[fleshweasel]

This is great. Seems less cumbersome than TypeScript's user defined type guards for distinguishing cases of a union. The sentinel value concept is something you could at least steal when creating user defined type guards to make them simpler, at least.

[wereHamster]

TypeScript union types and Flow tagged unions are nice to have, but to make them really useful you also have to provide a good way to pattern match on them. And there they both are still way behind Haskell.