Type-Level Error Messages

Related Skills: See typescript for general TypeScript conventions. See arktype for runtime regex validation and where this pattern surfaces in arktype itself.

The pattern in one example

// 1. Predicate on the literal type.
type IsSnakeCase<S extends string> = /* recursive template literal walk */;

// 2. Branded error type. The trailing ​ is a zero-width space.
type InvalidKey<S extends string> =
  `Invalid action key "${S}", must be snake_case ASCII matching /^[a-z][a-z0-9_]*$/​`;