Why does it have Either? Doesn't TypeScript have "A | B" style sum types?

Either is biased, union is not.

Probably we should say "union" instead of sum, as typescript unions are not discriminated. string | string in typescript is exactly the same as just string, while Either[String, String] is a type which is exactly a sum of two string types. Plus Either is biased towards R, the happy path value.

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