Unknown, Never, Null, and Undefined

These types each have a distinct role.

Unknown

Use Unknown for values whose shape is not yet trusted. It is especially useful at host boundaries before parsing or validation has finished.

Never

Never represents code paths that do not produce a value, such as a helper that always throws.

fail = (message: String): Never => {
  throw { tag: 'fatal', message }
}

Null

Null is an explicit value-level absence marker. It is not silently assignable everywhere.

Undefined

Undefined is also explicit and distinct from Null. It often appears as the result of helpers that exist for side effects rather than meaningful values.

Why the distinction matters

FScript does not want JavaScript’s blurred absence model. Keeping these types separate improves clarity and type safety.