Checkpoint: Type-Safe Client Logic

The sandbox runs JavaScript, so this checkpoint verifies the RUNTIME halves of the module's patterns — the exact guards and shapes your TypeScript types will describe. If the logic is right, the types write themselves.

1. isUser(v) — a validator returning true only for well-formed user objects.
2. parseWith(v, validator, fallback) — validate-or-fallback wrapper.
3. apiGet(kind) — a fake client returning discriminated results and a handler that must check ok before touching data.

Types as the last module exam

TypeScript is only useful when it describes decisions, and this checkpoint asks you to make four: what unknown means at an API boundary, how a union makes invalid states unrepresentable, where a generic earns its parameter, and why a type guard beats an as cast. Each is a judgment call you will make daily on the job — now you have made them under grading conditions.