> On Jun 30, 2017, at 5:38 AM, Matthew Johnson <[email protected]> wrote:
>
>> 3. If we later change `throw` from being a statement to being a
>> `Never`-returning expression, you could use `throw` on the right-hand side
>> of `??`.
>
> What do you have in mind here? I don't recall any discussion of `throw`
> return Never. It seems like a novel use of a bottom type that might preclude
> the possibility of ever having a Result type that seamlessly bridges to
> Swift's error handling.
`throw` is currently a statement. Imagine, for sake of illustration, that it
was instead a function. This function would take an `Error` as a parameter and
throw it. It would never return normally—it would only return by throwing—so
its return type would be `Never`:
@_implicitlyTry
func throw(_ error: Error) throws -> Never {
try Builtin.throw(error)
unreachable()
}
What I'm suggesting is that `throw` should remain a keyword, but should have
the semantics of this `throw(_:)` function. The parser should allow it in
expression context, the `try` checker should treat it as though it was already
marked `try`, and the type checker should treat it as an expression that
returns `Never` but can throw.
That would then allow you to say things like:
let lastItem = array.last ?? throw MyError.arrayEmpty
It would not have any negative effect I can think of on `Result`. In fact,
trying to directly wrap a `throw SomeError.foo` statement in a `Result` would
produce a `Result<Never, SomeError>`, correctly expressing the fact that the
result of that particular expression can never be successful.
--
Brent Royal-Gordon
Architechies
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