Roc's polymorphic tagged unions seem to be the sweet spot of not requiring specially-crafted enums or multiple levels of unwrapping, yet having lots of statical

Roc's polymorphic tagged unions seem to be the sweet spot of not requiring specially-crafted enums or multiple levels of unwrapping, yet having lots of statically-checked guarantees. (Zig is similar but doesn't support different types of payloads on the different error values).