jac-types

E1001 occurs when an any value flows into a declared non-any destination. Fix it by typing the source with a return annotation, narrowing with isinstance before assignment, or casting at the use site with value as Type.

Use value as Type to re-type a value for the checker; it is unchecked and type-erased at runtime like typing.cast. It is most useful for landing any values from walker reports or JSON into concrete types such as result.reports[0] as list[TweetView].

Yes, Jac supports declared type params like obj Result[T, E = Exception] and def first[T](items: list[T]) -> T. However, type-param defaults do not apply at subscripted construction, and the checker treats T opaquely, so cast results back to concrete types.

No, use Jac-native syntax instead: write X | None instead of Optional[X] and X | Y instead of Union[X, Y]. Lowercase built-ins like list[X] and dict[K, V] replace List and Dict, and the any keyword replaces typing.Any.

E1032 means type inference failed, often from an unresolved import or a Self return type, which the current checker resolves to Unknown. Add an explicit annotation or return the concrete archetype name instead of Self.

Use import type to break circular imports between modules whose types reference each other, since it compiles to a TYPE_CHECKING-guarded import. Do not use it for names you construct, isinstance-check, or use in has field types, as those need runtime resolution.