Contracts and Errors Decision Examples

Positive example — semantic boundary parsing

• Context: an external command contains identifiers, quantities, and an optional timestamp.
• Mode: strict-boundaries.
• Decision: parse it once into validated semantic values, reject ambiguity, and expose a stable failure result.
• Why it fits: trusted code cannot receive a partial invalid command.
• Rejected alternative: pass a loosely checked representation through multiple layers.

Negative example — indiscriminate defensiveness

• Context: controlled pipeline stages exchange values created only by a validated constructor.
• Mode: trusted-internal-pipeline.
• Bad decision: reparse and catch every possible failure at every local handoff.
• Why it fails: duplicated checks obscure invariant ownership and can conceal programming errors.
• Better decision: validate provenance changes and fail visibly if an internal invariant is violated.