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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.

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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.

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