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Discussion on: IO: to be a Monad or not to be, that's the question!

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Adam Warski

Thanks for the link! I think my reservations come from the fact that with IO you have the following signature: race(a: IO[A], b: IO[B]): IO[Either[A, B]]. While with suspensions, you have: race(a: suspend () -> A, b: suspend () -> B): Either[A, B].

Note that the return type doesn't return our "effect type", that would be () -> Either[A, B], but an eagerly evaluated value (Either[A, B]). And this matters for composition, meaning that if you want to compose that process later with others, you'll have to keep that in mind when defining it.

Hence it seems we're trading the uniformity of IO and some composition properties for the better readability and performance of suspensions. As always, tradeoffs :)

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