions using stable framework types, without dependencies on effectful adapter wiring
• 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 boundary right now?" If not, distinguish repeated validation of an established invariant from an operation-specific check required by the contract.
• "Can this be a pure function that the shell just calls?" If yes, extract it.