principle-boundary-discipline

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Apply when wiring validation, error handling, or framework adapters. Concentrate guards at system boundaries (CLI, config, network, external APIs); trust internal types and keep business logic in pure functions.

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Added on2026-06-02

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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. Validate data once at the boundary. 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.

Applications:

Validation and error handling:

Validate config at parse time (the boundary), not inside business logic
Store raw data at boundaries; parse lazily at use-site
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.

principle-boundary-discipline

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Boundary Discipline