| name | code-conventions |
| description | General code conventions that keep a codebase predictable — reversible side effects, switching on discriminant tags, explicit-over-implicit at boundaries, configuration over hardcoded tunables, branded opaque ids, trusting types at typed boundaries, and honest empty catches. Use when writing, reviewing, or refactoring code in a typed language with an explicit boundary structure (services, plugins, modules, packages). |
Code Conventions
A set of conventions distilled from a large, heavily-gated TypeScript codebase. Each one prevents a recurring class of mistake. They are not style preferences; they are rules that keep behavior explicit, reversible, and checked at the right boundary. Apply the ones that fit your codebase's shape; most assume a typed language and module/package boundaries.
The defining constraint: every convention makes one thing unambiguous — where a side effect can be undone, where a decision is made, where a value is validated, and what a swallow actually swallowed.
Reversible side effects
Registrations and contributions are effects, and every effect has a disposer. When a plugin, module, or service contributes something to a shared context — a listener, a schema, a provider, a slot — the registration path returns the function that undoes it. This makes reload and teardown unwind predictably. If a contribution cannot be undone, that is a design gap to name, not something to hide.
Switch on discriminant tags
When a union is closed, exhaustiveness is checked by ending the switch in assertNever. When the union is deliberately open to extension, fall through a documented default rather than a bare default that silently does nothing. The reader should always be able to tell "every case handled and proven" from "unknown case falls through on purpose."
Explicit over implicit at boundaries
Defaulting and choice-making happen at one named step, never hidden inside a downstream call. If a request carries optional fields that must resolve to concrete values, the resolution is an explicit resolve(request) → spec step in the owning implementation — not a ?? default tucked inside run(). A caller reading run() should never have to hunt for the default it applied.
Configuration over hardcoded tunables
Anything that varies by deployment is a validated configuration field, changeable by the operator — not a hardcoded constant or a test-only hook. A DEFAULT_* constant is a code smell for a choice that belongs in config. The inverse also holds: protocol constants, external specifications, and security invariants stay fixed in code; they are not configurable because someone might someday want to change them.
Fail loud, at the earliest resolvable point
Misconfiguration fails loudly at load when it is self-contained, and at the earliest resolvable point otherwise. Never silently skip a missing referent — a config that names something absent is an error to surface, not a feature to ignore.
Brand opaque cross-boundary ids
An opaque identifier that crosses a module, service, or wire boundary is a branded type (Branded<B>), never a bare string. The brand keeps two unrelated id spaces from silently interoperating, and it tells the reader what domain the value belongs to.
Trust types at typed same-process boundaries
At a typed, same-process boundary — a direct call the compiler checks — do not add runtime validation, defensive fallbacks, or hostile-input tests for values the static interface already requires. Add runtime validation only where the boundary is genuinely untrusted: parser/config input, queued data, model or tool JSON, durable files, worker or process messages, and wire boundaries. Over-validating a typed boundary is redundant cost; under-validating an untyped one is a crash waiting.
Honest empty catches
An empty catch names what it swallows and why nothing else can reach it — a one-line comment that states the swallowed class of error and the argument that no other error can occur there. Keep the try to one statement so the catch's scope is obvious. Never write a bare catch {} that could be hiding a real failure.
Symmetry for parallel values
Parallel constructs (twin functions, sibling cases, paired options) are written symmetrically. An unexplained asymmetry usually means an extraction was missed — one path gained behavior the other silently lacks. When you see two things that "should" match and don't, ask which one is wrong before accepting the difference.
Tests describe behavior, not correctness
Tests pin behavior. When behavior intentionally changes, the tests change with it — do not preserve a "correct" test for obsolete behavior. Explain the behavior change in the PR; do not let a stale test masquerade as the source of truth.
Workflow
- Name the boundaries. For the code in hand, list the explicit boundaries: typed same-process calls, and the genuinely untrusted ones (config, wire, worker, file, model/tool JSON).
- Trace reversibility. Every contribution has a disposer; if one does not, flag it.
- Trace decision points. Every default, every switch fall-through, every catch is either explicit and documented, or an open bug.
- Apply the positive rule at each site.
Completion criteria
- Every side effect has a returned disposer.
- Every closed union ends in
assertNever; every open union falls through a documented default.
- Every default is an explicit resolution step, never a hidden
?? default in a downstream call.
- Every deployment-varying choice is a validated config field; protocol and security constants stay fixed.
- Every opaque cross-boundary id is branded.
- Runtime validation exists exactly at untrusted boundaries and nowhere else.
- Every empty catch names what it swallows and why nothing else can reach it.
- Parallel constructs are symmetric; every asymmetry is accounted for.
