[SE-0413: Typed throws] needs union types

It is hard to use if it allow throw only one type Error.

throws(ErrorTypeA | ErrorTypeB) should be allowed

Most of the cases are throwing multiple type Error.

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The proposal says:


Yes. Thanks for pointing out. Without Union type (or sum type), the typed error means nothing for most developer using cases.

I’ll repeat the argument from the original proposal discussion against this: if you throw two errors today, how do you know you won’t throw three errors tomorrow? A dedicated enum type (wrapping the two errors in their own cases) can handle this in a source-compatible way*; untyped throws or even a custom protocol can as well, with no claims of exhaustive case matching; but plain old anonymous union types, like tuples, would break callers as soon as you need to add a third possibility.

* …if we get around to adding a frozen/non-frozen distinction for enums in libraries that aren’t using the binary-compatible library evolution mode. Which admittedly is not much less hypothetical than anonymous union types, but is a little more scoped, and anyway the language recommendation is still to prefer untyped errors.


I'm pretty sure this accepted SE feature needs a sum type. We'll see when developer or framework team using this typed error feature without sum type, will make a lot of unnecessary enum error types. Declaring and naming these kind of Enums is ridiculous.

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It sounds like you should just be throwing Error. You don't have to use typed throws just because it was added to the language.


You can use Either<ErrorA, ErrorB> or similar OneOfThree<A, B, C> enum for ability to throw two / three Error Types. Does it cover your cases?