Hey Aleksander, On Thu, Mar 11, 2021 at 8:25 AM Aleksander Machniak <a...@alec.pl> wrote:
> On 10.03.2021 20:28, Ben Ramsey wrote: > >> I don't like that type covariance would be allowed. Why such an > >> exception to the rules? > > > > It’s not an exception. Returns are covariant. Parameters are > > contravariant. Since `noreturn` is a subtype of all other types, it > > behaves as expected. > > I see it's a subtype, I don't get why. Wouldn't it be better to be a > separate type so return type covariance is not allowed (as it is with > void)? Was it a design decision or a side product of the implementation? > `noreturn` is what's called the "bottom type", and it's a subtype of all types. It can sound counter-intuitive, but it is true that all methods in a subclass can be re-implemented with the bottom type as their return type (instead of their original one), and the system is still sound from a type perspective. Marco Pivetta http://twitter.com/Ocramius http://ocramius.github.com/
