[swift-evolution] [Pitch] Typed throws

[Tino Heth]

> I disagree with that, it works if you only have a single function parameter type that throws an error, but if there are more than one inferring the type won’t be possible anymore: func foo(_: () throws(T) -> Void, _: () throws(S) -> Void) rethrows(S) (here, we’re assuming that T is handled inside foo).
> 
Well, isn't it actually incorrect that "rethrows" is a property of the function, and not of its parameters?

Just imagine this

func catches(catchMe: () throws -> Void, throwMe: () throws -> Void) rethrows {
	try? catchMe()
	try throwMe()
	print("Nothing thrown")
}

func doesThrow() throws {
	throw NSError()
}

func doesntThrow() {
	print("I'm a nice function")
}

try catches(catchMe: doesThrow, throwMe: doesntThrow)

The last call can't throw, but the compiler still needs the "try" on it.
I guess in real-world code, this isn't an issue — but still, it's wrong… so if we want a really sophisticated model for error handling, imho this should be addressed.

Maybe we should also take into account that a higher-order function might catch some errors, and only rethrow for special cases, and this adds complexity as well?
In this case, each function-parameter would require a full list a errors that are filtered out, or even a translation-table… I guess most developers would agree that this is overkill, and that keeping things simple has merit as well.
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