Provenance first: FormGenerator began as a hiring exercise and outgrew it. It takes a JSON schema describing fields and renders a working form — validation, conditional visibility, error announcement and focus management included — with no form library and no validation library underneath. Hand-rolling both is the point; the data modeling is the exhibit.

Fields are a discriminated union on their type, so an invalid schema does not compile — a select without options is unrepresentable — and the renderer narrows per case with no casts anywhere. The same compile-time-checklist trick appears three times over: the renderer's switch, the validation registry and the visibility operator dispatch each assign an unhandled case to never, so adding a field type, a rule or an operator stays a type error until every consumer handles it.

The engine is a headless hook. It owns state, validation timing and submission, and is exported on its own; the shipped component is only the default renderer over it. Fields the user cannot interact with — hidden by a visibility condition, or carrying a type the renderer does not know — are skipped by validation and dropped from the payload, so an invisible required field can never block a submit with nothing on screen to fix.