Yup, it's the Either monad. But I intentionally omitted these terms (and others, such as "Algebraic Data Type") to show that functional programming can be practical and useful (not to say that it isn't).
The thing with Either is that it's really generic, and doesn't necessarily represent "the outcome of a computation that might fail". And although it's more convenient to others who are very familiar with functional programming, it adds to the slope of the learning curve (and makes functional programming seem again less practical).
I also chose to omit more useful APIs from the neverthrow package since simultaneous mapping can already be achieved through simpler means (using match), without having to teach people what a Bifunctor is. I took this approach because I am inspired by Elm's minimal design that tries to make functional programming as approachable and pragmatic as possible.
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