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principle-boundary-discipline

Skill2 starsChanged 28 days agoMITView on GitHub

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.

We split this skill into the rules it gives an agent

3 rules. 3 are common, 0 are distinctive, and 0 are very specific to this project.

  • 3

    Common

    Most of the 42 edited versions we read keep these.

  • 0

    Distinctive

    This author's own choices. Worth a look.

  • 0

    Very specific

    Tied to their project's names, files or tools. Probably skip.

  • Common
  • Distinctive
  • Very specific
  1. Common:

    Place validation, type narrowing, and error handling at system boundaries.

    line 9
  2. Common:

    Do not re-export transport, storage, framework, or wire types through the public surface

    line 23
  3. Common:

    No redundant nil checks deep in call chains if the boundary already validated

    line 24

“Common” is measured: kept by at least 6 in 10 of the 42 edited versions of this file we could read. “Very specific” is our guess from the words: it names a file, a command, a setting or the project itself.

Keep what fits and drop the rest. Your version, ready for your agents.
Files
SKILL.md

Boundary Discipline

Place validation, type narrowing, and error handling at system boundaries. Trust validated internal types where their invariants actually hold; retain necessary error handling and synchronization. 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.

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