Agent skill · software engineering · cursor
principle-type-system-discipline
Apply when designing types, reviewing a function signature, or writing code in any statically-typed language. Make illegal states unrepresentable, brand semantic primitives, parse external data at boundaries, refuse to lie to the compiler, exhaust variants, derive from authoritative schemas.
Why this skill is useful
Provides domain-specific knowledge on type system design patterns that the AI wouldn't reliably generate on its own.
What it needs
About 3k tokens when loaded. Last updated 2026-08-06. 2,555 stars on the source repository.
What this skill does
Type System Discipline The type checker is a proof assistant. Use it to eliminate impossible states, mismatched primitives, and unhandled variants at compile time. A case the types let you ignore becomes a runtime failure the compiler could have stopped. Prefer defining errors and special cases out of existence over proliferating handlers. Unrepresentable states, total functions, and interface redesign (the patterns below) are the tools. Applies to any typed language. Skills like typescript-best-practices ground it in specific syntax. The patterns: Make illegal states unrepresentable. Model variants as sum types: discriminated unions in TypeScript, enums with payloads in Rust/Swift/Kotlin, sealed classes in Scala, ADTs in Haskell/OCaml. Don't model state as a bag of optional fields where contradictory combinations compile. A subtle anti-pattern: { completed: boolean; completedAt?: Date } admits completed: true; completedAt: undefined, which is meaningless. Derive the boolean from a single source like completedAt !== null, or model the variants explicitly as { kind: 'open' } { kind: 'done'; at: Date }. If a bug forces the question "wait, can this combination actually happen?", the type is too loose. Types are constructions, not restrictions. Build the type up from the values you want instead of carving them out of a looser type with checks. The invariant that seems to need a refinement type is usually a construction away. A non-empty list is a head plus a rest, not a list with a length check. A valid time range is a start plus a duration, not two timestamps you must keep ordered. No representation is privileged. A list of pairs is an even-length list if you interpret it that way, so choose the shape that cannot build the illegal value and expose the interface callers need on top. Brand semantic primitives. UserId and OrderId are strings underneath but should not be interchangeable. …
How to use it
Reference it in AdaL, Claude Code, Cursor or any coding agent — nothing to install:
@skills cursor/principle-type-system-discipline